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