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DASH(1) General Commands Manual DASH(1)
dash — command interpreter (shell)
dash [-aCefnuvxIimqVEb] [+aCefnuvxIimqVEb] [-o option_name]
[+o option_name] [command_file [argument ...]] dash -c
[-aCefnuvxIimqVEb] [+aCefnuvxIimqVEb] [-o option_name]
[+o option_name] command_string [command_name [argument ...]]
dash -s [-aCefnuvxIimqVEb] [+aCefnuvxIimqVEb] [-o option_name]
[+o option_name] [argument ...]
dash is the standard command interpreter for the system. The
current version of dash is in the process of being changed to
conform with the POSIX 1003.2 and 1003.2a specifications for the
shell. This version has many features which make it appear
similar in some respects to the Korn shell, but it is not a Korn
shell clone (see ksh(1)). Only features designated by POSIX,
plus a few Berkeley extensions, are being incorporated into this
shell. This man page is not intended to be a tutorial or a
complete specification of the shell.
Overview
The shell is a command that reads lines from either a file or the
terminal, interprets them, and generally executes other commands.
It is the program that is running when a user logs into the
system (although a user can select a different shell with the
chsh(1) command). The shell implements a language that has flow
control constructs, a macro facility that provides a variety of
features in addition to data storage, along with built in history
and line editing capabilities. It incorporates many features to
aid interactive use and has the advantage that the interpretative
language is common to both interactive and non-interactive use
(shell scripts). That is, commands can be typed directly to the
running shell or can be put into a file and the file can be
executed directly by the shell.
Invocation
If no args are present and if the standard input of the shell is
connected to a terminal (or if the -i flag is set), and the -c
option is not present, the shell is considered an interactive
shell. An interactive shell generally prompts before each
command and handles programming and command errors differently
(as described below). When first starting, the shell inspects
argument 0, and if it begins with a dash ‘-’, the shell is also
considered a login shell. This is normally done automatically by
the system when the user first logs in. A login shell first
reads commands from the files /etc/profile and .profile if they
exist. If the environment variable ENV is set on entry to an
interactive shell, or is set in the .profile of a login shell,
the shell next reads commands from the file named in ENV.
Therefore, a user should place commands that are to be executed
only at login time in the .profile file, and commands that are
executed for every interactive shell inside the ENV file. To set
the ENV variable to some file, place the following line in your
.profile of your home directory
ENV=$HOME/.shinit; export ENV
substituting for “.shinit” any filename you wish.
If command line arguments besides the options have been
specified, then the shell treats the first argument as the name
of a file from which to read commands (a shell script), and the
remaining arguments are set as the positional parameters of the
shell ($1, $2, etc). Otherwise, the shell reads commands from
its standard input.
Argument List Processing
All of the single letter options that have a corresponding name
can be used as an argument to the -o option. The set -o name is
provided next to the single letter option in the description
below. Specifying a dash “-” turns the option on, while using a
plus “+” disables the option. The following options can be set
from the command line or with the set builtin (described later).
-a allexport Export all variables assigned to.
-c Read commands from the command_string
operand instead of from the standard
input. Special parameter 0 will be set
from the command_name operand and the
positional parameters ($1, $2, etc.) set
from the remaining argument operands.
-C noclobber Don't overwrite existing files with “>”.
-e errexit If not interactive, exit immediately if
any untested command fails. The exit
status of a command is considered to be
explicitly tested if the command is used
to control an if, elif, while, or until;
or if the command is the left hand operand
of an “&&” or “||” operator.
-f noglob Disable pathname expansion.
-n noexec If not interactive, read commands but do
not execute them. This is useful for
checking the syntax of shell scripts.
-u nounset Write a message to standard error when
attempting to expand a variable that is
not set, and if the shell is not
interactive, exit immediately.
-v verbose The shell writes its input to standard
error as it is read. Useful for
debugging.
-x xtrace Write each command to standard error
(preceded by a ‘+ ’) before it is
executed. Useful for debugging.
-I ignoreeof Ignore EOF's from input when interactive.
-i interactive Force the shell to behave interactively.
-l Make dash act as if it had been invoked as
a login shell.
-m monitor Turn on job control (set automatically
when interactive).
-s stdin Read commands from standard input (set
automatically if no file arguments are
present). This option has no effect when
set after the shell has already started
running (i.e. with set).
-V vi Enable the built-in vi(1) command line
editor (disables -E if it has been set).
-E emacs Enable the built-in emacs(1) command line
editor (disables -V if it has been set).
-b notify Enable asynchronous notification of
background job completion. (UNIMPLEMENTED
for 4.4alpha)
Lexical Structure
The shell reads input in terms of lines from a file and breaks it
up into words at whitespace (blanks and tabs), and at certain
sequences of characters that are special to the shell called
“operators”. There are two types of operators: control operators
and redirection operators (their meaning is discussed later).
Following is a list of operators:
Control operators:
& && ( ) ; ;; | || <newline>
Redirection operators:
< > >| << >> <& >& <<- <>
Quoting
Quoting is used to remove the special meaning of certain
characters or words to the shell, such as operators, whitespace,
or keywords. There are three types of quoting: matched single
quotes, matched double quotes, and backslash.
Backslash
A backslash preserves the literal meaning of the following
character, with the exception of ⟨newline⟩. A backslash
preceding a ⟨newline⟩ is treated as a line continuation.
Single Quotes
Enclosing characters in single quotes preserves the literal
meaning of all the characters (except single quotes, making it
impossible to put single-quotes in a single-quoted string).
Double Quotes
Enclosing characters within double quotes preserves the literal
meaning of all characters except dollarsign ($), backquote (`),
and backslash (\). The backslash inside double quotes is
historically weird, and serves to quote only the following
characters:
$ ` " \ <newline>.
Otherwise it remains literal.
Reserved Words
Reserved words are words that have special meaning to the shell
and are recognized at the beginning of a line and after a control
operator. The following are reserved words:
! elif fi while case
else for then { }
do done until if esac
Their meaning is discussed later.
