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6.11 Bash POSIX Mode

Starting Bash with the ‘--posix’ command-line option or executing ‘set -o posix’ while Bash is running will cause Bash to conform more closely to the POSIX standard by changing the behavior to match that specified by POSIX in areas where the Bash default differs.

When invoked as sh, Bash enters POSIX mode after reading the startup files.

The following list is what’s changed when ‘POSIX mode’ is in effect:

  1. When a command in the hash table no longer exists, Bash will re-search $PATH to find the new location. This is also available with ‘shopt -s checkhash’.
  2. The message printed by the job control code and builtins when a job exits with a non-zero status is ‘Done(status)’.
  3. The message printed by the job control code and builtins when a job is stopped is ‘Stopped(signame)’, where signame is, for example, SIGTSTP.
  4. The bg builtin uses the required format to describe each job placed in the background, which does not include an indication of whether the job is the current or previous job.
  5. Reserved words appearing in a context where reserved words are recognized do not undergo alias expansion.
  6. The POSIX PS1 and PS2 expansions of ‘!’ to the history number and ‘!!’ to ‘!’ are enabled, and parameter expansion is performed on the values of PS1 and PS2 regardless of the setting of the promptvars option.
  7. The POSIX startup files are executed ($ENV) rather than the normal Bash files.
  8. Tilde expansion is only performed on assignments preceding a command name, rather than on all assignment statements on the line.
  9. The command builtin does not prevent builtins that take assignment statements as arguments from expanding them as assignment statements; when not in POSIX mode, assignment builtins lose their assignment statement expansion properties when preceded by command.
  10. The default history file is ‘~/.sh_history’ (this is the default value of $HISTFILE).
  11. The output of ‘kill -l’ prints all the signal names on a single line, separated by spaces, without the ‘SIG’ prefix.
  12. The kill builtin does not accept signal names with a ‘SIG’ prefix.
  13. Non-interactive shells exit if filename in . filename is not found.
  14. Non-interactive shells exit if a syntax error in an arithmetic expansion results in an invalid expression.
  15. Non-interactive shells exit if there is a syntax error in a script read with the . or source builtins, or in a string processed by the eval builtin.
  16. Redirection operators do not perform filename expansion on the word in the redirection unless the shell is interactive.
  17. Redirection operators do not perform word splitting on the word in the redirection.
  18. Function names must be valid shell names. That is, they may not contain characters other than letters, digits, and underscores, and may not start with a digit. Declaring a function with an invalid name causes a fatal syntax error in non-interactive shells.
  19. Function names may not be the same as one of the POSIX special builtins.
  20. POSIX special builtins are found before shell functions during command lookup.
  21. The time reserved word may be used by itself as a command. When used in this way, it displays timing statistics for the shell and its completed children. The TIMEFORMAT variable controls the format of the timing information.
  22. When parsing and expanding a ${…} expansion that appears within double quotes, single quotes are no longer special and cannot be used to quote a closing brace or other special character, unless the operator is one of those defined to perform pattern removal. In this case, they do not have to appear as matched pairs.
  23. The parser does not recognize time as a reserved word if the next token begins with a ‘-’.
  24. If a POSIX special builtin returns an error status, a non-interactive shell exits. The fatal errors are those listed in the POSIX standard, and include things like passing incorrect options, redirection errors, variable assignment errors for assignments preceding the command name, and so on.
  25. A non-interactive shell exits with an error status if a variable assignment error occurs when no command name follows the assignment statements. A variable assignment error occurs, for example, when trying to assign a value to a readonly variable.
  26. A non-interactive shell exits with an error status if a variable assignment error occurs in an assignment statement preceding a special builtin, but not with any other simple command.
  27. A non-interactive shell exits with an error status if the iteration variable in a for statement or the selection variable in a select statement is a readonly variable.
  28. Process substitution is not available.
  29. While variable indirection is available, it may not be applied to the ‘#’ and ‘?’ special parameters.
  30. Assignment statements preceding POSIX special builtins persist in the shell environment after the builtin completes.
  31. Assignment statements preceding shell function calls persist in the shell environment after the function returns, as if a POSIX special builtin command had been executed.
  32. The export and readonly builtin commands display their output in the format required by POSIX.
  33. The trap builtin displays signal names without the leading SIG.
  34. The trap builtin doesn’t check the first argument for a possible signal specification and revert the signal handling to the original disposition if it is, unless that argument consists solely of digits and is a valid signal number. If users want to reset the handler for a given signal to the original disposition, they should use ‘-’ as the first argument.
  35. The . and source builtins do not search the current directory for the filename argument if it is not found by searching PATH.
  36. Subshells spawned to execute command substitutions inherit the value of the ‘-e’ option from the parent shell. When not in POSIX mode, Bash clears the ‘-e’ option in such subshells.
  37. Alias expansion is always enabled, even in non-interactive shells.
  38. When the alias builtin displays alias definitions, it does not display them with a leading ‘alias ’ unless the ‘-p’ option is supplied.
  39. When the set builtin is invoked without options, it does not display shell function names and definitions.
  40. When the set builtin is invoked without options, it displays variable values without quotes, unless they contain shell metacharacters, even if the result contains nonprinting characters.
  41. When the cd builtin is invoked in logical mode, and the pathname constructed from $PWD and the directory name supplied as an argument does not refer to an existing directory, cd will fail instead of falling back to physical mode.
  42. The pwd builtin verifies that the value it prints is the same as the current directory, even if it is not asked to check the file system with the ‘-P’ option.
  43. When listing the history, the fc builtin does not include an indication of whether or not a history entry has been modified.
  44. The default editor used by fc is ed.
  45. The type and command builtins will not report a non-executable file as having been found, though the shell will attempt to execute such a file if it is the only so-named file found in $PATH.
  46. The vi editing mode will invoke the vi editor directly when the ‘v’ command is run, instead of checking $VISUAL and $EDITOR.
  47. When the xpg_echo option is enabled, Bash does not attempt to interpret any arguments to echo as options. Each argument is displayed, after escape characters are converted.
  48. The ulimit builtin uses a block size of 512 bytes for the ‘-c’ and ‘-f’ options.
  49. The arrival of SIGCHLD when a trap is set on SIGCHLD does not interrupt the wait builtin and cause it to return immediately. The trap command is run once for each child that exits.
  50. The read builtin may be interrupted by a signal for which a trap has been set. If Bash receives a trapped signal while executing read, the trap handler executes and read returns an exit status greater than 128.

There is other POSIX behavior that Bash does not implement by default even when in POSIX mode. Specifically:

  1. The fc builtin checks $EDITOR as a program to edit history entries if FCEDIT is unset, rather than defaulting directly to ed. fc uses ed if EDITOR is unset.
  2. As noted above, Bash requires the xpg_echo option to be enabled for the echo builtin to be fully conformant.

Bash can be configured to be POSIX-conformant by default, by specifying the ‘--enable-strict-posix-default’ to configure when building (see section Optional Features).

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