DASH(1) | General Commands Manual | DASH(1) |
dash
— command
interpreter (shell)
dash |
[-aCefnuvxIimqVEbp ]
[+aCefnuvxIimqVEbp ]
[-o option_name]
[+o option_name]
[command_file [argument ...]] |
dash |
-c [-aCefnuvxIimqVEbp ]
[+aCefnuvxIimqVEbp ]
[-o option_name]
[+o option_name]
command_string
[command_name [argument ...]] |
dash |
-s [-aCefnuvxIimqVEbp ]
[+aCefnuvxIimqVEbp ]
[-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.
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.
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, $2, etc). Otherwise, the shell reads commands from its standard input.
All of the single letter options that have a corresponding name
can be used as an argument to the -o
option. The set
-o
name is provided next to the single letter option
in the description below. Specifying a dash “-” turns the
option on, while using a plus “+” disables the option. The
following options can be set from the command line or with the
set
builtin (described later).
-a
allexport-c
-C
noclobber-e
errexitif
,
elif
, while
, or
until
; or if the command is the left hand operand
of an “&&” or “||” operator.-f
noglob-n
noexec-u
nounset-v
verbose-x
xtrace-I
ignoreeof-i
interactive-l
-m
monitor-s
stdinset
).-V
vi-E
if it has been set).-E
emacs-V
if it has been set).-b
notify-p
privThe shell reads input in terms of lines from a file and breaks it up into words at whitespace (blanks and tabs), and at certain sequences of characters that are special to the shell called “operators”. There are two types of operators: control operators and redirection operators (their meaning is discussed later). Following is a list of operators:
& && ( ) ; ;; | ||
<newline>
< > >| << >>
<& >& <<- <>
Quoting is used to remove the special meaning of certain characters or words to the shell, such as operators, whitespace, or keywords. There are three types of quoting: matched single quotes, matched double quotes, and backslash.
A backslash preserves the literal meaning of the following character, with the exception of ⟨newline⟩. A backslash preceding a ⟨newline⟩ is treated as a line continuation.
Enclosing characters in single quotes preserves the literal meaning of all the characters (except single quotes, making it impossible to put single-quotes in a single-quoted string).
Enclosing characters within double quotes preserves the literal meaning of all characters except dollarsign ($), backquote (`), and backslash (\). The backslash inside double quotes is historically weird, and serves to quote only the following characters:
$ ` " \
<newline>
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.
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.
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.
If a simple command has been recognized, the shell performs the following actions:
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.
-C
option.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.
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".
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:
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.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 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:
Unless otherwise stated, the exit status of a command is that of the last simple command executed by the command.
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).
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.
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.
“&&” 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.
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.
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.
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.
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.
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.
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.
IFS
variable, or by a ⟨space⟩ if
IFS
is unset."abc" "def ghi"
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:
IFS
variable is null.-f
is in
effect).The $ character is used to introduce parameter expansion, command substitution, or arithmetic evaluation.
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).
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:
In addition, a parameter expansion can be modified by using one of the following formats.
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.
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.
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 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.
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.
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.
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.
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).
alias
builtin prints the names and values of all
defined aliases (see unalias
).-p
] [-v
]
[-V
] command
[arg ...]-p
PATH
that
guarantees to find all the standard utilities.-V
-v
-LP
] [directory]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.-n
] args...-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
\c
echo
would otherwise output.\e
\f
\n
\r
\t
\v
\0
digits\\
All other backslash sequences elicit undefined behaviour.
exec
command
are marked as permanent, so that they are not undone when the
exec
command finishes.-p
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.
-e
editor]
[first [last]]-l
[-nr
]
[first [last]]-s
[old=new]
[first]fc
builtin lists, or edits and re-executes,
commands previously entered to an interactive shell.
-e
editorPATH
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)-r
, with each command preceded
by the command number.-n
-r
-l
) or edited (with neither
-l
nor -s
).-s
HISTSIZE
variable. The value of first or last
or both are one of the following:
-l
option.-number
-s
, the string form of the first operand
cannot contain an embedded equal sign.The following environment variables affect the execution of fc:
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.
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 `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
-rv
command ...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.
-LP
]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.-p
prompt]
[-r
] variable
[...]-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.
-p
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
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:
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
\b
\e
\f
\n
\r
\t
\v
\\
\
numEach format specification is introduced by the percent character (``%''). The remainder of the format specification includes, in the following order:
#
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.-
+
0
.
’,
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;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
f
.
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
.
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
f
or in style e
(E
) whichever gives full precision in minimum
space.b
c
s
%
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.
-options
| +options
| -- }
] arg ...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
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
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-c
file-d
file-e
file-f
file-g
file-h
file-k
file-n
string-p
file-r
file-s
file-t
file_descriptor-u
file-w
file-x
file-z
string-L
file-h
instead.-O
file-G
file-S
file-nt
file2-ot
file2-ef
file2=
s2!=
s2<
s2>
s2-eq
n2-ne
n2-gt
n2-ge
n2-lt
n2-le
n2These primaries can be combined with the following operators:
!
expression-a
expression2-o
expression2(
expression)
The -a
operator has higher precedence
than the -o
operator.
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
-H
| -S
]
[-a
| -tfdscmlpnv
[value]]-H
-S
-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
-t
-f
-d
-s
-c
-m
-l
-p
-n
-v
-r
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.
-a
] [name]-a
is specified, all aliases are removed.-fv
] name ...-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.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
PATH
CDPATH
MAIL
MAILPATH
.MAILCHECK
MAILPATH
or the
MAIL
file. If set to 0, the check will occur at
each prompt.MAILPATH
MAIL
setting. There is a maximum of 10 mailboxes
that can be monitored at once.PS1
PS2
PS4
IFS
TERM
HISTSIZE
PWD
cd
command.OLDPWD
cd
command.PPID
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.
PS1, PS2, and PS4 should be subject to parameter expansion before being displayed.
January 19, 2003 | Debian |