= Parameters and variables
:encoding: UTF-8
:lang: en
//:title: Yash manual - Parameters and variables
:description: This page describes parameters and variables supported by yash.

dfn:[Parameters] are string values that are expanded in
link:expand.html[parameter expansion]. There are three types of parameters:
<<positional,positional parameters>>, <<special,special parameters>> and
<<variables,variables>>.

[[positional]]
== Positional parameters

dfn:[Positional parameters] are parameters that are identified by natural
numbers. If there are three positional parameters, for example, they are
identified as +1+, +2+, and +3+. You can obtain the number of positional
parameters by <<sp-hash,the +#+ special parameter>>. The +*+ and +@+
special parameters are expanded to all positional parameters.

Positional parameters are initialized from the shell's command line arguments
when the shell is started (see link:invoke.html#arguments[Command line
arguments]). In the initialization, the order of the operands are preserved as
the order of the positional parameters.

When the shell executes a link:exec.html#function[function] call, positional
parameters are changed to the arguments to the function call so that you can
access the arguments while the function is being executed. Positional
parameters are restored to the original values when the execution of the
function is finished.

Positional parameters can be manipulated by built-in commands like
link:_set.html[set] and link:_shift.html[shift].

Note that +0+ is not a positional parameter but a special parameter.

[[special]]
== Special parameters

dfn:[Special parameters] are parameters each identified by a single symbol.
They cannot be directly assigned to by the user.

Yash provides the following special parameters:

[[sp-zero]]+0+::
The name of the shell executable file or the script file that was specified in
the invocation of the shell.

[[sp-hash]]+#+::
The number of current positional parameters.
The value is a non-negative integer.

[[sp-dollar]]+$+::
The process ID of the shell. The value is a positive integer and is never
changed even in subshells.

[[sp-hyphen]]+-+::
Currently enabled shell options. The value is a concatenation of alphabet
characters that are the names of currently enabled single-character options
that can be specified in shell invocation. The value reflects changes of
enabled options when you enable or disable options using the
link:_set.html[set built-in].

[[sp-question]]+?+::
The exit status of the last executed link:syntax.html#pipelines[pipeline].
The value is a non-negative integer.

[[sp-exclamation]]+!+::
The process ID of the last executed link:syntax.html#async[asynchronous list].

[[sp-asterisk]]+*+::
This special parameter represents the whole <<positional,positional
parameters>>. When there is no positional parameters, the value of this special
parameter is the empty string. When there is more than one positional
parameter, the value is a concatenation of all the positional parameters, each
of which is separated as follows:
+
- If the <<sv-ifs,+IFS+>> variable exists and its value is not empty,
  positional parameters are each separated by the first character of the
  value of the +IFS+ variable. 
- If the +IFS+ variable exists and has an empty value, positional parameters
  are just concatenated without any separator.
- If the +IFS+ variable does not exist, positional parameters are each
  separated by a space character.

[[sp-at]]+@+::
This special parameter represents the whole <<positional,positional
parameters>> like the +*+ special parameter above. The difference between the
two is the results of expansion that occurs between a pair of
link:syntax.html#quotes[double-quotation marks]. If the +@+ special parameter
is expanded inside double-quotations, positional parameters are
link:expand.html#split[field-split] rather than concatenated (in spite of the
quotation). If there are no positional parameters, the expansion yields no
word rather than an empty word.
+
--
- When there are no positional parameters, the command words +echo 1 "$@" 2+
  is expanded to the three words +echo+, +1+, and +2+.
- When positional
  parameters are the three words +1+, +2  2+, and +3+, the command words
  +echo "$@"+ is expanded to the four words +echo+, +1+, +2  2+, and +3+,
  and the words +echo "a$@b"+ to the four words +echo+, +a1+, +2  2+, and
  +3b+.
--

[[variables]]
== Variables

dfn:[Variables] are parameters the user can assign values to.
Each variable has a name that identifies it and a value that defines the
results of expansion.

