ksh - public domain Korn shell
ksh [-+abCefhiklmnprsuvxX] [-+o option]
[[-c command-string [command-name] | -s | file]
[argument ...]]
ksh is a command interpreter intended for both interactive
and shell
script use. Its command language is a superset of the sh(1)
shell language.
Shell startup [Toc] [Back]
The following options can be specified only on the command
line:
-c command-string
ksh will execute the command(s) contained in
command-string.
-i Interactive mode; see below.
-l Login shell; see below.
-s The shell reads commands from standard input; all
non-option arguments
are positional parameters.
-r Restricted mode; see below.
In addition to the above, the options described in the set
built-in command
can also be used on the command line.
If neither the -c nor the -s option is specified, the first
non-option
argument specifies the name of a file the shell reads commands from. If
there are no non-option arguments, the shell reads commands
from the
standard input. The name of the shell (i.e., the contents
of $0) is determined
as follows: if the -c option is used and there is a
non-option
argument, it is used as the name; if commands are being read
from a file,
the file is used as the name; otherwise, the name the shell
was called
with (i.e., argv[0]) is used.
A shell is ``interactive'' if the -i option is used or if
both standard
input and standard error are attached to a tty. An interactive shell has
job control enabled (if available), ignores the SIGINT,
SIGQUIT, and
SIGTERM signals, and prints prompts before reading input
(see PS1 and PS2
parameters). For non-interactive shells, the trackall option is on by
default (see the set command below).
A shell is ``restricted'' if the -r option is used or if either the basename
of the name the shell was invoked with or the SHELL parameter match
the pattern ``*r*sh'' (e.g., ``rsh'', ``rksh'', ``rpdksh'',
etc.). The
following restrictions come into effect after the shell processes any
profile and ENV files:
+o The cd command is disabled.
+o The SHELL, ENV, and PATH parameters cannot be changed.
+o Command names can't be specified with absolute or relative paths.
+o The -p option of the built-in command command can't be
used.
+o Redirections that create files can't be used (i.e., `>',
`>|', `>>',
`<>').
A shell is ``privileged'' if the -p option is used or if the
real user ID
or group ID does not match the effective user ID or group ID
(see
getuid(2) and getgid(2)). A privileged shell does not process
$HOME/.profile nor the ENV parameter (see below). Instead,
the file
/etc/suid_profile is processed. Clearing the privileged option causes
the shell to set its effective user ID (group ID) to its real user ID
(group ID).
If the basename of the name the shell is called with (i.e.,
argv[0])
starts with `-' or if the -l option is used, the shell is
assumed to be a
login shell and the shell reads and executes the contents of
/etc/profile
and $HOME/.profile if they exist and are readable.
If the ENV parameter is set when the shell starts (or, in
the case of login
shells, after any profiles are processed), its value is
subjected to
parameter, command, arithmetic, and tilde (`~') substitution
and the resulting
file (if any) is read and executed. If the ENV parameter is not
set (and not NULL) and pdksh was compiled with the DEFAULT_ENV macro defined,
the file named in that macro is included (after the
above mentioned
substitutions have been performed).
The exit status of the shell is 127 if the command file
specified on the
command line could not be opened, or non-zero if a fatal
syntax error occurred
during the execution of a script. In the absence of
fatal errors,
the exit status is that of the last command executed, or zero, if no command
is executed.
Command syntax [Toc] [Back]
The shells begins parsing its input by breaking it into
words. Words,
which are sequences of characters, are delimited by unquoted
whitespace
characters (space, tab, and newline) or meta-characters
(`<', `>', `|',
`;', `(', and `)'). Aside from delimiting words, spaces and
tabs are ignored,
while newlines usually delimit commands. The metacharacters are
used in building the following tokens: `<', `<&', `<<', `>',
`>&', `>>',
etc. are used to specify redirections (see Input/output
redirection below);
`|' is used to create pipelines; `|&' is used to create co-processes
(see Co-processes below); `;' is used to separate commands; `&' is
used to create asynchronous pipelines; `&&' and `||' are
used to specify
conditional execution; `;;' is used in case statements; `((
.. ))' is
used in arithmetic expressions; and lastly, `( .. )' is used
to create
subshells.
Whitespace and meta-characters can be quoted individually
using a backslash
(`'), or in groups using double (`"') or single (`'')
quotes.
Note that the following characters are also treated specially by the
shell and must be quoted if they are to represent themselves: `', `"',
`'', `#', `$', ``', `~', `{', `}', `*', `?', and `['. The
first three of
these are the above mentioned quoting characters (see
Quoting below);
`#', if used at the beginning of a word, introduces a comment -- everything
after the `#' up to the nearest newline is ignored;
`$' is used to
introduce parameter, command, and arithmetic substitutions
(see
Substitution below); ``' introduces an old-style command
substitution
(see Substitution below); `~' begins a directory expansion
(see Tilde
expansion below); `{' and `}' delimit csh(1) style alterations (see Brace
expansion below); and finally, `*', `?', and `[' are used in
file name
generation (see File name patterns below).