Aliases
An alias is a name and corresponding value set using the alias(1)
builtin command. Whenever a reserved word may occur (see above),
and after checking for reserved words, the shell checks the word
to see if it matches an alias. If it does, it replaces it in the
input stream with its value. For example, if there is an alias
called “lf” with the value “ls -F”, then the input:
lf foobar ⟨return⟩
would become
ls -F foobar ⟨return⟩
Aliases provide a convenient way for naive users to create
shorthands for commands without having to learn how to create
functions with arguments. They can also be used to create
lexically obscure code. This use is discouraged.
Commands
The shell interprets the words it reads according to a language,
the specification of which is outside the scope of this man page
(refer to the BNF in the POSIX 1003.2 document). Essentially
though, a line is read and if the first word of the line (or
after a control operator) is not a reserved word, then the shell
has recognized a simple command. Otherwise, a complex command or
some other special construct may have been recognized.
Simple Commands
If a simple command has been recognized, the shell performs the
following actions:
1. Leading words of the form “name=value” are stripped
off and assigned to the environment of the simple
command. Redirection operators and their arguments
(as described below) are stripped off and saved for
processing.
2. The remaining words are expanded as described in the
section called “Expansions”, and the first remaining
word is considered the command name and the command is
located. The remaining words are considered the
arguments of the command. If no command name
resulted, then the “name=value” variable assignments
recognized in item 1 affect the current shell.
3. Redirections are performed as described in the next
section.
Redirections
Redirections are used to change where a command reads its input
or sends its output. In general, redirections open, close, or
duplicate an existing reference to a file. The overall format
used for redirection is:
[n] redir-op file
where redir-op is one of the redirection operators mentioned
previously. Following is a list of the possible redirections.
The [n] is an optional number between 0 and 9, as in ‘3’ (not
‘[3]’), that refers to a file descriptor.
[n]> file Redirect standard output (or n) to file.
[n]>| file Same, but override the -C option.
[n]>> file Append standard output (or n) to file.
[n]< file Redirect standard input (or n) from file.
[n1]<&n2 Copy file descriptor n2 as stdin (or fd n1).
[n]<&- Close standard input (or n).
[n1]>&n2 Copy file descriptor n2 as stdout (or fd n1).
[n]>&- Close standard output (or n).
[n]<> file Open file for reading and writing on standard
input (or n).
The following redirection is often called a “here-document”.
[n]<< delimiter
here-doc-text ...
delimiter
All the text on successive lines up to the delimiter is saved
away and made available to the command on standard input, or file
descriptor n if it is specified. If the delimiter as specified
on the initial line is quoted, then the here-doc-text is treated
literally, otherwise the text is subjected to parameter
expansion, command substitution, and arithmetic expansion (as
described in the section on “Expansions”). If the operator is
“<<-” instead of “<<”, then leading tabs in the here-doc-text are
stripped.
Search and Execution
There are three types of commands: shell functions, builtin
commands, and normal programs – and the command is searched for
(by name) in that order. They each are executed in a different
way.
When a shell function is executed, all of the shell positional
parameters (except $0, which remains unchanged) are set to the
arguments of the shell function. The variables which are
explicitly placed in the environment of the command (by placing
assignments to them before the function name) are made local to
the function and are set to the values given. Then the command
given in the function definition is executed. The positional
parameters are restored to their original values when the command
completes. This all occurs within the current shell.
Shell builtins are executed internally to the shell, without
spawning a new process.
Otherwise, if the command name doesn't match a function or
builtin, the command is searched for as a normal program in the
file system (as described in the next section). When a normal
program is executed, the shell runs the program, passing the
arguments and the environment to the program. If the program is
not a normal executable file (i.e., if it does not begin with the
"magic number" whose ASCII representation is "#!", so execve(2)
returns ENOEXEC then) the shell will interpret the program in a
subshell. The child shell will reinitialize itself in this case,
so that the effect will be as if a new shell had been invoked to
handle the ad-hoc shell script, except that the location of
hashed commands located in the parent shell will be remembered by
the child.
Note that previous versions of this document and the source code
itself misleadingly and sporadically refer to a shell script
without a magic number as a "shell procedure".
Path Search
When locating a command, the shell first looks to see if it has a
shell function by that name. Then it looks for a builtin command
by that name. If a builtin command is not found, one of two
things happen:
1. Command names containing a slash are simply executed without
performing any searches.
2. The shell searches each entry in PATH in turn for the
command. The value of the PATH variable should be a series
of entries separated by colons. Each entry consists of a
directory name. The current directory may be indicated
implicitly by an empty directory name, or explicitly by a
single period.
Command Exit Status
Each command has an exit status that can influence the behaviour
of other shell commands. The paradigm is that a command exits
with zero for normal or success, and non-zero for failure, error,
or a false indication. The man page for each command should
indicate the various exit codes and what they mean.
Additionally, the builtin commands return exit codes, as does an
executed shell function.
If a command consists entirely of variable assignments then the
exit status of the command is that of the last command
substitution if any, otherwise 0.
Complex Commands
Complex commands are combinations of simple commands with control
operators or reserved words, together creating a larger complex
command. More generally, a command is one of the following:
• simple command
• pipeline
• list or compound-list
• compound command
• function definition
Unless otherwise stated, the exit status of a command is that of
the last simple command executed by the command.
Pipelines
A pipeline is a sequence of one or more commands separated by the
control operator |. The standard output of all but the last
command is connected to the standard input of the next command.
The standard output of the last command is inherited from the
shell, as usual.
The format for a pipeline is:
[!] command1 [| command2 ...]
The standard output of command1 is connected to the standard
input of command2. The standard input, standard output, or both
of a command is considered to be assigned by the pipeline before
any redirection specified by redirection operators that are part
of the command.
If the pipeline is not in the background (discussed later), the
shell waits for all commands to complete.
If the pipefail option was enabled when the shell began execution
of the pipeline, the pipeline's exit status is the exit status of
the last command specified in the pipeline that exited with non-
zero status, or zero if all commands in the pipeline exited with
a status of zero. If the pipefail option was not enabled, the
pipeline's exit status is the exit status of the last command
specified in the pipeline; the exit statuses of any other
commands are not used. If the reserved word ! precedes the
pipeline, its exit status is the logical NOT of the exit status
described above.
