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(zsh.info)Compatibility

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Compatibility
=============

Zsh tries to emulate `sh' or `ksh' when it is invoked as sh or ksh
respectively; more precisely, it looks at the first letter of the name
by which it was invoked, excluding any initial `r' (assumed to stand
for `restricted'), and if that is `s' or `k' it will emulate `sh' or
`ksh'.  Furthermore, if invoked as su (which happens on certain systems
when the shell is executed by the su command), the shell will try to
find an alternative name from the SHELL environment variable and
perform emulation based on that.

In `sh' and `ksh' compatibility modes the following parameters are not
special and not initialized by the shell: ARGC, argv, cdpath, fignore,
fpath, HISTCHARS, mailpath, MANPATH, manpath, path, prompt, PROMPT,
PROMPT2, PROMPT3, PROMPT4, psvar, status, watch.

The usual zsh startup/shutdown scripts are not executed.  Login shells
source /etc/profile followed by $HOME/.profile.  If the ENV environment
variable is set on invocation, $ENV is sourced after the profile
scripts.  The value of ENV is subjected to parameter expansion, command
substitution, and arithmetic expansion before being interpreted as a
pathname.  Note that the PRIVILEGED option also affects the execution
of startup files.

The following options are set if the shell is invoked as sh or ksh:
NO_BAD_PATTERN, NO_BANG_HIST, NO_BG_NICE, NO_EQUALS,
NO_FUNCTION_ARGZERO, GLOB_SUBST, NO_GLOBAL_EXPORT, NO_HUP,
INTERACTIVE_COMMENTS, KSH_ARRAYS, NO_MULTIOS, NO_NOMATCH, NO_NOTIFY,
POSIX_BUILTINS, NO_PROMPT_PERCENT, RM_STAR_SILENT, SH_FILE_EXPANSION,
SH_GLOB, SH_OPTION_LETTERS, SH_WORD_SPLIT.  Additionally the BSD_ECHO
and IGNORE_BRACES options are set if zsh is invoked as sh.  Also, the
KSH_OPTION_PRINT, LOCAL_OPTIONS, PROMPT_BANG, PROMPT_SUBST and
SINGLE_LINE_ZLE options are set if zsh is invoked as ksh.