As words and tokens are parsed, the shell builds commands,
of which there
are two basic types: simple-commands, typically programs
that are executed,
and compound-commands, such as for and if statements,
grouping constructs,
and function definitions.
A simple-command consists of some combination of parameter
assignments
(see Parameters below), input/output redirections (see
Input/output
redirections below), and command words; the only restriction
is that parameter
assignments come before any command words. The command words, if
any, define the command that is to be executed and its arguments. The
command may be a shell built-in command, a function or an
external command
(i.e., a separate executable file that is located using
the PATH parameter
(see Command execution below)). Note that all command constructs
have an exit status: for external commands, this is related
to the status
returned by wait(2) (if the command could not be found, the
exit status
is 127; if it could not be executed, the exit status is
126); the exit
status of other command constructs (built-in commands, functions, compound-commands,
pipelines, lists, etc.) are all well-defined
and are described
where the construct is described. The exit status
of a command
consisting only of parameter assignments is that of the last
command substitution
performed during the parameter assignment or 0 is
there were no
command substitutions.
Commands can be chained together using the `|' token to form
pipelines,
in which the standard output of each command but the last is
piped (see
pipe(2)) to the standard input of the following command.
The exit status
of a pipeline is that of its last command. A pipeline may
be prefixed by
the `!' reserved word, which causes the exit status of the
pipeline to be
logically complemented: if the original status was 0, the
complemented
status will be 1; if the original status was not 0, the complemented status
will be 0.
Lists of commands can be created by separating pipelines by
any of the
following tokens: `&&', `||', `&', `|&', and `;'. The first
two are for
conditional execution: ``cmd1 && cmd2'' executes cmd2 only
if the exit
status of cmd1 is zero; `||' is the opposite -- cmd2 is executed only if
the exit status of cmd1 is non-zero. `&&' and `||' have
equal precedence
which is higher than that of `&', `|&', and `;', which also
have equal
precedence. Note that the `&&' and `||' operators are
"leftassociative".
For example, both of these commands will
print only "bar":
false && echo foo || echo bar
true || echo foo && echo bar
The `&' token causes the preceding command to be executed
asynchronously;
that is, the shell starts the command but does not wait for
it to complete
(the shell does keep track of the status of asynchronous commands,
see Job control below). When an asynchronous command is
started when job
control is disabled (i.e., in most scripts), the command is
started with
signals SIGINT and SIGQUIT ignored and with input redirected
from
/dev/null (however, redirections specified in the asynchronous command
have precedence). The `|&' operator starts a co-process
which is a special
kind of asynchronous process (see Co-processes below).
Note that a
command must follow the `&&' and `||' operators, while it
need not follow
`&', `|&', or `;'. The exit status of a list is that of the
last command
executed, with the exception of asynchronous lists, for
which the exit
status is 0.
Compound commands are created using the following reserved
words. These
words are only recognized if they are unquoted and if they
are used as
the first word of a command (i.e., they can't be preceded by
parameter
assignments or redirections):
case else function then !
do esac if time [[
done fi in until {
elif for select while }
Note: Some shells (but not this one) execute control structure commands
in a subshell when one or more of their file descriptors are
redirected,
so any environment changes inside them may fail. To be
portable, the
exec statement should be used instead to redirect file descriptors before
the control structure.
In the following compound command descriptions, command
lists (denoted as
list) that are followed by reserved words must end with a
semicolon, a
newline, or a (syntactically correct) reserved word. For
example,
{ echo foo; echo bar; }
{ echo foo; echo bar<newline> }
{ { echo foo; echo bar; } }
are all valid, but
{ echo foo; echo bar }
is not.
( list )
Execute list in a subshell. There is no implicit
way to pass environment
changes from a subshell back to its parent.
{ list }
Compound construct; list is executed, but not in a
subshell.
Note that { and } are reserved words, not meta-characters.
case word in [[(] pattern [| pattern] ... ) list ;; ] ...
esac
The case statement attempts to match word against
the specified
patterns; the list associated with the first successfully matched
pattern is executed. Patterns used in case statements are the
same as those used for file name patterns except
that the restrictions
regarding `.' and `/' are dropped. Note
that any unquoted
space before and after a pattern is stripped;
any space
within a pattern must be quoted. Both the word and
the patterns
are subject to parameter, command, and arithmetic
substitution,
as well as tilde substitution. For historical reasons, open and
close braces may be used instead of in and esac
(e.g., case $foo
{ *) echo bar; }). The exit status of a case statement is that
of the executed list; if no list is executed, the
exit status is
zero.
for name [in word ... term] do list done
For each word in the specified word list, the parameter name is
set to the word and list is executed. If in is not
used to specify
a word list, the positional parameters ($1, $2,
etc.) are
used instead. For historical reasons, open and
close braces may
be used instead of do and done (e.g., for i; { echo
$i; }). The
exit status of a for statement is the last exit status of list;
if list is never executed, the exit status is zero.
term is either
a newline or a `;'.
