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 '$PATH' to find the new location.  This is also available
  18      with 'shopt -s checkhash'.
  19
  20   3. Bash will not insert a command without the execute bit set into the
  21      command hash table, even if it returns it as a (last-ditch) result
  22      from a '$PATH' search.
  23
  24   4. The message printed by the job control code and builtins when a job
  25      exits with a non-zero status is 'Done(status)'.
  26
  27   5. The message printed by the job control code and builtins when a job
  28      is stopped is 'Stopped(SIGNAME)', where SIGNAME is, for example,
  29      'SIGTSTP'.
  30
  31   6. Alias expansion is always enabled, even in non-interactive shells.
  32
  33   7. Reserved words appearing in a context where reserved words are
  34      recognized do not undergo alias expansion.
  35
  36   8. The POSIX 'PS1' and 'PS2' expansions of '!' to the history number
  37      and '!!' to '!' are enabled, and parameter expansion is performed
  38      on the values of 'PS1' and 'PS2' regardless of the setting of the
  39      'promptvars' option.
  40
  41   9. The POSIX startup files are executed ('$ENV') rather than the
  42      normal Bash files.
  43
  44   10. Tilde expansion is only performed on assignments preceding a
  45      command name, rather than on all assignment statements on the line.
  46
  47   11. The default history file is '~/.sh_history' (this is the default
  48      value of '$HISTFILE').
  49
  50   12. Redirection operators do not perform filename expansion on the
  51      word in the redirection unless the shell is interactive.
  52
  53   13. Redirection operators do not perform word splitting on the word in
  54      the redirection.
  55
  56   14. Function names must be valid shell 'name's.  That is, they may not
  57      contain characters other than letters, digits, and underscores, and
  58      may not start with a digit.  Declaring a function with an invalid
  59      name causes a fatal syntax error in non-interactive shells.
  60
  61   15. Function names may not be the same as one of the POSIX special
  62      builtins.
  63
  64   16. POSIX special builtins are found before shell functions during
  65      command lookup.
  66
  67   17. When printing shell function definitions (e.g., by 'type'), Bash
  68      does not print the 'function' keyword.
  69
  70   18. Literal tildes that appear as the first character in elements of
  71      the 'PATH' variable are not expanded as described above under *note
  72      Tilde Expansion::.
  73
  74   19. The 'time' reserved word may be used by itself as a command.  When
  75      used in this way, it displays timing statistics for the shell and
  76      its completed children.  The 'TIMEFORMAT' variable controls the
  77      format of the timing information.
  78
  79   20. When parsing and expanding a ${...} expansion that appears within
  80      double quotes, single quotes are no longer special and cannot be
  81      used to quote a closing brace or other special character, unless
  82      the operator is one of those defined to perform pattern removal.
  83      In this case, they do not have to appear as matched pairs.
  84
  85   21. The parser does not recognize 'time' as a reserved word if the
  86      next token begins with a '-'.
  87
  88   22. The '!' character does not introduce history expansion within a
  89      double-quoted string, even if the 'histexpand' option is enabled.
  90
  91   23. If a POSIX special builtin returns an error status, a
  92      non-interactive shell exits.  The fatal errors are those listed in
  93      the POSIX standard, and include things like passing incorrect
  94      options, redirection errors, variable assignment errors for
  95      assignments preceding the command name, and so on.
  96
  97   24. A non-interactive shell exits with an error status if a variable
  98      assignment error occurs when no command name follows the assignment
  99      statements.  A variable assignment error occurs, for example, when
 100      trying to assign a value to a readonly variable.
 101
 102   25. A non-interactive shell exits with an error status if a variable
 103      assignment error occurs in an assignment statement preceding a
 104      special builtin, but not with any other simple command.
 105
 106   26. A non-interactive shell exits with an error status if the
 107      iteration variable in a 'for' statement or the selection variable
 108      in a 'select' statement is a readonly variable.
 109
 110   27. Non-interactive shells exit if FILENAME in '.'  FILENAME is not
 111      found.
 112
 113   28. Non-interactive shells exit if a syntax error in an arithmetic
 114      expansion results in an invalid expression.
 115
 116   29. Non-interactive shells exit if a parameter expansion error occurs.
 117
 118   30. Non-interactive shells exit if there is a syntax error in a script
 119      read with the '.' or 'source' builtins, or in a string processed by
 120      the 'eval' builtin.
 121
 122   31. While variable indirection is available, it may not be applied to
 123      the '#' and '?' special parameters.
 124
 125   32. When expanding the '*' special parameter in a pattern context
 126      where the expansion is double-quoted does not treat the '$*' as if
 127      it were double-quoted.
 128
 129   33. Assignment statements preceding POSIX special builtins persist in
 130      the shell environment after the builtin completes.
 131
 132   34. The 'command' builtin does not prevent builtins that take
 133      assignment statements as arguments from expanding them as
 134      assignment statements; when not in POSIX mode, assignment builtins
 135      lose their assignment statement expansion properties when preceded
 136      by 'command'.