Because pipeline assignment of standard input or standard output
or both takes place before redirection, it can be modified by
redirection. For example:
$ command1 2>&1 | command2
sends both the standard output and standard error of command1 to
the standard input of command2.
A ; or ⟨newline⟩ terminator causes the preceding AND-OR-list
(described next) to be executed sequentially; a & causes
asynchronous execution of the preceding AND-OR-list.
Note that unlike some other shells, each process in the pipeline
is a child of the invoking shell (unless it is a shell builtin,
in which case it executes in the current shell – but any effect
it has on the environment is wiped).
Background Commands – &
If a command is terminated by the control operator ampersand (&),
the shell executes the command asynchronously – that is, the
shell does not wait for the command to finish before executing
the next command.
The format for running a command in background is:
command1 & [command2 & ...]
If the shell is not interactive, the standard input of an
asynchronous command is set to /dev/null.
Lists – Generally Speaking
A list is a sequence of zero or more commands separated by
newlines, semicolons, or ampersands, and optionally terminated by
one of these three characters. The commands in a list are
executed in the order they are written. If command is followed
by an ampersand, the shell starts the command and immediately
proceeds onto the next command; otherwise it waits for the
command to terminate before proceeding to the next one.
Short-Circuit List Operators
“&&” and “||” are AND-OR list operators. “&&” executes the first
command, and then executes the second command if and only if the
exit status of the first command is zero. “||” is similar, but
executes the second command if and only if the exit status of the
first command is nonzero. “&&” and “||” both have the same
priority.
Flow-Control Constructs – if, while, for, case
The syntax of the if command is
if list
then list
[ elif list
then list ] ...
[ else list ]
fi
The syntax of the while command is
while list
do list
done
The two lists are executed repeatedly while the exit status of
the first list is zero. The until command is similar, but has
the word until in place of while, which causes it to repeat until
the exit status of the first list is zero.
The syntax of the for command is
for variable [ in [ word ... ] ]
do list
done
The words following in are expanded, and then the list is
executed repeatedly with the variable set to each word in turn.
Omitting in word ... is equivalent to in "$@".
The syntax of the break and continue command is
break [ num ]
continue [ num ]
Break terminates the num innermost for or while loops. Continue
continues with the next iteration of the innermost loop. These
are implemented as builtin commands.
The syntax of the case command is
case word in
[(]pattern) list ;;
...
esac
The pattern can actually be one or more patterns (see “Shell
Patterns” described later), separated by “|” characters. The “(”
character before the pattern is optional.
Grouping Commands Together
Commands may be grouped by writing either
(list)
or
{ list; }
The first of these executes the commands in a subshell. Builtin
commands grouped into a (list) will not affect the current shell.
The second form does not fork another shell so is slightly more
efficient. Grouping commands together this way allows you to
redirect their output as though they were one program:
{ printf " hello " ; printf " world\n" ; } > greeting
Note that “}” must follow a control operator (here, “;”) so that
it is recognized as a reserved word and not as another command
argument.
Functions
The syntax of a function definition is
name () command
A function definition is an executable statement; when executed
it installs a function named name and returns an exit status of
zero. The command is normally a list enclosed between “{” and
“}”.
Variables may be declared to be local to a function by using a
local command. This should appear as the first statement of a
function, and the syntax is
local [variable | -] ...
Local is implemented as a builtin command.
When a variable is made local, it inherits the initial value and
exported and readonly flags from the variable with the same name
in the surrounding scope, if there is one. Otherwise, the
variable is initially unset. The shell uses dynamic scoping, so
that if you make the variable x local to function f, which then
calls function g, references to the variable x made inside g will
refer to the variable x declared inside f, not to the global
variable named x.
The only special parameter that can be made local is “-”. Making
“-” local any shell options that are changed via the set command
inside the function to be restored to their original values when
the function returns.
The syntax of the return command is
return [exitstatus]
It terminates the currently executing function. Return is
implemented as a builtin command.
Variables and Parameters
The shell maintains a set of parameters. A parameter denoted by
a name is called a variable. When starting up, the shell turns
all the environment variables into shell variables. New
variables can be set using the form
name=value
Variables set by the user must have a name consisting solely of
alphabetics, numerics, and underscores - the first of which must
not be numeric. A parameter can also be denoted by a number or a
special character as explained below.
Positional Parameters
A positional parameter is a parameter denoted by a number (n >
0). The shell sets these initially to the values of its command
line arguments that follow the name of the shell script. The set
builtin can also be used to set or reset them.
Special Parameters
A special parameter is a parameter denoted by one of the
following special characters. The value of the parameter is
listed next to its character.
* Expands to the positional parameters, starting from
one. When the expansion occurs within a double-
quoted string it expands to a single field with the
value of each parameter separated by the first
character of the IFS variable, or by a ⟨space⟩ if
IFS is unset.
@ Expands to the positional parameters, starting from
one. When the expansion occurs within double-
quotes, each positional parameter expands as a
separate argument. If there are no positional
parameters, the expansion of @ generates zero
arguments, even when @ is double-quoted. What this
basically means, for example, is if $1 is “abc” and
$2 is “def ghi”, then "$@" expands to the two
arguments:
"abc" "def ghi"
# Expands to the number of positional parameters.
? Expands to the exit status of the most recent
pipeline.
- (Hyphen.) Expands to the current option flags (the single-
letter option names concatenated into a string) as
specified on invocation, by the set builtin command,
or implicitly by the shell.
$ Expands to the process ID of the invoked shell. A
subshell retains the same value of $ as its parent.
! Expands to the process ID of the most recent
background command executed from the current shell.
For a pipeline, the process ID is that of the last
command in the pipeline.
0 (Zero.) Expands to the name of the shell or shell script.
Word Expansions
This clause describes the various expansions that are performed
on words. Not all expansions are performed on every word, as
explained later.
Tilde expansions, parameter expansions, command substitutions,
arithmetic expansions, and quote removals that occur within a
single word expand to a single field. It is only field splitting
or pathname expansion that can create multiple fields from a
single word. The single exception to this rule is the expansion
of the special parameter @ within double-quotes, as was described
above.
The order of word expansion is:
1. Tilde Expansion, Parameter Expansion, Command Substitution,
Arithmetic Expansion (these all occur at the same time).