if list then list [elif list then list] ... [else list] fi
If the exit status of the first list is zero, the
second list is
executed; otherwise, the list following the elif, if
any, is executed
with similar consequences. If all the lists
following the
if and elifs fail (i.e., exit with non-zero status),
the list
following the else is executed. The exit status of
an if statement
is that of non-conditional list that is executed; if no nonconditional
list is executed, the exit status is zero.
select name [in word ... term] do list done
The select statement provides an automatic method of
presenting
the user with a menu and selecting from it. An enumerated list
of the specified words is printed on standard error,
followed by
a prompt (PS3, normally ``#? ''). A number corresponding to one
of the enumerated words is then read from standard
input, name is
set to the selected word (or unset if the selection
is not
valid), REPLY is set to what was read (leading/trailing space is
stripped), and list is executed. If a blank line
(i.e., zero or
more IFS characters) is entered, the menu is
reprinted without
executing list. When list completes, the enumerated
list is
printed if REPLY is NULL, the prompt is printed and
so on. This
process continues until an end-of-file is read, an
interrupt is
received, or a break statement is executed inside
the loop. If
in word ... is omitted, the positional parameters
are used (i.e.,
$1, $2, etc.). For historical reasons, open and
close braces may
be used instead of do and done (e.g., select i; {
echo $i; }).
The exit status of a select statement is zero if a
break statement
is used to exit the loop, non-zero otherwise.
until list do list done
This works like while, except that the body is executed only
while the exit status of the first list is non-zero.
while list do list done
A while is a pre-checked loop. Its body is executed
as often as
the exit status of the first list is zero. The exit
status of a
while statement is the last exit status of the list
in the body
of the loop; if the body is not executed, the exit
status is zero.
function name { list }
Defines the function name (see Functions below).
Note that redirections
specified after a function definition are
performed
whenever the function is executed, not when the
function definition
is executed.
name () command
Mostly the same as function (see Functions below).
time [-p] [pipeline]
The time reserved word is described in the Command
execution section.
(( expression ))
The arithmetic expression expression is evaluated;
equivalent to
let expression (see Arithmetic expressions and the
let command
below).
[[ expression ]]
Similar to the test and [ ... ] commands (described
later), with
the following exceptions:
+o Field splitting and file name generation
are not performed
on arguments.
+o The -a (AND) and -o (OR) operators are replaced with
`&&' and `||', respectively.
+o Operators (e.g., `-f', `=', `!', etc.)
must be unquoted.
+o The second operand of the `!=' and `=' expressions are
patterns (e.g., the comparison [[ foobar =
f*r ]] succeeds).
+o There are two additional binary operators:
`<' and `>'
which return true if their first string
operand is less
than, or greater than, their second string
operand, respectively.
+o The single argument form of test, which
tests if the
argument has a non-zero length, is not
valid; explicit
operators must always be used (e.g., instead of [ str ]
use [[ -n str ]]).
+o Parameter, command, and arithmetic substitutions are
performed as expressions are evaluated and
lazy expression
evaluation is used for the `&&' and
`||' operators.
This means that in the statement
[[ -r foo && $(< foo) = b*r ]]
the $(< foo) is evaluated if and only if
the file foo
exists and is readable.
Quoting [Toc] [Back]
Quoting is used to prevent the shell from treating characters or words
specially. There are three methods of quoting. First, `'
quotes the
following character, unless it is at the end of a line, in
which case
both the `' and the newline are stripped. Second, a single
quote (`'')
quotes everything up to the next single quote (this may span
lines).
Third, a double quote (`"') quotes all characters, except
`$', ``' and
`', up to the next unquoted double quote. `$' and ``' inside double
quotes have their usual meaning (i.e., parameter, command or
arithmetic
substitution) except no field splitting is carried out on
the results of
double-quoted substitutions. If a `' inside a double-quoted
string is
followed by `', `$', ``', or `"', it is replaced by the second character;
if it is followed by a newline, both the `' and the
newline are
stripped; otherwise, both the `' and the character following
are unchanged.
Note: See POSIX mode below for a special rule regarding sequences of the
form "...`...
Aliases [Toc] [Back]
There are two types of aliases: normal command aliases and
tracked aliases.
Command aliases are normally used as a short hand for a
long or often
used command. The shell expands command aliases (i.e.,
substitutes
the alias name for its value) when it reads the first word
of a command.
An expanded alias is re-processed to check for more aliases.
If a command
alias ends in a space or tab, the following word is also checked for
alias expansion. The alias expansion process stops when a
word that is
not an alias is found, when a quoted word is found or when
an alias word
that is currently being expanded is found.
The following command aliases are defined automatically by
the shell:
autoload='typeset -fu'
functions='typeset -f'
hash='alias -t'
history='fc -l'
integer='typeset -i'
local='typeset'
login='exec login'
nohup='nohup '
r='fc -e -'
stop='kill -STOP'
suspend='kill -STOP $$'
type='whence -v'
Tracked aliases allow the shell to remember where it found a
particular
command. The first time the shell does a path search for a
command that
is marked as a tracked alias, it saves the full path of the
command. The
next time the command is executed, the shell checks the
saved path to see
that it is still valid, and if so, avoids repeating the path
search.
