POSIX

   1 6.11 Bash POSIX Mode
   2 ====================
   3
   4 Starting Bash with the `--posix' command-line option or executing `set
   5 -o posix' while Bash is running will cause Bash to conform more closely
   6 to the POSIX standard by changing the behavior to match that specified
   7 by POSIX in areas where the Bash default differs.
   8
   9 When invoked as `sh', Bash enters POSIX mode after reading the startup
  10 files.
  11
  12 The following list is what's changed when `POSIX mode' is in effect:
  13
  14   1. When a command in the hash table no longer exists, Bash will
  15      re-search `$PATH' to find the new location.  This is also
  16      available with `shopt -s checkhash'.
  17
  18   2. The message printed by the job control code and builtins when a job
  19      exits with a non-zero status is `Done(status)'.
  20
  21   3. The message printed by the job control code and builtins when a job
  22      is stopped is `Stopped(SIGNAME)', where SIGNAME is, for example,
  23      `SIGTSTP'.
  24
  25   4. The `bg' builtin uses the required format to describe each job
  26      placed in the background, which does not include an indication of
  27      whether the job is the current or previous job.
  28
  29   5. Reserved words appearing in a context where reserved words are
  30      recognized do not undergo alias expansion.
  31
  32   6. The POSIX `PS1' and `PS2' expansions of `!' to the history number
  33      and `!!' to `!' are enabled, and parameter expansion is performed
  34      on the values of `PS1' and `PS2' regardless of the setting of the
  35      `promptvars' option.
  36
  37   7. The POSIX startup files are executed (`$ENV') rather than the
  38      normal Bash files.
  39
  40   8. Tilde expansion is only performed on assignments preceding a
  41      command name, rather than on all assignment statements on the line.
  42
  43   9. The default history file is `~/.sh_history' (this is the default
  44      value of `$HISTFILE').
  45
  46  10. The output of `kill -l' prints all the signal names on a single
  47      line, separated by spaces, without the `SIG' prefix.
  48
  49  11. The `kill' builtin does not accept signal names with a `SIG'
  50      prefix.
  51
  52  12. Non-interactive shells exit if FILENAME in `.' FILENAME is not
  53      found.
  54
  55  13. Non-interactive shells exit if a syntax error in an arithmetic
  56      expansion results in an invalid expression.
  57
  58  14. Redirection operators do not perform filename expansion on the word
  59      in the redirection unless the shell is interactive.
  60
  61  15. Redirection operators do not perform word splitting on the word in
  62      the redirection.
  63
  64  16. Function names must be valid shell `name's.  That is, they may not
  65      contain characters other than letters, digits, and underscores, and
  66      may not start with a digit.  Declaring a function with an invalid
  67      name causes a fatal syntax error in non-interactive shells.
  68
  69  17. POSIX special builtins are found before shell functions during
  70      command lookup.
  71
  72  18. If a POSIX special builtin returns an error status, a
  73      non-interactive shell exits.  The fatal errors are those listed in
  74      the POSIX standard, and include things like passing incorrect
  75      options, redirection errors, variable assignment errors for
  76      assignments preceding the command name, and so on.
  77
  78  19. If `CDPATH' is set, the `cd' builtin will not implicitly append
  79      the current directory to it.  This means that `cd' will fail if no
  80      valid directory name can be constructed from any of the entries in
  81      `$CDPATH', even if the a directory with the same name as the name
  82      given as an argument to `cd' exists in the current directory.
  83
  84  20. A non-interactive shell exits with an error status if a variable
  85      assignment error occurs when no command name follows the assignment
  86      statements.  A variable assignment error occurs, for example, when
  87      trying to assign a value to a readonly variable.
  88
  89  21. A non-interactive shell exits with an error status if the iteration
  90      variable in a `for' statement or the selection variable in a
  91      `select' statement is a readonly variable.
  92
  93  22. Process substitution is not available.
  94
  95  23. Assignment statements preceding POSIX special builtins persist in
  96      the shell environment after the builtin completes.
  97
  98  24. Assignment statements preceding shell function calls persist in the
  99      shell environment after the function returns, as if a POSIX
 100      special builtin command had been executed.
 101
 102  25. The `export' and `readonly' builtin commands display their output
 103      in the format required by POSIX.
 104
 105  26. The `trap' builtin displays signal names without the leading `SIG'.
 106
 107  27. The `trap' builtin doesn't check the first argument for a possible
 108      signal specification and revert the signal handling to the original
 109      disposition if it is, unless that argument consists solely of
 110      digits and is a valid signal number.  If users want to reset the
 111      handler for a given signal to the original disposition, they
 112      should use `-' as the first argument.
 113
 114  28. The `.' and `source' builtins do not search the current directory
 115      for the filename argument if it is not found by searching `PATH'.
 116
 117  29. Subshells spawned to execute command substitutions inherit the
 118      value of the `-e' option from the parent shell.  When not in POSIX
 119      mode, Bash clears the `-e' option in such subshells.
 120
 121  30. Alias expansion is always enabled, even in non-interactive shells.
 122
 123  31. When the `alias' builtin displays alias definitions, it does not
 124      display them with a leading `alias ' unless the `-p' option is
 125      supplied.
 126
 127  32. When the `set' builtin is invoked without options, it does not
 128      display shell function names and definitions.
 129
 130  33. When the `set' builtin is invoked without options, it displays
 131      variable values without quotes, unless they contain shell
 132      metacharacters, even if the result contains nonprinting characters.
 133
 134  34. When the `cd' builtin is invoked in LOGICAL mode, and the pathname
 135      constructed from `$PWD' and the directory name supplied as an
 136      argument does not refer to an existing directory, `cd' will fail
 137      instead of falling back to PHYSICAL mode.
 138
 139  35. When the `pwd' builtin is supplied the `-P' option, it resets
 140      `$PWD' to a pathname containing no symlinks.
 141
 142  36. The `pwd' builtin verifies that the value it prints is the same as
 143      the current directory, even if it is not asked to check the file
 144      system with the `-P' option.
 145
 146  37. When listing the history, the `fc' builtin does not include an
 147      indication of whether or not a history entry has been modified.
 148
 149  38. The default editor used by `fc' is `ed'.
 150
 151  39. The `type' and `command' builtins will not report a non-executable
 152      file as having been found, though the shell will attempt to
 153      execute such a file if it is the only so-named file found in
 154      `$PATH'.
 155
 156  40. The `vi' editing mode will invoke the `vi' editor directly when
 157      the `v' command is run, instead of checking `$FCEDIT' and
 158      `$EDITOR'.
 159
 160  41. When the `xpg_echo' option is enabled, Bash does not attempt to
 161      interpret any arguments to `echo' as options.  Each argument is
 162      displayed, after escape characters are converted.
 163
 164
 165 There is other POSIX behavior that Bash does not implement by default
 166 even when in POSIX mode.  Specifically:
 167
 168   1. The `fc' builtin checks `$EDITOR' as a program to edit history
 169      entries if `FCEDIT' is unset, rather than defaulting directly to
 170      `ed'.  `fc' uses `ed' if `EDITOR' is unset.
 171
 172   2. As noted above, Bash requires the `xpg_echo' option to be enabled
 173      for the `echo' builtin to be fully conformant.
 174
 175
 176 Bash can be configured to be POSIX-conformant by default, by specifying
 177 the `--enable-strict-posix-default' to `configure' when building (*note
 178 Optional Features::).
 179