= POSIXly-correct mode
:encoding: UTF-8
:lang: en
//:title: Yash manual - POSIXly-correct mode
:description: This page describes the behavior of yash's POSIXly-correct mode.

Yash behaves as defined in
link:http://pubs.opengroup.org/onlinepubs/9699919799/[POSIX.1-2008], Shell &
Utilities for the most part, but some functionalities disobey POSIX for
usability.
When full POSIX-conformance is needed, you can enable the dfn:[POSIXly-correct
mode] to make yash obey POSIX as mush as possible.

If yash is started in the name of ``sh'', the POSIXly-correct mode is
automatically enabled.
The +-o posixly-correct+ command-line option also enables the POSIXly-correct
mode.
After yash has been started, the POSIXly-correct mode can be enabled by
executing the command string +link:_set.html[set] -o posixly-correct+.

When the POSIXly-correct mode is on, yash not only tries to obey the
requirements by POSIX, but also treats as errors most conditions where the
behavior is _undefined_ or _unspecified_ by POSIX.
As a result, most yash-specific functionalities are disabled in the
POSIXly-correct mode.

Below is the complete list of the behavioral differences between when yash is
in the POSIXly-correct mode and when not.
When the POSIXly-correct mode is enabled:

- Different link:invoke.html#init[initialization scripts] are used.
- Global link:syntax.html#aliases[aliases] are not substituted.
- Nested commands in a link:syntax.html#compound[compound command] must not be
  empty.
- Words expanded in a link:syntax.html#for[for loop] are assigned as a global
  variable rather than a local.
- The first pattern in a link:syntax.html#case[case command] cannot be +esac+.
- The +function+ keyword cannot be used for link:syntax.html#funcdef[function
  definition].
- link:syntax.html#simple[Simple commands] cannot assign to
  link:params.html#arrays[arrays].
- Changing the value of the link:params.html#sv-lc_ctype[+LC_CTYPE+ variable]
  after the shell has been initialized does not affect the shell's locale.
- The link:params.html#sv-random[+RANDOM+ variable] cannot be used to generate
  random numbers.
- link:expand.html#tilde[Tilde expansion] only expands +~+ and
  +~{{username}}+.
- link:expand.html#params[Parameter expansion] cannot be nested.
  No link:expand.html#param-index[indexes] are allowed.
- The commands in a link:expand.html#cmdsub[command substitution] of the form
  +$({{commands}})+ are parsed every time the substitution is executed.
- Fractional numbers cannot be used in link:expand.html#arith[arithmetic
  expansion].
- In a link:redir.html#file[redirection to a file], if the
  link:expand.html#glob[pathname expansion] yielded more than one or no
  pathname, it is not immediately treated as an error. Instead, the shell
  tries to treat the word before the expansion as a pathname.
- link:redir.html#socket[Socket redirection], link:redir.html#here[here
  strings], link:redir.html#pipe[pipe redirection], and
  link:redir.html#process[process redirection] cannot be used.
- When link:exec.html#simple[executing a simple command], failure in command
  search does not trigger execution of the
  link:params.html#sv-command_not_found_handler[+COMMAND_NOT_FOUND_HANDLER+
  variable].
- In link:exec.html#search[command search], a link:builtin.html#types[regular
  built-in] needs to have a corresponding external command for the built-in to
  be found.
- link:syntax.html#async[asynchronous commands] ignore the SIGINT and SIGQUIT
  signals even when link:job.html[job control] is active.
  The standard input of asynchronous commands are redirected to /dev/null if
  the shell is not link:interact.html[interactive], regardless of whether job
  control is active or not.
- Some link:builtin.html[built-ins] behave differently.
  Especially, some command-line options cannot be used.
- A link:interact.html[non-interactive] shell exits when a
  link:builtin.html#types[special built-in] is given a syntactically wrong
  arguments or when an error occurs in assignment or redirection with a
  special built-in.
- An link:interact.html[interactive] shell does not execute the
  link:params.html#sv-prompt_command[+PROMPT_COMMAND+ variable] before
  printing a prompt.
  The values of the link:params.html#sv-ps1[+PS1+],
  link:params.html#sv-ps2[+PS2+], and link:params.html#sv-ps4[+PS4+] variables
  are parsed differently.
- In link:interact.html#mailcheck[mail checking], a notification message is
  printed if the file has been modified, regardless of whether the file is
  empty.

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