2. Field Splitting is performed on fields generated by step (1)
unless the IFS variable is null.
3. Pathname Expansion (unless set -f is in effect).
4. Quote Removal.
The $ character is used to introduce parameter expansion, command
substitution, or arithmetic evaluation.
Tilde Expansion (substituting a user's home directory)
A word beginning with an unquoted tilde character (~) is
subjected to tilde expansion. All the characters up to a slash
(/) or the end of the word are treated as a username and are
replaced with the user's home directory. If the username is
missing (as in ~/foobar), the tilde is replaced with the value of
the HOME variable (the current user's home directory).
Parameter Expansion
The format for parameter expansion is as follows:
${expression}
where expression consists of all characters until the matching
“}”. Any “}” escaped by a backslash or within a quoted string,
and characters in embedded arithmetic expansions, command
substitutions, and variable expansions, are not examined in
determining the matching “}”.
The simplest form for parameter expansion is:
${parameter}
The value, if any, of parameter is substituted.
The parameter name or symbol can be enclosed in braces, which are
optional except for positional parameters with more than one
digit or when parameter is followed by a character that could be
interpreted as part of the name. If a parameter expansion occurs
inside double-quotes:
1. Pathname expansion is not performed on the results of the
expansion.
2. Field splitting is not performed on the results of the
expansion, with the exception of @.
In addition, a parameter expansion can be modified by using one
of the following formats.
${parameter:-word} Use Default Values. If parameter is unset
or null, the expansion of word is
substituted; otherwise, the value of
parameter is substituted.
${parameter:=word} Assign Default Values. If parameter is
unset or null, the expansion of word is
assigned to parameter. In all cases, the
final value of parameter is substituted.
Only variables, not positional parameters
or special parameters, can be assigned in
this way.
${parameter:?[word]} Indicate Error if Null or Unset. If
parameter is unset or null, the expansion
of word (or a message indicating it is
unset if word is omitted) is written to
standard error and the shell exits with a
nonzero exit status. Otherwise, the value
of parameter is substituted. An
interactive shell need not exit.
${parameter:+word} Use Alternative Value. If parameter is
unset or null, null is substituted;
otherwise, the expansion of word is
substituted.
In the parameter expansions shown previously, use of the colon in
the format results in a test for a parameter that is unset or
null; omission of the colon results in a test for a parameter
that is only unset.
${#parameter} String Length. The length in characters of
the value of parameter.
The following four varieties of parameter expansion provide for
substring processing. In each case, pattern matching notation
(see “Shell Patterns”), rather than regular expression notation,
is used to evaluate the patterns. If parameter is * or @, the
result of the expansion is unspecified. Enclosing the full
parameter expansion string in double-quotes does not cause the
following four varieties of pattern characters to be quoted,
whereas quoting characters within the braces has this effect.
${parameter%word} Remove Smallest Suffix Pattern. The word
is expanded to produce a pattern. The
parameter expansion then results in
parameter, with the smallest portion of the
suffix matched by the pattern deleted.
${parameter%%word} Remove Largest Suffix Pattern. The word is
expanded to produce a pattern. The
parameter expansion then results in
parameter, with the largest portion of the
suffix matched by the pattern deleted.
${parameter#word} Remove Smallest Prefix Pattern. The word
is expanded to produce a pattern. The
parameter expansion then results in
parameter, with the smallest portion of the
prefix matched by the pattern deleted.
${parameter##word} Remove Largest Prefix Pattern. The word is
expanded to produce a pattern. The
parameter expansion then results in
parameter, with the largest portion of the
prefix matched by the pattern deleted.
Command Substitution
Command substitution allows the output of a command to be
substituted in place of the command name itself. Command
substitution occurs when the command is enclosed as follows:
$(command)
or (“backquoted” version):
`command`
The shell expands the command substitution by executing command
in a subshell environment and replacing the command substitution
with the standard output of the command, removing sequences of
one or more ⟨newline⟩s at the end of the substitution. (Embedded
⟨newline⟩s before the end of the output are not removed; however,
during field splitting, they may be translated into ⟨space⟩s,
depending on the value of IFS and quoting that is in effect.)
Arithmetic Expansion
Arithmetic expansion provides a mechanism for evaluating an
arithmetic expression and substituting its value. The format for
arithmetic expansion is as follows:
$((expression))
The expression is treated as if it were in double-quotes, except
that a double-quote inside the expression is not treated
specially. The shell expands all tokens in the expression for
parameter expansion, command substitution, and quote removal.
Next, the shell treats this as an arithmetic expression and
substitutes the value of the expression.
White Space Splitting (Field Splitting)
After parameter expansion, command substitution, and arithmetic
expansion the shell scans the results of expansions and
substitutions that did not occur in double-quotes for field
splitting and multiple fields can result.
The shell treats each character of the IFS as a delimiter and
uses the delimiters to split the results of parameter expansion
and command substitution into fields.
If IFS is empty, field splitting yields no fields if the input
string was empty, and one string with the unchanged value of the
input otherwise. For example, with the default IFS, “read -r l”
will remove any initial whitespace, but “IFS= read -r l” will
leave the entire line in l.
Pathname Expansion (File Name Generation)
Unless the -f flag is set, file name generation is performed
after word splitting is complete. Each word is viewed as a
series of patterns, separated by slashes. The process of
expansion replaces the word with the names of all existing files
whose names can be formed by replacing each pattern with a string
that matches the specified pattern. There are two restrictions
on this: first, a pattern cannot match a string containing a
slash, and second, a pattern cannot match a string starting with
a period unless the first character of the pattern is a period.
The next section describes the patterns used for both Pathname
Expansion and the case command.
Shell Patterns
A pattern consists of normal characters, which match themselves,
and meta-characters. The meta-characters are “!”, “*”, “?”, and
“[”. These characters lose their special meanings if they are
quoted. When command or variable substitution is performed and
the dollar sign or back quotes are not double quoted, the value
of the variable or the output of the command is scanned for these
characters and they are turned into meta-characters.
An asterisk (“*”) matches any string of characters. A question
mark matches any single character. A left bracket (“[”)
introduces a character class. The end of the character class is
indicated by a (“]”); if the “]” is missing then the “[” matches
a “[” rather than introducing a character class. A character
class matches any of the characters between the square brackets.