A variable name is composed of one or more alphanumeric characters and
underscores (+_+). A name cannot start with a digit.
Other characters may be used in a name depending on internationalization
support of your environment.

Variables that are exported to external commands are called dfn:[environment
variables].
They are passed to all external commands the shell invokes.
Variables passed to the shell in invocation will be automatically exported.

You can assign to variables by a link:syntax.html#simple[simple command] as
well as the link:_typeset.html[typeset built-in].
You can remove variables by using the link:_unset.html[unset built-in].

[[shellvars]]
=== Variables used by the shell

The following variables are used by the shell for special purposes.

[[sv-cdpath]]+CDPATH+::
This variable is used by the cd built-in to find a destination directory.

[[sv-columns]]+COLUMNS+::
This variable specifies the width (the number of character columns) of the
terminal screen. The value affects the display of
link:lineedit.html[line-editing].

[[sv-command_not_found_handler]]+COMMAND_NOT_FOUND_HANDLER+::
When the shell cannot find a command to be executed,
the value of this variable is interpreted and executed instead.
You can override the shell's error handling behavior with this variable.
See link:exec.html#simple[Execution of simple commands] for detail.
+
This feature is disabled in the link:posix.html[POSIXly-correct mode].

[[sv-dirstack]]+DIRSTACK+::
This array variable is used by the shell to store the directory stack
contents.
If you modify the value of this variable, the directory stack may be
corrupted.

[[sv-echo_style]]+ECHO_STYLE+::
This variable specifies the behavior of the link:_echo.html[echo built-in].

[[sv-env]]+ENV+::
When an link:interact.html[interactive] shell is started in the
link:posix.html[POSIXly-correct mode],
the value of this variable is used to find the initialization file.
See link:invoke.html#init[Initialization of yash].

[[sv-fcedit]]+FCEDIT+::
This variable specifies an editor program used to edit command lines during
execution of the link:_fc.html[fc built-in].

[[sv-handled]]+HANDLED+::
This variable can be set in the
<<sv-command_not_found_handler,command-not-found handler>> to tell the shell
not to produce a further error message.
See link:exec.html#simple[Execution of simple commands] for detail.

[[sv-histfile]]+HISTFILE+::
This variable specifies the pathname of the file to save the
link:interact.html#history[command history] in.

[[sv-histrmdup]]+HISTRMDUP+::
This variable specifies the number of link:interact.html#history[command
history] items to be checked for duplication.
When the shell is adding a new history item to the command history, if some of
the most recent {{n}} items have the same contents as the new one, then the
duplicate existing items are removed from the history before the new one is
added, where {{n}} is the value of this variable.
+
If the value of this variable is +1+, for example, the most recent item is
removed when a new item that have the same contents is added.
+
Items older than the {{n}}th recent item are not removed.
No items are removed if the value of this variable is +0+.
All items are subject to removal if the variable value is greater than or
equal to the value of the <<sv-histsize,+HISTSIZE+ variable>>.

[[sv-histsize]]+HISTSIZE+::
This variable specifies the maximum number of items in the
link:interact.html#history[command history].

[[sv-home]]+HOME+::
This variable specifies the pathname of the user's home directory and affects
results of link:expand.html#tilde[tilde expansion] and link:_cd.html[cd
built-in].

[[sv-ifs]]+IFS+::
This variable specifies separators used in link:expand.html#split[field
splitting].
The variable value is initialized to the three characters of a space, a tab,
and a newline when the shell is started.

[[sv-lang]]+LANG+::
[[sv-lc_all]]+LC_ALL+::
[[sv-lc_collate]]+LC_COLLATE+::
[[sv-lc_ctype]]+LC_CTYPE+::
[[sv-lc_messages]]+LC_MESSAGES+::
[[sv-lc_monetary]]+LC_MONETARY+::
[[sv-lc_numeric]]+LC_NUMERIC+::
[[sv-lc_time]]+LC_TIME+::
These variables specify a locale in which the shell runs.
The shell chooses the file input/output encoding, the error message language,
etc. according to the locale specified.
+
Unless the shell is link:interact.html[interactive] and not in the
link:posix.html[POSIXly-correct mode], the value of the +LC_CTYPE+ variable is
considered only when the shell is started. Once the shell has been
initialized, changing the value of +LC_CTYPE+ will have no effect on the
shell's behavior.