Tracked aliases can be listed and created using alias -t.
Note that
changing the PATH parameter clears the saved paths for all
tracked aliases.
If the trackall option is set (i.e., set -o trackall or
set -h), the
shell tracks all commands. This option is set automatically
for non-interactive
shells. For interactive shells, only the following commands
are automatically tracked: cat, cc, chmod, cp, date, ed,
emacs, grep, ls,
mail, make, mv, pr, rm, sed, sh, vi, and who.
Substitution [Toc] [Back]
The first step the shell takes in executing a simple-command
is to perform
substitutions on the words of the command. There are
three kinds of
substitution: parameter, command, and arithmetic. Parameter
substitutions,
which are described in detail in the next section,
take the form
$name or ${...}; command substitutions take the form
$(command) or
`command`; and arithmetic substitutions take the form
$((expression)).
If a substitution appears outside of double quotes, the results of the
substitution are generally subject to word or field splitting according
to the current value of the IFS parameter. The IFS parameter specifies a
list of characters which are used to break a string up into
several
words; any characters from the set space, tab, and newline
that appear in
the IFS characters are called ``IFS whitespace''. Sequences
of one or
more IFS whitespace characters, in combination with zero or
one non-IFS
whitespace characters, delimit a field. As a special case,
leading and
trailing IFS whitespace is stripped (i.e., no leading or
trailing empty
field is created by it); leading or trailing non-IFS whitespace does create
an empty field.
Example: If IFS is set to ``<space>:'', the sequence of
characters
``<space>A<space>:<space><space>B::D'' contains four fields:
`A', `B',
`,' and `D'. Note that if the IFS parameter is set to the
NULL string,
no field splitting is done; if the parameter is unset, the
default value
of space, tab, and newline is used.
The results of substitution are, unless otherwise specified,
also subject
to brace expansion and file name expansion (see the relevant
sections below).
A command substitution is replaced by the output generated
by the specified
command, which is run in a subshell. For $(command)
substitutions,
normal quoting rules are used when command is parsed; however, for the
`command` form, a `' followed by any of `$', ``', or `' is
stripped (a
`' followed by any other character is unchanged). As a special case in
command substitutions, a command of the form < file is interpreted to
mean substitute the contents of file (note that $(< foo) has
the same effect
as $(cat foo), but it is carried out more efficiently
because no
process is started).
Note: $(command) expressions are currently parsed by finding
the matching
parenthesis, regardless of quoting. This should be fixed
soon.
Arithmetic substitutions are replaced by the value of the
specified expression.
For example, the command echo $((2+3*4)) prints
14. See
Arithmetic expressions for a description of an expression.
Parameters [Toc] [Back]
Parameters are shell variables; they can be assigned values
and their
values can be accessed using a parameter substitution. A
parameter name
is either one of the special single punctuation or digit
character parameters
described below, or a letter followed by zero or more
letters or
digits (`_' counts as a letter). The later form can be
treated as arrays
by appending an array index of the form [expr] where expr is
an arithmetic
expression. Array indices are currently limited to
the range 0
through 1023, inclusive. Parameter substitutions take the
form $name,
${name}, or ${name[expr]}, where name is a parameter name.
If substitution
is performed on a parameter (or an array parameter element) that is
not set, a null string is substituted unless the nounset option (set -o
nounset or set -u) is set, in which case an error occurs.
Parameters can be assigned values in a number of ways.
First, the shell
implicitly sets some parameters like #, PWD, etc.; this is
the only way
the special single character parameters are set. Second,
parameters are
imported from the shell's environment at startup. Third,
parameters can
be assigned values on the command line, for example, FOO=bar
sets the parameter
FOO to ``bar''; multiple parameter assignments can
be given on a
single command line and they can be followed by a simplecommand, in
which case the assignments are in effect only for the duration of the
command (such assignments are also exported, see below for
implications
of this). Note that both the parameter name and the `='
must be unquoted
for the shell to recognize a parameter assignment. The
fourth way of
setting a parameter is with the export, readonly and typeset
commands;
see their descriptions in the Command execution section.
Fifth, for and
select loops set parameters as well as the getopts, read and
set -A commands.
Lastly, parameters can be assigned values using assignment operators
inside arithmetic expressions (see Arithmetic
expressions below) or
using the ${name=value} form of the parameter substitution
(see below).
Parameters with the export attribute (set using the export
or typeset -x
commands, or by parameter assignments followed by simple
commands) are
put in the environment (see environ(7)) of commands run by
the shell as
name=value pairs. The order in which parameters appear in
the environment
of a command is unspecified. When the shell starts up,
it extracts
parameters and their values from its environment and automatically sets
the export attribute for those parameters.
Modifiers can be applied to the ${name} form of parameter
substitution:
${name:-word}
If name is set and not NULL, it is substituted; otherwise, word
is substituted.
${name:+word}
If name is set and not NULL, word is substituted;
otherwise,
nothing is substituted.
${name:=word}
If name is set and not NULL, it is substituted; otherwise, it is
assigned word and the resulting value of name is
substituted.
