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. Bash ensures that the 'POSIXLY_CORRECT' variable is set.
  15
  16   2. When a command in the hash table no longer exists, Bash will
  17      re-search '$PATH' to find the new location.  This is also available
  18      with 'shopt -s checkhash'.
  19
  20   3. The message printed by the job control code and builtins when a job
  21      exits with a non-zero status is 'Done(status)'.
  22
  23   4. The message printed by the job control code and builtins when a job
  24      is stopped is 'Stopped(SIGNAME)', where SIGNAME is, for example,
  25      'SIGTSTP'.
  26
  27   5. Alias expansion is always enabled, even in non-interactive shells.
  28
  29   6. Reserved words appearing in a context where reserved words are
  30      recognized do not undergo alias expansion.
  31
  32   7. The POSIX 'PS1' and 'PS2' expansions of '!' to the history number
  33      and '!!' to '!' are enabled, and parameter expansion is performed
  34      on the values of 'PS1' and 'PS2' regardless of the setting of the
  35      'promptvars' option.
  36
  37   8. The POSIX startup files are executed ('$ENV') rather than the
  38      normal Bash files.
  39
  40   9. Tilde expansion is only performed on assignments preceding a
  41      command name, rather than on all assignment statements on the line.
  42
  43   10. The default history file is '~/.sh_history' (this is the default
  44      value of '$HISTFILE').
  45
  46   11. Redirection operators do not perform filename expansion on the
  47      word in the redirection unless the shell is interactive.
  48
  49   12. Redirection operators do not perform word splitting on the word in
  50      the redirection.
  51
  52   13. Function names must be valid shell 'name's.  That is, they may not
  53      contain characters other than letters, digits, and underscores, and
  54      may not start with a digit.  Declaring a function with an invalid
  55      name causes a fatal syntax error in non-interactive shells.
  56
  57   14. Function names may not be the same as one of the POSIX special
  58      builtins.
  59
  60   15. POSIX special builtins are found before shell functions during
  61      command lookup.
  62
  63   16. When printing shell function definitions (e.g., by 'type'), Bash
  64      does not print the 'function' keyword.
  65
  66   17. Literal tildes that appear as the first character in elements of
  67      the 'PATH' variable are not expanded as described above under *note
  68      Tilde Expansion::.
  69
  70   18. The 'time' reserved word may be used by itself as a command.  When
  71      used in this way, it displays timing statistics for the shell and
  72      its completed children.  The 'TIMEFORMAT' variable controls the
  73      format of the timing information.
  74
  75   19. When parsing and expanding a ${...} expansion that appears within
  76      double quotes, single quotes are no longer special and cannot be
  77      used to quote a closing brace or other special character, unless
  78      the operator is one of those defined to perform pattern removal.
  79      In this case, they do not have to appear as matched pairs.
  80
  81   20. The parser does not recognize 'time' as a reserved word if the
  82      next token begins with a '-'.
  83
  84   21. The '!' character does not introduce history expansion within a
  85      double-quoted string, even if the 'histexpand' option is enabled.
  86
  87   22. If a POSIX special builtin returns an error status, a
  88      non-interactive shell exits.  The fatal errors are those listed in
  89      the POSIX standard, and include things like passing incorrect
  90      options, redirection errors, variable assignment errors for
  91      assignments preceding the command name, and so on.
  92
  93   23. A non-interactive shell exits with an error status if a variable
  94      assignment error occurs when no command name follows the assignment
  95      statements.  A variable assignment error occurs, for example, when
  96      trying to assign a value to a readonly variable.
  97
  98   24. A non-interactive shell exits with an error status if a variable
  99      assignment error occurs in an assignment statement preceding a
 100      special builtin, but not with any other simple command.
 101
 102   25. A non-interactive shell exits with an error status if the
 103      iteration variable in a 'for' statement or the selection variable
 104      in a 'select' statement is a readonly variable.
 105
 106   26. Non-interactive shells exit if FILENAME in '.'  FILENAME is not
 107      found.
 108
 109   27. Non-interactive shells exit if a syntax error in an arithmetic
 110      expansion results in an invalid expression.
 111
 112   28. Non-interactive shells exit if a parameter expansion error occurs.
 113
 114   29. Non-interactive shells exit if there is a syntax error in a script
 115      read with the '.' or 'source' builtins, or in a string processed by
 116      the 'eval' builtin.
 117
 118   30. Process substitution is not available.
 119
 120   31. While variable indirection is available, it may not be applied to
 121      the '#' and '?' special parameters.
 122
 123   32. When expanding the '*' special parameter in a pattern context
 124      where the expansion is double-quoted does not treat the '$*' as if
 125      it were double-quoted.
 126
 127   33. Assignment statements preceding POSIX special builtins persist in
 128      the shell environment after the builtin completes.
 129
 130   34. Assignment statements preceding shell function calls persist in
 131      the shell environment after the function returns, as if a POSIX
 132      special builtin command had been executed.