A range of characters may be specified using a minus sign. The
character class may be complemented by making an exclamation
point the first character of the character class.
To include a “]” in a character class, make it the first
character listed (after the “!”, if any). To include a minus
sign, make it the first or last character listed.
Builtins
This section lists the builtin commands which are builtin because
they need to perform some operation that can't be performed by a
separate process. In addition to these, there are several other
commands that may be builtin for efficiency (e.g. printf(1),
echo(1), test(1), etc).
:
true A null command that returns a 0 (true) exit value.
false A null command that returns a 1 (false) exit value.
. file
The commands in the specified file are read and executed
by the shell.
alias [name[=string ...]]
If name=string is specified, the shell defines the alias
name with value string. If just name is specified, the
value of the alias name is printed. With no arguments,
the alias builtin prints the names and values of all
defined aliases (see unalias).
bg [job] ...
Continue the specified jobs (or the current job if no jobs
are given) in the background.
command [-p] [-v] [-V] command [arg ...]
Execute the specified command but ignore shell functions
when searching for it. (This is useful when you have a
shell function with the same name as a builtin command.)
-p search for command using a PATH that guarantees to
find all the standard utilities.
-V Do not execute the command but search for the
command and print the resolution of the command
search. This is the same as the type builtin.
-v Do not execute the command but search for the
command and print the absolute pathname of
utilities, the name for builtins or the expansion
of aliases.
cd|chdir -
cd|chdir [-LP] [directory]
Switch to the specified directory (default HOME). If an
entry for CDPATH appears in the environment of the cd
command or the shell variable CDPATH is set and the
directory name does not begin with a slash, then the
directories listed in CDPATH will be searched for the
specified directory. The format of CDPATH is the same as
that of PATH. If a single dash is specified as the
argument, it will be replaced by the value of OLDPWD. The
cd command will print out the name of the directory that
it actually switched to if this is different from the name
that the user gave. These may be different either because
the CDPATH mechanism was used or because the argument is a
single dash. The -P option causes the physical directory
structure to be used, that is, all symbolic links are
resolved to their respective values. The -L option turns
off the effect of any preceding -P options.
echo [-n] args...
Print the arguments on the standard output, separated by
spaces. Unless the -n option is present, a newline is
output following the arguments.
If any of the following sequences of characters is
encountered during output, the sequence is not output.
Instead, the specified action is performed:
\b A backspace character is output.
\c Subsequent output is suppressed. This is normally
used at the end of the last argument to suppress
the trailing newline that echo would otherwise
output.
\f Output a form feed.
\n Output a newline character.
\r Output a carriage return.
\t Output a (horizontal) tab character.
\v Output a vertical tab.
\0digits
Output the character whose value is given by zero
to three octal digits. If there are zero digits,
a nul character is output.
\\ Output a backslash.
All other backslash sequences elicit undefined behaviour.
eval string ...
Concatenate all the arguments with spaces. Then re-parse
and execute the command.
exec [command arg ...]
Unless command is omitted, the shell process is replaced
with the specified program (which must be a real program,
not a shell builtin or function). Any redirections on the
exec command are marked as permanent, so that they are not
undone when the exec command finishes.
exit [exitstatus]
Terminate the shell process. If exitstatus is given it is
used as the exit status of the shell; otherwise the exit
status of the preceding command is used.
export name ...
export -p
The specified names are exported so that they will appear
in the environment of subsequent commands. The only way
to un-export a variable is to unset it. The shell allows
the value of a variable to be set at the same time it is
exported by writing
export name=value
With no arguments the export command lists the names of
all exported variables. With the -p option specified the
output will be formatted suitably for non-interactive use.
fc [-e editor] [first [last]]
fc -l [-nr] [first [last]]
fc -s [old=new] [first]
The fc builtin lists, or edits and re-executes, commands
previously entered to an interactive shell.
-e editor
Use the editor named by editor to edit the
commands. The editor string is a command name,
subject to search via the PATH variable. The value
in the FCEDIT variable is used as a default when -e
is not specified. If FCEDIT is null or unset, the
value of the EDITOR variable is used. If EDITOR is
null or unset, ed(1) is used as the editor.
-l (ell)
List the commands rather than invoking an editor on
them. The commands are written in the sequence
indicated by the first and last operands, as
affected by -r, with each command preceded by the
command number.
-n Suppress command numbers when listing with -l.
-r Reverse the order of the commands listed (with -l)
or edited (with neither -l nor -s).
-s Re-execute the command without invoking an editor.
first
last Select the commands to list or edit. The number of
previous commands that can be accessed are
determined by the value of the HISTSIZE variable.
The value of first or last or both are one of the
following:
[+]number
A positive number representing a command
number; command numbers can be displayed
with the -l option.
-number
A negative decimal number representing the
command that was executed number of commands
previously. For example, -1 is the
immediately previous command.
string
A string indicating the most recently entered
command that begins with that string. If the
old=new operand is not also specified with -s, the
string form of the first operand cannot contain an
embedded equal sign.
The following environment variables affect the execution
of fc:
FCEDIT Name of the editor to use.
HISTSIZE The number of previous commands that are
accessible.
fg [job]
Move the specified job or the current job to the
foreground.
getopts optstring var [arg ...]
The POSIX getopts command, not to be confused with the
Bell Labs-derived getopt(1).
The first argument should be a series of letters, each of
which may be optionally followed by a colon to indicate
that the option requires an argument. The variable
specified is set to the parsed option.
The getopts command deprecates the older getopt(1) utility
due to its handling of arguments containing whitespace.
The getopts builtin may be used to obtain options and
their arguments from a list of parameters. When invoked,
getopts places the value of the next option from the
option string in the list in the shell variable specified
by var and its index in the shell variable OPTIND. When
the shell is invoked, OPTIND is initialized to 1. For
each option that requires an argument, the getopts builtin
will place it in the shell variable OPTARG. If an option
is not allowed for in the optstring, then OPTARG will be
unset.