${name:?word}
If name is set and not NULL, it is substituted; otherwise, word
is printed on standard error (preceded by name:) and
an error occurs
(normally causing termination of a shell
script, function or
.-script). If word is omitted the string ``parameter null or not
set'' is used instead.
In the above modifiers, the `:' can be omitted, in which
case the conditions
only depend on name being set (as opposed to set and
not NULL). If
word is needed, parameter, command, arithmetic, and tilde
substitution
are performed on it; if word is not needed, it is not evaluated.
The following forms of parameter substitution can also be
used:
${#name}
The number of positional parameters if name is `*',
`@', not
specified, or the length of the string value of parameter name.
${#name[*]}, ${#name[@]}
The number of elements in the array name.
${name#pattern}, ${name##pattern}
If pattern matches the beginning of the value of parameter name,
the matched text is deleted from the result of substitution. A
single `#' results in the shortest match, and two of
them result
in the longest match.
${name%pattern}, ${name%%pattern}
Like ${..#..} substitution, but it deletes from the
end of the
value.
The following special parameters are implicitly set by the
shell and cannot
be set directly using assignments:
! Process ID of the last background process started.
If no background
processes have been started, the parameter
is not set.
# The number of positional parameters (i.e., $1, $2,
etc.).
$ The process ID of the shell, or the PID of the
original shell if
it is a subshell. Do NOT use this mechanism for
generating temporary
file names; see mktemp(1) instead.
- The concatenation of the current single letter options (see the
set command below for a list of options).
? The exit status of the last non-asynchronous command executed.
If the last command was killed by a signal, $? is
set to 128
plus the signal number.
0 The name the shell was invoked with (i.e.,
argv[0]), or the
command-name if it was invoked with the -c option
and the
command-name was supplied, or the file argument, if
it was supplied.
If the posix option is not set, $0 is the
name of the
current function or script.
1 ... 9 The first nine positional parameters that were supplied to the
shell, function or .-script. Further positional
parameters may
be accessed using ${number}.
* All positional parameters (except parameter 0);
i.e., $1, $2,
$3, ... If used outside of double quotes, parameters are separate
words (which are subjected to word splitting);
if used
within double quotes, parameters are separated by
the first
character of the IFS parameter (or the empty string
if IFS is
NULL).
@ Same as $*, unless it is used inside double quotes,
in which
case a separate word is generated for each positional parameter.
If there are no positional parameters, no word is
generated. $@
can be used to access arguments, verbatim, without
losing NULL
arguments or splitting arguments with spaces.
The following parameters are set and/or used by the shell:
_ (underscore)
When an external command is executed by the
shell, this parameter
is set in the environment of the new process
to the path
of the executed command. In interactive use,
this parameter
is also set in the parent shell to the last word
of the previous
command. When MAILPATH messages are evaluated, this parameter
contains the name of the file that
changed (see
MAILPATH parameter below).
CDPATH Search path for the cd built-in command. Works
the same way
as PATH for those directories not beginning with
`/' in cd
commands. Note that if CDPATH is set and does
not contain
``.'' or contains an empty path, the current directory is not
searched. Also, the cd built-in command will
display the resulting
directory when a match is found in any
search path
other than the empty path.
COLUMNS Set to the number of columns on the terminal or
window. Currently
set to the ``cols'' value as reported by
stty(1) if
that value is non-zero. This parameter is used
by the interactive
line editing modes, and by the select, set
-o, and kill
-l commands to format information columns.
EDITOR If the VISUAL parameter is not set, this parameter controls
the command-line editing mode for interactive
shells. See
VISUAL parameter below for how this works.
ENV If this parameter is found to be set after any
profile files
are executed, the expanded value is used as a
shell startup
file. It typically contains function and alias
definitions.
ERRNO Integer value of the shell's errno variable. It
indicates the
reason the last system call failed. Not yet implemented.
EXECSHELL If set, this parameter is assumed to contain the
shell that is
to be used to execute commands that execve(2)
fails to execute
and which do not start with a ``#!shell'' sequence.
FCEDIT The editor used by the fc command (see below).
FPATH Like PATH, but used when an undefined function is
executed to
locate the file defining the function. It is also searched
when a command can't be found using PATH. See
Functions below
for more information.
HISTFILE The name of the file used to store command history. When assigned
to, history is loaded from the specified
file. Also,
several invocations of the shell running on the
same machine
will share history if their HISTFILE parameters
all point to
the same file.
Note: If HISTFILE isn't set, no history file is
used. This is
different from the original Korn shell, which uses
$HOME/.sh_history; in future, pdksh may also use
a default
history file.
HISTSIZE The number of commands normally stored for history. The default
is 128.
HOME The default directory for the cd command and the
value substituted
for an unqualified ~ (see Tilde expansion
below).
IFS Internal field separator, used during substitution and by the
read command, to split values into distinct arguments; normally
set to space, tab and newline. See
Substitution above for
details.
Note: This parameter is not imported from the environment when
the shell is started.
KSH_VERSION
The version of the shell and the date the version
was created
(read-only). See also the version commands in
Emacs editing
mode and Vi editing mode sections, below.