[[sv-lineno]]+LINENO+::
The value of this variable is automatically set to the line number in which
the currently executed command appears in the file.
+
In the link:interact.html[interactive shell], the line number is reset to 1
each time the shell reads and executes a command.
+
If you assign to or remove this variable, it will no longer provide line
numbers.

[[sv-lines]]+LINES+::
This variable specifies the height (the number of character lines) of the
terminal screen.
The value affects the display of link:lineedit.html[line-editing].

[[sv-mail]]+MAIL+::
This variable specifies the pathname of a file that is checked in
link:interact.html#mailcheck[mail checking].

[[sv-mailcheck]]+MAILCHECK+::
This variable specifies how often the shell should do
link:interact.html#mailcheck[mail checking].
The value has to be specified as a positive integer in seconds.
The value is initialized to the default value of +600+ when the shell is
started.

[[sv-mailpath]]+MAILPATH+::
This variable specifies the pathnames of files that are checked in
link:interact.html#mailcheck[mail checking].

[[sv-nlspath]]+NLSPATH+::
The POSIX standard prescribes that the value of this variable specifies
pathname templates of locale-dependent message data files, but yash does not
use it.

[[sv-oldpwd]]+OLDPWD+::
This variable is set to the previous working directory path when you change
the working directory by using the link:_cd.html[cd] or other built-ins.
This variable is exported by default.

[[sv-optarg]]+OPTARG+::
When the link:_getopts.html[getopts built-in] parses an option that takes an
argument, the argument value is assigned to this variable.

[[sv-optind]]+OPTIND+::
The value of this variable specifies the index of an option that is to be
parsed by the next link:_getopts.html[getopts built-in] execution.
This variable is initialized to +1+ when the shell is started.

[[sv-path]]+PATH+::
This variable specifies paths that are searched for a command in
link:exec.html#search[command search].

[[sv-ppid]]+PPID+::
The value of this variable is the process ID of the shell's parent process,
which is a positive integer.
This variable is initialized when the shell is started.
The value is not changed when the shell makes a new
link:exec.html#subshell[subshell].

[[sv-prompt_command]]+PROMPT_COMMAND+::
The shell interprets and executes the value of this variable before printing
each command prompt if the shell is link:interact.html[interactive] and not in
the link:posix.html[POSIXly-correct mode].
This behavior is equivalent to executing the command
ifdef::basebackend-html[]
pass:[<code><a href="_eval.html">eval</a> -i -- "${PROMPT_COMMAND-}"</code>]
endif::basebackend-html[]
ifndef::basebackend-html[`eval -i -- "${PROMPT_COMMAND-}"`]
before each command prompt, but its exit status does not affect the expansion
of the +?+ special parameter in the next command.

[[sv-ps1]]+PS1+::
This variable specifies the main command prompt string printed by an
link:interact.html[interactive] shell.
See link:interact.html#prompt[Prompts] for the format of the variable value.
The value is initialized to +\$&#x20;+ when the shell is started.
(In the link:posix.html[POSIXly-correct mode], the initial value is either
+$&#x20;+ or +#&#x20;+ depending on whether the effective user ID of the shell
process is zero or not.)

[[sv-ps1r]]+PS1R+::
This variable specifies the auxiliary prompt string printed to the right of
the cursor when you input a command line to an link:interact.html[interactive]
shell.
See link:interact.html#prompt[Prompts] for the format of the variable value.

[[sv-ps1s]]+PS1S+::
This variable specifies the font style of command strings you enter to an
link:interact.html[interactive] shell.
See link:interact.html#prompt[Prompts] for the format of the variable value.

[[sv-ps2]]+PS2+::
This variable is like the <<sv-ps1,+PS1+>> variable, but it is used for the
second and following lines of a command that is longer than one line.
See link:interact.html#prompt[Prompts] for the format of the variable value.
The value is initialized to +>&#x20;+ when the shell is started.