By default, the variables $1, ..., $n are inspected; if
args are specified, they'll be parsed instead.
optstring is a string of recognized option letters (see
getopt(3)). If a letter is followed by a colon, the
option is expected to have an argument which may or may
not be separated from it by white space. If an option
character is not found where expected, getopts will set
the variable var to a “?”; getopts will then unset OPTARG
and write output to standard error. By specifying a colon
as the first character of optstring all errors will be
ignored.
After the last option getopts will return a non-zero value
and set var to “?”.
The following code fragment shows how one might process
the arguments for a command that can take the options [a]
and [b], and the option [c], which requires an argument.
while getopts abc: f
do
case $f in
a | b) flag=$f;;
c) carg=$OPTARG;;
\?) echo $USAGE; exit 1;;
esac
done
shift $((OPTIND - 1))
This code will accept any of the following as equivalent:
cmd -acarg file file
cmd -a -c arg file file
cmd -carg -a file file
cmd -a -carg -- file file
hash [command ...]
hash -r
The shell maintains a hash table which remembers the
locations of commands. With no arguments whatsoever, the
hash command prints out the contents of this table.
Entries which have not been looked at since the last cd
command are marked with an asterisk; it is possible for
these entries to be invalid.
With arguments, the hash command removes the specified
commands from the hash table (unless they are functions)
and then locates them. The -r option causes the hash
command to delete all the entries in the hash table except
for functions.
jobs [-lp] [job ...]
Display the status of all, or just the specified, jobs:
By default display the job number, currency (+-)
status, if any, the job state, and its
shell command.
-l also output the PID of the group leader,
and just the PID and shell commands of
other members of the job.
-p Display only leader PIDs, one per line.
kill [-s sigspec | -signum | -sigspec] [pid | job ...]
Equivalent to kill(1), but a job spec may also be
specified. Signals can be either case-insensitive names
without SIG prefixes or decimal numbers; the default is
TERM.
kill -l [signum | exitstatus]
List available signal names without the SIG prefix
(sigspecs). If signum specified, display just the sigspec
for that signal. If exitstatus specified (> 128), display
just the sigspec that caused it.
pwd [-LP]
builtin command remembers what the current directory is
rather than recomputing it each time. This makes it
faster. However, if the current directory is renamed, the
builtin version of pwd will continue to print the old name
for the directory. The -P option causes the physical
value of the current working directory to be shown, that
is, all symbolic links are resolved to their respective
values. The -L option turns off the effect of any
preceding -P options.
read [-p prompt] [-r] variable [...]
The prompt is printed if the -p option is specified and
the standard input is a terminal. Then a line is read
from the standard input. The trailing newline is deleted
from the line and the line is split as described in the
section on word splitting above, and the pieces are
assigned to the variables in order. At least one variable
must be specified. If there are more pieces than
variables, the remaining pieces (along with the characters
in IFS that separated them) are assigned to the last
variable. If there are more variables than pieces, the
remaining variables are assigned the null string. The
read builtin will indicate success unless EOF is
encountered on input, in which case failure is returned.
By default, unless the -r option is specified, the
backslash “\” acts as an escape character, causing the
following character to be treated literally. If a
backslash is followed by a newline, the backslash and the
newline will be deleted.
readonly name ...
readonly -p
The specified names are marked as read only, so that they
cannot be subsequently modified or unset. The shell
allows the value of a variable to be set at the same time
it is marked read only by writing
readonly name=value
With no arguments the readonly command lists the names of
all read only variables. With the -p option specified the
output will be formatted suitably for non-interactive use.
printf format [value]...
printf formats and prints its arguments according to
format, a character string which contains three types of
objects: plain characters, which are simply copied to
standard output, character escape sequences which are
converted and copied to the standard output, and format
specifications, each of which causes printing of the next
successive value.
Each value is treated as a string if the corresponding
format specification is either b, c, or s; otherwise it is
evaluated as a C constant, with the following additions:
• A leading plus or minus sign is allowed.
• If the leading character is a single or double
quote, the value of the next byte.
The format string is reused as often as necessary until
all values are consumed. Any extra format specifications
are evaluated with zero or the null string.
Character escape sequences are in backslash notation as
defined in ANSI X3.159-1989 (“ANSI C89”). The characters
and their meanings are as follows:
\a Write a <bell> character.
\b Write a <backspace> character.
\f Write a <form-feed> character.
\n Write a <new-line> character.
\r Write a <carriage return> character.
\t Write a <tab> character.
\v Write a <vertical tab> character.
\\ Write a backslash character.
\num Write an 8-bit character whose ASCII value
is the 1-, 2-, or 3-digit octal number num.
Each format specification is introduced by the percent
character (``%''). The remainder of the format
specification includes, in the following order:
Zero or more of the following flags:
# A `#' character specifying that the value
should be printed in an ``alternative
form''. For b, c, d, and s formats, this
option has no effect. For the o format
the precision of the number is increased
to force the first character of the output
string to a zero. For the x (X) format, a
non-zero result has the string 0x (0X)
prepended to it. For e, E, f, g, and G
formats, the result will always contain a
decimal point, even if no digits follow
the point (normally, a decimal point only
appears in the results of those formats if
a digit follows the decimal point). For g
and G formats, trailing zeros are not
removed from the result as they would
otherwise be.
- A minus sign `-' which specifies left
adjustment of the output in the indicated
field;
+ A `+' character specifying that there
should always be a sign placed before the
number when using signed formats.
‘ ’ A space specifying that a blank should be
left before a positive number for a signed
format. A `+' overrides a space if both
are used;
0 A zero `0' character indicating that zero-
padding should be used rather than blank-
padding. A `-' overrides a `0' if both
are used;
Field Width:
An optional digit string specifying a field width;
if the output string has fewer characters than the
field width it will be blank-padded on the left
(or right, if the left-adjustment indicator has
been given) to make up the field width (note that
a leading zero is a flag, but an embedded zero is
part of a field width);
Precision:
An optional period, ‘.’, followed by an optional
digit string giving a precision which specifies
the number of digits to appear after the decimal
point, for e and f formats, or the maximum number
of bytes to be printed from a string (b and s
formats); if the digit string is missing, the
precision is treated as zero;
Format:
A character which indicates the type of format to
use (one of diouxXfwEgGbcs).
A field width or precision may be ‘*’ instead of a digit
string. In this case an argument supplies the field width
or precision.
