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