[[sv-ps2r]]+PS2R+::
This variable is like the <<sv-ps1r,+PS1R+>> variable, but it is used when
<<sv-ps2,+PS2+>> is used.
See link:interact.html#prompt[Prompts] for the format of the variable value.

[[sv-ps2s]]+PS2S+::
This variable is like the <<sv-ps1s,+PS1S+>> variable, but it is used when
<<sv-ps2,+PS2+>> is used.
See link:interact.html#prompt[Prompts] for the format of the variable value.

[[sv-ps4]]+PS4+::
The value of this variable is printed before each command trace output when
the link:_set.html#so-xtrace[xtrace option] is enabled.
The value is subject to link:expand.html#params[parameter expansion],
link:expand.html#cmdsub[command substitution],
link:expand.html#arith[arithmetic expansion].
You can also use backslash notations if the shell is not in the
link:posix.html[POSIXly-correct mode].
The value is initialized to +&#x2B;&#x20;+ when the shell is started.

[[sv-ps4s]]+PS4S+::
This variable is like the <<sv-ps1s,+PS1S+>> variable, but it is used when
<<sv-ps4,+PS4+>> is used.
You can use this variable to modify font style of command trace output.

[[sv-pwd]]+PWD+::
The value of this variable is the pathname of the current working directory.
The value is set when the shell is started and reset each time the working
directory is changed by the link:_cd.html[cd] or other built-ins.
This variable is exported by default.

[[sv-random]]+RANDOM+::
You can use this variable to get random numbers.
The value of this variable is a uniformly distributed random integer between 0
and 32767 (inclusive). You will get a different number each time the variable
is expanded.
+
You can set the ``seed'' of random numbers by assigning a non-negative integer
to the variable.
+
If you remove this variable, it will no longer work as a random number
generator. If the shell was invoked in the link:posix.html[POSIXly-correct
mode], this variable does not work as a random number generator.

[[sv-term]]+TERM+::
This variable specifies the type of the terminal in which the shell is
running.
The value affects the behavior of link:lineedit.html[line-editing].

[[sv-yash_after_cd]]+YASH_AFTER_CD+::
The shell interprets and executes the value of this variable after each time
the shell's working directory is changed by the link:_cd.html[cd] or other
built-ins.
This behavior is equivalent to executing the command
ifdef::basebackend-html[]
pass:[<code><a href="_eval.html">eval</a> -i -- "${YASH_AFTER_CD-}"</code>]
endif::basebackend-html[]
ifndef::basebackend-html[`eval -i -- "${YASH_AFTER_CD-}"`]
after the directory was changed.

[[sv-yash_loadpath]]+YASH_LOADPATH+::
This variable specifies directories the dot built-in searches
for a script file.
More than one directory can be specified by separating them by colons like the
<<sv-path,+PATH+>> variable.
When the shell is started, this variable is initialized to the pathname of the
directory where common script files are installed.

[[sv-yash_le_timeout]]+YASH_LE_TIMEOUT+::
This variable specifies how long the shell should wait for a next possible
input from the terminal when it encountered an ambiguous control sequence
while link:lineedit.html[line-editing].
The value must be specified in milliseconds.
If you do not define this variable, the default value of 100 milliseconds is
assumed.

[[sv-yash_version]]+YASH_VERSION+::
The value is initialized to the version number of the shell
when the shell is started.

[[arrays]]
=== Arrays

An dfn:[array] is a variable that contains zero or more strings.
The string values of an array are identified by natural numbers (like
<<positional,positional parameters>>).

You can assign values to an array by using a
link:syntax.html#simple[simple command] as well as the
link:_array.html[array built-in].
You can use the link:_unset.html[unset built-in] to remove arrays.

Arrays cannot be exported as arrays.
When an array is exported, it is treated as a normal variable whose value is
a concatenation of all the array values, each separated by a colon.

Arrays are not supported in the link:posix.html[POSIXly-correct mode].

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