The format characters and their meanings are:
diouXx The argument is printed as a signed decimal (d
or i), unsigned octal, unsigned decimal, or
unsigned hexadecimal (X or x), respectively.
f The argument is printed in the style
[-]ddd.ddd where the number of d's after the
decimal point is equal to the precision
specification for the argument. If the
precision is missing, 6 digits are given; if
the precision is explicitly 0, no digits and
no decimal point are printed.
eE The argument is printed in the style
[-]d.ddde±dd where there is one digit before
the decimal point and the number after is
equal to the precision specification for the
argument; when the precision is missing, 6
digits are produced. An upper-case E is used
for an `E' format.
gG The argument is printed in style f or in style
e (E) whichever gives full precision in
minimum space.
b Characters from the string argument are
printed with backslash-escape sequences
expanded.
The following additional backslash-escape
sequences are supported:
\c Causes dash to ignore any remaining
characters in the string operand
containing it, any remaining string
operands, and any additional
characters in the format operand.
\0num Write an 8-bit character whose ASCII
value is the 1-, 2-, or 3-digit octal
number num.
c The first character of argument is printed.
s Characters from the string argument are
printed until the end is reached or until the
number of bytes indicated by the precision
specification is reached; if the precision is
omitted, all characters in the string are
printed.
% Print a `%'; no argument is used.
In no case does a non-existent or small field width cause
truncation of a field; padding takes place only if the
specified field width exceeds the actual width.
set [{ -options | +options | -- }] arg ...
The set command performs three different functions.
With no arguments, it lists the values of all shell
variables.
If options are given, it sets the specified option flags,
or clears them as described in the section called
“Argument List Processing”. As a special case, if the
option is -o or +o and no argument is supplied, the shell
prints the settings of all its options. If the option is
-o, the settings are printed in a human-readable format;
if the option is +o, the settings are printed in a format
suitable for reinput to the shell to affect the same
option settings.
In addition to the option names listed in the “Argument
List Processing” section, the following options may be
specified as arguments to -o or +o:
pipefail Derive the exit status of a pipeline from the
exit statuses of all of the commands in the
pipeline, not just the last command, as
described in the “Pipelines” section.
The third use of the set command is to set the values of
the shell's positional parameters to the specified args.
To change the positional parameters without changing any
options, use “--” as the first argument to set. If no
args are present, the set command will clear all the
positional parameters (equivalent to executing “shift
$#”.)
shift [n]
Shift the positional parameters n times. A shift sets the
value of $1 to the value of $2, the value of $2 to the
value of $3, and so on, decreasing the value of $# by one.
If n is greater than the number of positional parameters,
shift will issue an error message, and exit with return
status 2.
test expression
[ expression ]
The test utility evaluates the expression and, if it
evaluates to true, returns a zero (true) exit status;
otherwise it returns 1 (false). If there is no
expression, test also returns 1 (false).
All operators and flags are separate arguments to the test
utility.
The following primaries are used to construct expression:
-b file True if file exists and is a block special
file.
-c file True if file exists and is a character
special file.
-d file True if file exists and is a directory.
-e file True if file exists (regardless of type).
-f file True if file exists and is a regular file.
-g file True if file exists and its set group ID
flag is set.
-h file True if file exists and is a symbolic link.
-k file True if file exists and its sticky bit is
set.
-n string True if the length of string is nonzero.
-p file True if file is a named pipe (FIFO).
-r file True if file exists and is readable.
-s file True if file exists and has a size greater
than zero.
-t file_descriptor
True if the file whose file descriptor
number is file_descriptor is open and is
associated with a terminal.
-u file True if file exists and its set user ID flag
is set.
-w file True if file exists and is writable. True
indicates only that the write flag is on.
The file is not writable on a read-only file
system even if this test indicates true.
-x file True if file exists and is executable. True
indicates only that the execute flag is on.
If file is a directory, true indicates that
file can be searched.
-z string True if the length of string is zero.
-L file True if file exists and is a symbolic link.
This operator is retained for compatibility
with previous versions of this program. Do
not rely on its existence; use -h instead.
-O file True if file exists and its owner matches
the effective user id of this process.
-G file True if file exists and its group matches
the effective group id of this process.
-S file True if file exists and is a socket.
file1 -nt file2
True if file1 and file2 exist and file1 is
newer than file2.
file1 -ot file2
True if file1 and file2 exist and file1 is
older than file2.
file1 -ef file2
True if file1 and file2 exist and refer to
the same file.
string True if string is not the null string.
s1 = s2 True if the strings s1 and s2 are identical.
s1 != s2 True if the strings s1 and s2 are not
identical.
s1 < s2 True if string s1 comes before s2 based on
the ASCII value of their characters.
s1 > s2 True if string s1 comes after s2 based on
the ASCII value of their characters.
n1 -eq n2 True if the integers n1 and n2 are
algebraically equal.
n1 -ne n2 True if the integers n1 and n2 are not
algebraically equal.
n1 -gt n2 True if the integer n1 is algebraically
greater than the integer n2.
n1 -ge n2 True if the integer n1 is algebraically
greater than or equal to the integer n2.
n1 -lt n2 True if the integer n1 is algebraically less
than the integer n2.
n1 -le n2 True if the integer n1 is algebraically less
than or equal to the integer n2.
These primaries can be combined with the following
operators:
! expression True if expression is false.
expression1 -a expression2
True if both expression1 and expression2 are
true.
expression1 -o expression2
True if either expression1 or expression2
are true.
(expression) True if expression is true.
The -a operator has higher precedence than the -o
operator.
times Print the accumulated user and system times for the shell
and for processes run from the shell. The return status
is 0.
trap [action signal ...]
Cause the shell to parse and execute action when any of
the specified signals are received. The signals are
specified by signal number or as the name of the signal.
If signal is 0 or EXIT, the action is executed when the
shell exits. action may be empty (''), which causes the
specified signals to be ignored. With action omitted or
set to `-' the specified signals are set to their default
action. When the shell forks off a subshell, it resets
trapped (but not ignored) signals to the default action.
The trap command has no effect on signals that were
ignored on entry to the shell. trap without any arguments
cause it to write a list of signals and their associated
action to the standard output in a format that is suitable
as an input to the shell that achieves the same trapping
results.