LINENO The line number of the function or shell script
that is currently
being executed.
LINES Set to the number of lines on the terminal or
window. Not yet
implemented.
MAIL If set, the user will be informed of the arrival
of mail in
the named file. This parameter is ignored if the
MAILPATH parameter
is set.
MAILCHECK How often, in seconds, the shell will check for
mail in the
file(s) specified by MAIL or MAILPATH. If set to
0, the shell
checks before each prompt. The default is 600
(10 minutes).
MAILPATH A list of files to be checked for mail. The list
is colon
separated, and each file may be followed by a `?'
and a message
to be printed if new mail has arrived. Command, parameter
and arithmetic substitution is performed on
the message,
and, during substitution, the parameter $_ contains the name
of the file. The default message is ``you have
mail in $_''.
OLDPWD The previous working directory. Unset if cd has
not successfully
changed directories since the shell started, or if the
shell doesn't know where it is.
OPTARG When using getopts, it contains the argument for
a parsed option,
if it requires one.
OPTIND The index of the last argument processed when using getopts.
Assigning 1 to this parameter causes getopts to
process arguments
from the beginning the next time it is invoked.
PATH A colon separated list of directories that are
searched when
looking for commands and .'d files. An empty
string resulting
from a leading or trailing colon, or two adjacent
colons, is
treated as a ``.'', the current directory.
POSIXLY_CORRECT
If set, this parameter causes the posix option to
be enabled.
See POSIX mode below.
PPID The process ID of the shell's parent (read-only).
PS1 The primary prompt for interactive shells. Parameter, command,
and arithmetic substitutions are performed,
and `!' is
replaced with the current command number (see the
fc command
below). A literal `!' can be put in the prompt
by placing
`!!' in PS1. Note that since the command-line
editors try to
figure out how long the prompt is (so they know
how far it is
to the edge of the screen), escape codes in the
prompt tend to
mess things up. You can tell the shell not to
count certain
sequences (such as escape codes) by prefixing
your prompt with
a non-printing character (such as control-A) followed by a
carriage return and then delimiting the escape
codes with this
non-printing character. If you don't have any
non-printing
characters, you're out of luck. By the way,
don't blame me
for this hack; it's in the original ksh. Default
is ``$ ''
for non-root users, ``# '' for root.
PS2 Secondary prompt string, by default ``> '', used
when more input
is needed to complete a command.
PS3 Prompt used by select statement when reading a
menu selection.
Default is ``#? ''.
PS4 Used to prefix commands that are printed during
execution
tracing (see the set -x command below). Parameter, command,
and arithmetic substitutions are performed before
it is printed.
Default is ``+ ''.
PWD The current working directory. May be unset or
NULL if the
shell doesn't know where it is.
RANDOM A random number generator. Every time RANDOM is
referenced,
it is assigned the next random number in the
range 0-32767.
By default, arc4random(3) is used to produce values. If the
variable RANDOM is assigned a value, the value is
used as seed
to srand(3) and subsequent references of RANDOM
will use
rand(3) to produce values, resulting in a predictable sequence.
REPLY Default parameter for the read command if no
names are given.
Also used in select loops to store the value that
is read from
standard input.
SECONDS The number of seconds since the shell started or,
if the parameter
has been assigned an integer value, the
number of seconds
since the assignment plus the value that was
assigned.
TMOUT If set to a positive integer in an interactive
shell, it specifies
the maximum number of seconds the shell
will wait for
input after printing the primary prompt (PS1).
If the time is
exceeded, the shell exits.
TMPDIR The directory shell temporary files are created
in. If this
parameter is not set, or does not contain the absolute path of
a writable directory, temporary files are created
in /tmp.
VISUAL If set, this parameter controls the command-line
editing mode
for interactive shells. If the last component of
the path
specified in this parameter contains the string
``vi'',
``emacs'' or ``gmacs'', the vi, emacs or gmacs
(Gosling emacs)
editing mode is enabled, respectively.
Tilde expansion [Toc] [Back]
Tilde expansion, which is done in parallel with parameter
substitution,
is done on words starting with an unquoted `~'. The characters following
the tilde, up to the first `/', if any, are assumed to be a
login name.
If the login name is empty, `+' or `-', the value of the
HOME, PWD, or
OLDPWD parameter is substituted, respectively. Otherwise,
the password
file is searched for the login name, and the tilde expression is substituted
with the user's home directory. If the login name is
not found in
the password file or if any quoting or parameter substitution occurs in
the login name, no substitution is performed.
In parameter assignments (those preceding a simple-command
or those occurring
in the arguments of alias, export, readonly, and
typeset), tilde
expansion is done after any unquoted colon (`:'), and login
names are also
delimited by colons.
The home directory of previously expanded login names are
cached and reused.
The alias -d command may be used to list, change, and
add to this
cache (e.g., alias -d fac=/usr/local/facilities; cd
~fac/bin).