 137
 138   35. The 'bg' builtin uses the required format to describe each job
 139      placed in the background, which does not include an indication of
 140      whether the job is the current or previous job.
 141
 142   36. The output of 'kill -l' prints all the signal names on a single
 143      line, separated by spaces, without the 'SIG' prefix.
 144
 145   37. The 'kill' builtin does not accept signal names with a 'SIG'
 146      prefix.
 147
 148   38. The 'export' and 'readonly' builtin commands display their output
 149      in the format required by POSIX.
 150
 151   39. The 'trap' builtin displays signal names without the leading
 152      'SIG'.
 153
 154   40. The 'trap' builtin doesn't check the first argument for a possible
 155      signal specification and revert the signal handling to the original
 156      disposition if it is, unless that argument consists solely of
 157      digits and is a valid signal number.  If users want to reset the
 158      handler for a given signal to the original disposition, they should
 159      use '-' as the first argument.
 160
 161   41. 'trap -p' displays signals whose dispositions are set to SIG_DFL
 162      and those that were ignored when the shell started.
 163
 164   42. The '.' and 'source' builtins do not search the current directory
 165      for the filename argument if it is not found by searching 'PATH'.
 166
 167   43. Enabling POSIX mode has the effect of setting the
 168      'inherit_errexit' option, so subshells spawned to execute command
 169      substitutions inherit the value of the '-e' option from the parent
 170      shell.  When the 'inherit_errexit' option is not enabled, Bash
 171      clears the '-e' option in such subshells.
 172
 173   44. Enabling POSIX mode has the effect of setting the 'shift_verbose'
 174      option, so numeric arguments to 'shift' that exceed the number of
 175      positional parameters will result in an error message.
 176
 177   45. When the 'alias' builtin displays alias definitions, it does not
 178      display them with a leading 'alias ' unless the '-p' option is
 179      supplied.
 180
 181   46. When the 'set' builtin is invoked without options, it does not
 182      display shell function names and definitions.
 183
 184   47. When the 'set' builtin is invoked without options, it displays
 185      variable values without quotes, unless they contain shell
 186      metacharacters, even if the result contains nonprinting characters.
 187
 188   48. When the 'cd' builtin is invoked in LOGICAL mode, and the pathname
 189      constructed from '$PWD' and the directory name supplied as an
 190      argument does not refer to an existing directory, 'cd' will fail
 191      instead of falling back to PHYSICAL mode.
 192
 193   49. When the 'cd' builtin cannot change a directory because the length
 194      of the pathname constructed from '$PWD' and the directory name
 195      supplied as an argument exceeds PATH_MAX when all symbolic links
 196      are expanded, 'cd' will fail instead of attempting to use only the
 197      supplied directory name.
 198
 199   50. The 'pwd' builtin verifies that the value it prints is the same as
 200      the current directory, even if it is not asked to check the file
 201      system with the '-P' option.
 202
 203   51. When listing the history, the 'fc' builtin does not include an
 204      indication of whether or not a history entry has been modified.
 205
 206   52. The default editor used by 'fc' is 'ed'.
 207
 208   53. The 'type' and 'command' builtins will not report a non-executable
 209      file as having been found, though the shell will attempt to execute
 210      such a file if it is the only so-named file found in '$PATH'.
 211
 212   54. The 'vi' editing mode will invoke the 'vi' editor directly when
 213      the 'v' command is run, instead of checking '$VISUAL' and
 214      '$EDITOR'.
 215
 216   55. When the 'xpg_echo' option is enabled, Bash does not attempt to
 217      interpret any arguments to 'echo' as options.  Each argument is
 218      displayed, after escape characters are converted.
 219
 220   56. The 'ulimit' builtin uses a block size of 512 bytes for the '-c'
 221      and '-f' options.
 222
 223   57. The arrival of 'SIGCHLD' when a trap is set on 'SIGCHLD' does not
 224      interrupt the 'wait' builtin and cause it to return immediately.
 225      The trap command is run once for each child that exits.
 226
 227   58. The 'read' builtin may be interrupted by a signal for which a trap
 228      has been set.  If Bash receives a trapped signal while executing
 229      'read', the trap handler executes and 'read' returns an exit status
 230      greater than 128.
 231
 232   59. Bash removes an exited background process's status from the list
 233      of such statuses after the 'wait' builtin is used to obtain it.
 234
 235 There is other POSIX behavior that Bash does not implement by default
 236 even when in POSIX mode.  Specifically:
 237
 238   1. The 'fc' builtin checks '$EDITOR' as a program to edit history
 239      entries if 'FCEDIT' is unset, rather than defaulting directly to
 240      'ed'.  'fc' uses 'ed' if 'EDITOR' is unset.
 241
 242   2. As noted above, Bash requires the 'xpg_echo' option to be enabled
 243      for the 'echo' builtin to be fully conformant.
 244
 245 Bash can be configured to be POSIX-conformant by default, by specifying
 246 the '--enable-strict-posix-default' to 'configure' when building (*note
 247 Optional Features::).
 248