Examples:
trap
List trapped signals and their corresponding action
trap '' INT QUIT tstp 30
Ignore signals INT QUIT TSTP USR1
trap date INT
Print date upon receiving signal INT
type [name ...]
Interpret each name as a command and print the resolution
of the command search. Possible resolutions are: shell
keyword, alias, shell builtin, command, tracked alias and
not found. For aliases the alias expansion is printed;
for commands and tracked aliases the complete pathname of
the command is printed.
ulimit [-H | -S] [-a | -tfdscmlpnvwr [value]]
Inquire about or set the hard or soft limits on processes
or set new limits. The choice between hard limit (which
no process is allowed to violate, and which may not be
raised once it has been lowered) and soft limit (which
causes processes to be signaled but not necessarily
killed, and which may be raised) is made with these flags:
-H set or inquire about hard limits
-S set or inquire about soft limits. If neither
-H nor -S is specified, the soft limit is
displayed or both limits are set. If both are
specified, the last one wins.
The limit to be interrogated or set, then, is chosen by
specifying any one of these flags:
-a show all the current limits
-t show or set the limit on CPU time (in seconds)
-f show or set the limit on the largest file that
can be created (in 512-byte blocks)
-d show or set the limit on the data segment size
of a process (in kilobytes)
-s show or set the limit on the stack size of a
process (in kilobytes)
-c show or set the limit on the largest core dump
size that can be produced (in 512-byte blocks)
-m show or set the limit on the total physical
memory that can be in use by a process (in
kilobytes)
-l show or set the limit on how much memory a
process can lock with mlock(2) (in kilobytes)
-p show or set the limit on the number of
processes this user can have at one time
-n show or set the limit on the number files a
process can have open at once
-v show or set the limit on the total virtual
memory that can be in use by a process (in
kilobytes)
-w show or set the limit on the total number of
locks held by a process
-r show or set the limit on the real-time
scheduling priority of a process
If none of these is specified, it is the limit on file
size that is shown or set. If value is specified, the
limit is set to that number; otherwise the current limit
is displayed. The special value unlimited represents the
lack of any limit.
Limits of an arbitrary process can be displayed or set
using the sysctl(8) utility.
umask [mask]
Set the value of umask (see umask(2)) to the specified
octal value. If the argument is omitted, the umask value
is printed.
unalias [-a] [name]
If name is specified, the shell removes that alias. If -a
is specified, all aliases are removed.
unset [-fv] name ...
The specified variables and functions are unset and
unexported. If -f or -v is specified, the corresponding
function or variable is unset, respectively. If a given
name corresponds to both a variable and a function, and no
options are given, only the variable is unset.
wait [job]
Wait for the specified job to complete and return the exit
status of the last process in the job. If the argument is
omitted, wait for all jobs to complete and return an exit
status of zero.
Command Line Editing
When dash is being used interactively from a terminal, the
current command and the command history (see fc in “Builtins”)
can be edited using vi-mode command-line editing. This mode uses
commands, described below, similar to a subset of those described
in the vi man page. The command ‘set -o vi’ enables vi-mode
editing and places sh into vi insert mode. With vi-mode enabled,
sh can be switched between insert mode and command mode. It is
similar to vi: typing ⟨ESC⟩ enters vi command mode. Hitting
⟨return⟩ while in command mode will pass the line to the shell.
Errors that are detected by the shell, such as a syntax error,
will cause the shell to exit with a non-zero exit status. If the
shell is not an interactive shell, the execution of the shell
file will be aborted. Otherwise the shell will return the exit
status of the last command executed, or if the exit builtin is
used with a numeric argument, it will return the argument.
HOME Set automatically by login(1) from the user's login
directory in the password file (passwd(4)). This
environment variable also functions as the default
argument for the cd builtin.
PATH The default search path for executables. See the
above section “Path Search”.
CDPATH The search path used with the cd builtin.
MAIL The name of a mail file, that will be checked for the
arrival of new mail. Overridden by MAILPATH.
MAILCHECK The frequency in seconds that the shell checks for the
arrival of mail in the files specified by the MAILPATH
or the MAIL file. If set to 0, the check will occur
at each prompt.
MAILPATH A colon “:” separated list of file names, for the
shell to check for incoming mail. This environment
setting overrides the MAIL setting. There is a
maximum of 10 mailboxes that can be monitored at once.
PS1 The primary prompt string, which defaults to “$ ”,
unless you are the superuser, in which case it
defaults to “# ”.
PS2 The secondary prompt string, which defaults to “> ”.
PS4 Output before each line when execution trace (set -x)
is enabled, defaults to “+ ”.
IFS Input Field Separators. This is normally set to
⟨space⟩, ⟨tab⟩, and ⟨newline⟩. See the “White Space
Splitting” section for more details.
TERM The default terminal setting for the shell. This is
inherited by children of the shell, and is used in the
history editing modes.
HISTSIZE The number of lines in the history buffer for the
shell.
PWD The logical value of the current working directory.
This is set by the cd command.
OLDPWD The previous logical value of the current working
directory. This is set by the cd command.
PPID The process ID of the parent process of the shell.
$HOME/.profile
/etc/profile
csh(1), echo(1), getopt(1), ksh(1), login(1), printf(1), test(1),
getopt(3), passwd(5), environ(7), sysctl(8)
dash is a POSIX-compliant implementation of /bin/sh that aims to
be as small as possible. dash is a direct descendant of the
NetBSD version of ash (the Almquist SHell), ported to Linux in
early 1997. It was renamed to dash in 2002.
Setuid shell scripts should be avoided at all costs, as they are
a significant security risk.
This page is part of the dash (Debian Almquist shell) project.
Information about the project can be found at
http://gondor.apana.org.au/~herbert/dash/. If you have a bug
report for this manual page, send it to [email protected].
This page was obtained from the project's upstream Git repository
⟨git://git.kernel.org/pub/scm/utils/dash/dash.git⟩ on 2024-06-14.
(At that time, the date of the most recent commit that was found
in the repository was 2024-06-08.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
[email protected]
GNU January 19, 2003 DASH(1)
Pages that refer to this page: intro(1), systemctl(1), system(3)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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