NAME¶
dash —
command interpreter
(shell)
SYNOPSIS¶
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 ...] |
DESCRIPTION¶
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:
- 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.
- 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.
- 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, 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
- Duplicate standard input (or n1) from file descriptor
n2.
- [n]<&-
- Close standard input (or n).
- [n1]>&n2
- Duplicate standard output (or n1) to n2.
- [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:
- Command names containing a slash are simply executed
without performing any searches.
- 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 reserved word ! does not precede the pipeline, the exit status is the
exit status of the last command specified in the pipeline. Otherwise, the exit
status is the logical NOT of the exit status of the last command. That is, if
the last command returns zero, the exit status is 1; if the last command
returns greater than zero, the exit status is zero.
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 proceed
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 iff the exit status of the first command is zero.
“||” is similar, but executes the second command iff 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
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:
- Tilde Expansion, Parameter Expansion, Command
Substitution, Arithmetic Expansion (these all occur at the same
time).
- Field Splitting is performed on fields generated by step
(1) unless the
IFS
variable is null.
- Pathname Expansion (unless set -f is in
effect).
- 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:
- Pathname expansion is not performed on the results of the
expansion.
- 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.
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.
- . 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
-
-
- cd
[-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.
\0
digits
- 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
- 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.
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.
A nonzero value is returned when the last option is reached. If there are no
remaining arguments, getopts will set
var to the special option, “--”,
otherwise, it will 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 `expr $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 -rv
command ...
- 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. With the -v option, hash prints the locations of
the commands as it finds them. The -r option causes the
hash command to delete all the entries in the hash table except for
functions.
- 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 [arguments
...]
- printf formats and prints its arguments,
after the first, under control of the format. The
format is 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 argument.
The arguments after the first are treated as strings
if the corresponding format is either b,
c or s; otherwise it is evaluated as a
C constant, with the following extensions:
- A leading plus or minus
sign is allowed.
- If the leading character
is a single or double quote, the value is the ASCII code of the next
character.
The format string is reused as often as necessary to satisfy the
arguments. 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 characters 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
*(Pmdd 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 characters 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.
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
, the action is executed when the shell exits.
action may be null, which cause 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 | -tfdscmlpn
[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
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.
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 the 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
place sh into vi insert mode. With vi-mode enabled, sh can be switched between
insert mode and command mode. The editor is not described in full here, but
will be in a later document. It's similar to vi: typing ⟨ESC⟩
will throw you into command VI command mode. Hitting ⟨return⟩
while in command mode will pass the line to the shell.
EXIT STATUS¶
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.
ENVIRONMENT¶
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.
FILES¶
- $HOME/.profile
- /etc/profile
SEE ALSO¶
csh(1),
echo(1),
getopt(1),
ksh(1),
login(1),
printf(1),
test(1),
getopt(3),
passwd(5),
environ(7),
sysctl(8)
HISTORY¶
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.
BUGS¶
Setuid shell scripts should be avoided at all costs, as they are a significant
security risk.
PS1, PS2, and PS4 should be subject to parameter expansion before being
displayed.