 133
 134   35. The 'command' builtin does not prevent builtins that take
 135      assignment statements as arguments from expanding them as
 136      assignment statements; when not in POSIX mode, assignment builtins
 137      lose their assignment statement expansion properties when preceded
 138      by 'command'.
 139
 140   36. The 'bg' builtin uses the required format to describe each job
 141      placed in the background, which does not include an indication of
 142      whether the job is the current or previous job.
 143
 144   37. The output of 'kill -l' prints all the signal names on a single
 145      line, separated by spaces, without the 'SIG' prefix.
 146
 147   38. The 'kill' builtin does not accept signal names with a 'SIG'
 148      prefix.
 149
 150   39. The 'export' and 'readonly' builtin commands display their output
 151      in the format required by POSIX.
 152
 153   40. The 'trap' builtin displays signal names without the leading
 154      'SIG'.
 155
 156   41. The 'trap' builtin doesn't check the first argument for a possible
 157      signal specification and revert the signal handling to the original
 158      disposition if it is, unless that argument consists solely of
 159      digits and is a valid signal number.  If users want to reset the
 160      handler for a given signal to the original disposition, they should
 161      use '-' as the first argument.
 162
 163   42. The '.' and 'source' builtins do not search the current directory
 164      for the filename argument if it is not found by searching 'PATH'.
 165
 166   43. Enabling POSIX mode has the effect of setting the
 167      'inherit_errexit' option, so subshells spawned to execute command
 168      substitutions inherit the value of the '-e' option from the parent
 169      shell.  When the 'inherit_errexit' option is not enabled, Bash
 170      clears the '-e' option in such subshells.
 171
 172   44. Enabling POSIX mode has the effect of setting the 'shift_verbose'
 173      option, so numeric arguments to 'shift' that exceed the number of
 174      positional parameters will result in an error message.
 175
 176   45. When the 'alias' builtin displays alias definitions, it does not
 177      display them with a leading 'alias ' unless the '-p' option is
 178      supplied.
 179
 180   46. When the 'set' builtin is invoked without options, it does not
 181      display shell function names and definitions.
 182
 183   47. When the 'set' builtin is invoked without options, it displays
 184      variable values without quotes, unless they contain shell
 185      metacharacters, even if the result contains nonprinting characters.
 186
 187   48. When the 'cd' builtin is invoked in LOGICAL mode, and the pathname
 188      constructed from '$PWD' and the directory name supplied as an
 189      argument does not refer to an existing directory, 'cd' will fail
 190      instead of falling back to PHYSICAL mode.
 191
 192   49. When the 'cd' builtin cannot change a directory because the length
 193      of the pathname constructed from '$PWD' and the directory name
 194      supplied as an argument exceeds PATH_MAX when all symbolic links
 195      are expanded, 'cd' will fail instead of attempting to use only the
 196      supplied directory name.
 197
 198   50. The 'pwd' builtin verifies that the value it prints is the same as
 199      the current directory, even if it is not asked to check the file
 200      system with the '-P' option.
 201
 202   51. When listing the history, the 'fc' builtin does not include an
 203      indication of whether or not a history entry has been modified.
 204
 205   52. The default editor used by 'fc' is 'ed'.
 206
 207   53. The 'type' and 'command' builtins will not report a non-executable
 208      file as having been found, though the shell will attempt to execute
 209      such a file if it is the only so-named file found in '$PATH'.
 210
 211   54. The 'vi' editing mode will invoke the 'vi' editor directly when
 212      the 'v' command is run, instead of checking '$VISUAL' and
 213      '$EDITOR'.
 214
 215   55. When the 'xpg_echo' option is enabled, Bash does not attempt to
 216      interpret any arguments to 'echo' as options.  Each argument is
 217      displayed, after escape characters are converted.
 218
 219   56. The 'ulimit' builtin uses a block size of 512 bytes for the '-c'
 220      and '-f' options.
 221
 222   57. The arrival of 'SIGCHLD' when a trap is set on 'SIGCHLD' does not
 223      interrupt the 'wait' builtin and cause it to return immediately.
 224      The trap command is run once for each child that exits.
 225
 226   58. The 'read' builtin may be interrupted by a signal for which a trap
 227      has been set.  If Bash receives a trapped signal while executing
 228      'read', the trap handler executes and 'read' returns an exit status
 229      greater than 128.
 230
 231   59. Bash removes an exited background process's status from the list
 232      of such statuses after the 'wait' builtin is used to obtain it.
 233
 234 There is other POSIX behavior that Bash does not implement by default
 235 even when in POSIX mode.  Specifically:
 236
 237   1. The 'fc' builtin checks '$EDITOR' as a program to edit history
 238      entries if 'FCEDIT' is unset, rather than defaulting directly to
 239      'ed'.  'fc' uses 'ed' if 'EDITOR' is unset.
 240
 241   2. As noted above, Bash requires the 'xpg_echo' option to be enabled
 242      for the 'echo' builtin to be fully conformant.
 243
 244 Bash can be configured to be POSIX-conformant by default, by specifying
 245 the '--enable-strict-posix-default' to 'configure' when building (*note
 246 Optional Features::).
 247