Brace expansion (alteration) [Toc] [Back]
Brace expressions, which take the form
prefix{str1,...,strN}suffix
are expanded to N words, each of which is the concatenation
of prefix,
stri and suffix (e.g., ``a{c,b{X,Y},d}e'' expands to four
words: ``ace'',
``abXe'', ``abYe'', and ``ade''). As noted in the example,
brace expressions
can be nested and the resulting words are not sorted.
Brace expressions
must contain an unquoted comma (`,') for expansion
to occur
(i.e., {} and {foo} are not expanded). Brace expansion is
carried out
after parameter substitution and before file name generation.
File name patterns [Toc] [Back]
A file name pattern is a word containing one or more unquoted `?' or `*'
characters or ``[..]'' sequences. Once brace expansion has
been performed,
the shell replaces file name patterns with the sorted names of
all the files that match the pattern (if no files match, the
word is left
unchanged). The pattern elements have the following meaning:
? Matches any single character.
* Matches any sequence of characters.
[..] Matches any of the characters inside the brackets.
Ranges of
characters can be specified by separating two characters by a `-'
(e.g., ``[a0-9]'' matches the letter ``a'' or any
digit). In order
to represent itself, a `-' must either be quoted
or the first
or last character in the character list. Similarly,
a `]' must
be quoted or the first character in the list if it
is to represent
itself instead of the end of the list. Also, a
`!' appearing
at the start of the list has special meaning
(see below), so
to represent itself it must be quoted or appear later in the
list.
[!..] Like [..], except it matches any character not inside the brackets.
*(pattern| ... |pattern)
Matches any string of characters that matches zero
or more occurrences
of the specified patterns. Example: The pattern
*(foo|bar) matches the strings ``'', ``foo'',
``bar'',
``foobarfoo'', etc.
+(pattern| ... |pattern)
Matches any string of characters that matches one or
more occurrences
of the specified patterns. Example: The pattern
+(foo|bar) matches the strings ``foo'', ``bar'',
``foobar'', etc.
?(pattern| ... |pattern)
Matches the empty string or a string that matches
one of the
specified patterns. Example: The pattern ?(foo|bar)
only matches
the strings ``'', ``foo'' and ``bar''.
@(pattern| ... |pattern)
Matches a string that matches one of the specified
patterns. Example:
The pattern @(foo|bar) only matches the
strings ``foo''
and ``bar''.
!(pattern| ... |pattern)
Matches any string that does not match one of the
specified patterns.
Examples: The pattern !(foo|bar) matches all
strings except
``foo'' and ``bar''; the pattern !(*) matches
no strings;
the pattern !(?)* matches all strings (think about
it).
Note that pdksh currently never matches ``.'' and ``..'',
but the original
ksh, Bourne sh and bash do, so this may have to change
(too bad).
Note that none of the above pattern elements match either a
period (`.')
at the start of a file name or a slash (`/'), even if they
are explicitly
used in a [..] sequence; also, the names ``.'' and ``..''
are never
matched, even by the pattern ``.*''.
If the markdirs option is set, any directories that result
from file name
generation are marked with a trailing `/'.
The POSIX character classes (i.e., [:class-name:] inside a
[..] expression)
are not yet implemented.
Input/output redirection
When a command is executed, its standard input, standard
output, and
standard error (file descriptors 0, 1, and 2, respectively)
are normally
inherited from the shell. Three exceptions to this are commands in
pipelines, for which standard input and/or standard output
are those set
up by the pipeline, asynchronous commands created when job
control is
disabled, for which standard input is initially set to be
from /dev/null,
and commands for which any of the following redirections
have been specified:
> file Standard output is redirected to file. If file does
not exist,
it is created; if it does exist, is a regular file
and the
noclobber option is set, an error occurs; otherwise,
the file is
truncated. Note that this means the command cmd <
foo > foo will
open foo for reading and then truncate it when it
opens it for
writing, before cmd gets a chance to actually read
foo.
>| file
Same as >, except the file is truncated, even if the
noclobber
option is set.
>> file
Same as >, except if file exists it is appended to
instead of being
truncated. Also, the file is opened in append
mode, so
writes always go to the end of the file (see
open(2)).
< file Standard input is redirected from file, which is
opened for reading.
<> file
Same as <, except the file is opened for reading and
writing.
<< marker
After reading the command line containing this kind
of redirection
(called a ``here document''), the shell copies
lines from
the command source into a temporary file until a
line matching
marker is read. When the command is executed, standard input is
redirected from the temporary file. If marker contains no quoted
characters, the contents of the temporary file are
processed as
if enclosed in double quotes each time the command
is executed,
so parameter, command, and arithmetic substitutions
are performed,
along with backslash (`') escapes for `$',
``', `', and
`0wline'. If multiple here documents are used on
the same command
line, they are saved in order.
<<- marker
Same as <<, except leading tabs are stripped from
lines in the
here document.
<& fd Standard input is duplicated from file descriptor
fd. fd can be
a single digit, indicating the number of an existing
file descriptor;
the letter `p', indicating the file descriptor associated
with the output of the current co-process; or
the character
`-', indicating standard input is to be closed.
>& fd Same as <&, except the operation is done on standard
output.
