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