In any of the above redirections, the file descriptor that
is redirected
(i.e., standard input or standard output) can be explicitly
given by preceding
the redirection with a single digit. Parameter, command, and
arithmetic substitutions, tilde substitutions, and (if the
shell is interactive)
file name generation are all performed on the
file, marker,
and fd arguments of redirections. Note, however, that the
results of any
file name generation are only used if a single file is
matched; if multiple
files match, the word with the expanded file name generation characters
is used. Note that in restricted shells, redirections
which can
create files cannot be used.
For simple-commands, redirections may appear anywhere in the
command; for
compound-commands (if statements, etc.), any redirections
must appear at
the end. Redirections are processed after pipelines are
created and in
the order they are given, so
cat /foo/bar 2>&1 > /dev/null | cat -n
will print an error with a line number prepended to it.
Arithmetic expressions [Toc] [Back]
Integer arithmetic expressions can be used with the let command, inside
$((..)) expressions, inside array references (e.g.,
name[expr]), as numeric
arguments to the test command, and as the value of an
assignment to
an integer parameter.
Expressions may contain alpha-numeric parameter identifiers,
array references,
and integer constants and may be combined with the
following C operators
(listed and grouped in increasing order of precedence):
Unary operators:
+ - ! ~ ++ --
Binary operators:
,
= *= /= %= += -= <<= >>= &= ^= |=
||
&&
|
^
&
== !=
< <= >= >
<< >>
+ -
* / %
Ternary operators:
?: (precedence is immediately higher than assignment)
Grouping operators:
( )
Integer constants may be specified with arbitrary bases using the notation
base#number, where base is a decimal integer specifying
the base,
and number is a number in the specified base.
The operators are evaluated as follows:
unary +
Result is the argument (included for completeness).
unary -
Negation.
! Logical NOT; the result is 1 if argument is
zero, 0 if not.
~ Arithmetic (bit-wise) NOT.
++ Increment; must be applied to a parameter (not
a literal or
other expression). The parameter is incremented by 1.
When used as a prefix operator, the result is
the incremented
value of the parameter; when used as a
postfix operator,
the result is the original value of the
parameter.
-- Similar to ++, except the parameter is decremented by 1.
, Separates two arithmetic expressions; the
left-hand side is
evaluated first, then the right. The result
is the value
of the expression on the right-hand side.
= Assignment; variable on the left is set to the
value on the
right.
*= /= += -= <<= >>= &= ^= |=
Assignment operators. <var> <op> = <expr> is
the same as
<var> = <var> <op> ( <expr> ).
|| Logical OR; the result is 1 if either argument
is non-zero,
0 if not. The right argument is evaluated only if the left
argument is zero.
&& Logical AND; the result is 1 if both arguments
are non-zero,
0 if not. The right argument is evaluated
only if the
left argument is non-zero.
| Arithmetic (bit-wise) OR.
^ Arithmetic (bit-wise) XOR (exclusive-OR).
& Arithmetic (bit-wise) AND.
== Equal; the result is 1 if both arguments are
equal, 0 if
not.
!= Not equal; the result is 0 if both arguments
are equal, 1
if not.
< Less than; the result is 1 if the left argument is less
than the right, 0 if not.
<= >= >
Less than or equal, greater than or equal,
greater than.
See <.
<< >> Shift left (right); the result is the left argument with
its bits shifted left (right) by the amount
given in the
right argument.
+ - * /
Addition, subtraction, multiplication, and division.
% Remainder; the result is the remainder of the
division of
the left argument by the right. The sign of
the result is
unspecified if either argument is negative.
<arg1> ? <arg2> : <arg3>
If <arg1> is non-zero, the result is <arg2>,
otherwise
<arg3>.
Co-processes [Toc] [Back]
A co-process, which is a pipeline created with the |& operator, is an
asynchronous process that the shell can both write to (using
print -p)
and read from (using read -p). The input and output of the
co-process
can also be manipulated using >&p and <&p redirections, respectively.
Once a co-process has been started, another can't be started
until the
co-process exits, or until the co-process's input has been
redirected using
an exec n>&p redirection. If a co-process's input is
redirected in
this way, the next co-process to be started will share the
output with
the first co-process, unless the output of the initial coprocess has
been redirected using an exec n<&p redirection.
Some notes concerning co-processes:
+o The only way to close the co-process's input (so the coprocess reads
an end-of-file) is to redirect the input to a numbered
file descriptor
and then close that file descriptor (e.g., exec
3>&p; exec 3>&-).
+o In order for co-processes to share a common output, the
shell must
keep the write portion of the output pipe open. This
means that endof-file
will not be detected until all co-processes
sharing the coprocess's
output have exited (when they all exit, the
shell closes
its copy of the pipe). This can be avoided by redirecting the output
to a numbered file descriptor (as this also causes the
shell to close
its copy). Note that this behaviour is slightly different from the
original Korn shell which closes its copy of the write
portion of the
co-process output when the most recently started co-process (instead
of when all sharing co-processes) exits.
+o print -p will ignore SIGPIPE signals during writes if
the signal is
not being trapped or ignored; the same is true i
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