6.11.1 What is POSIX? ¶

POSIX is the name for a family of standards based on Unix. A number of Unix services, tools, and functions are part of the standard, ranging from the basic system calls and C library functions to common applications and tools to system administration and management.

The POSIX Shell and Utilities standard was originally developed by IEEE Working Group 1003.2 (POSIX.2). The first edition of the 1003.2 standard was published in 1992. It was merged with the original IEEE 1003.1 Working Group and is currently maintained by the Austin Group (a joint working group of the IEEE, The Open Group and ISO/IEC SC22/WG15). Today the Shell and Utilities are a volume within the set of documents that make up IEEE Std 1003.1-2024, and thus the former POSIX.2 (from 1992) is now part of the current unified POSIX standard.

The Shell and Utilities volume concentrates on the command interpreter interface and utility programs commonly executed from the command line or by other programs. The standard is freely available on the web at https://pubs.opengroup.org/onlinepubs/9799919799/utilities/contents.html.

Bash is concerned with the aspects of the shell’s behavior defined by the POSIX Shell and Utilities volume. The shell command language has of course been standardized, including the basic flow control and program execution constructs, I/O redirection and pipelines, argument handling, variable expansion, and quoting.

The special builtins, which must be implemented as part of the shell to provide the desired functionality, are specified as being part of the shell; examples of these are eval and export. Other utilities appear in the sections of POSIX not devoted to the shell which are commonly (and in some cases must be) implemented as builtin commands, such as read and test. POSIX also specifies aspects of the shell’s interactive behavior, including job control and command line editing. Only vi-style line editing commands have been standardized; emacs editing commands were left out due to objections.

6.11.2 Bash POSIX Mode ¶

Although Bash is an implementation of the POSIX shell specification, there are areas where the Bash default behavior differs from the specification. The Bash posix mode changes the Bash behavior in these areas so that it conforms more strictly to the standard.

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. Bash ensures that the POSIXLY_CORRECT variable is set.
2. Bash reads and executes the POSIX startup files ($ENV) rather than the normal Bash files (see Bash Startup Files.
3. Alias expansion is always enabled, even in non-interactive shells.
4. Reserved words appearing in a context where reserved words are recognized do not undergo alias expansion.
5. Alias expansion is performed when initially parsing a command substitution. The default (non-posix) mode generally defers it, when enabled, until the command substitution is executed. This means that command substitution will not expand aliases that are defined after the command substitution is initially parsed (e.g., as part of a function definition).
6. The time reserved word may be used by itself as a simple 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.
7. The parser does not recognize time as a reserved word if the next token begins with a ‘-’.
8. 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.
9. Redirection operators do not perform filename expansion on the word in a redirection unless the shell is interactive.
10. Redirection operators do not perform word splitting on the word in a redirection.
11. Function names may not be the same as one of the POSIX special builtins.
12. Tilde expansion is only performed on assignments preceding a command name, rather than on all assignment statements on the line.
13. While variable indirection is available, it may not be applied to the ‘#’ and ‘?’ special parameters.
14. Expanding the ‘*’ special parameter in a pattern context where the expansion is double-quoted does not treat the $* as if it were double-quoted.
15. A double quote character (‘"’) is treated specially when it appears in a backquoted command substitution in the body of a here-document that undergoes expansion. That means, for example, that a backslash preceding a double quote character will escape it and the backslash will be removed.
16. Command substitutions don’t set the ‘?’ special parameter. The exit status of a simple command without a command word is still the exit status of the last command substitution that occurred while evaluating the variable assignments and redirections in that command, but that does not happen until after all of the assignments and redirections.
17. Literal tildes that appear as the first character in elements of the PATH variable are not expanded as described above under Tilde Expansion.
18. Command lookup finds POSIX special builtins before shell functions, including output printed by the type and command builtins.
19. Even if a shell function whose name contains a slash was defined before entering POSIX mode, the shell will not execute a function whose name contains one or more slashes.
20. 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’.
21. Bash will not insert a command without the execute bit set into the command hash table, even if it returns it as a (last-ditch) result from a $PATH search.
22. The message printed by the job control code and builtins when a job exits with a non-zero status is ‘Done(status)’.
23. The message printed by the job control code and builtins when a job is stopped is ‘Stopped(signame)’, where signame is, for example, SIGTSTP.
24. If the shell is interactive, Bash does not perform job notifications between executing commands in lists separated by ‘;’ or newline. Non-interactive shells print status messages after a foreground job in a list completes.
25. If the shell is interactive, Bash waits until the next prompt before printing the status of a background job that changes status or a foreground job that terminates due to a signal. Non-interactive shells print status messages after a foreground job completes.
26. Bash permanently removes jobs from the jobs table after notifying the user of their termination via the wait or jobs builtins. It removes the job from the jobs list after notifying the user of its termination, but the status is still available via wait, as long as wait is supplied a PID argument.
27. The vi editing mode will invoke the vi editor directly when the ‘v’ command is run, instead of checking $VISUAL and $EDITOR.
28. Prompt expansion enables the POSIX PS1 and PS2 expansions of ‘!’ to the history number and ‘!!’ to ‘!’, and Bash performs parameter expansion on the values of PS1 and PS2 regardless of the setting of the promptvars option.
29. The default history file is ~/.sh_history (this is the default value the shell assigns to $HISTFILE).
30. The ‘!’ character does not introduce history expansion within a double-quoted string, even if the histexpand option is enabled.
31. When printing shell function definitions (e.g., by type), Bash does not print the function reserved word unless necessary.
32. Non-interactive shells exit if a syntax error in an arithmetic expansion results in an invalid expression.
33. Non-interactive shells exit if a parameter expansion error occurs.
34. 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.
35. 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.
36. 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. For any other simple command, the shell aborts execution of that command, and execution continues at the top level ("the shell shall not perform any further processing of the command in which the error occurred").
37. 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 or has an invalid name.
38. Non-interactive shells exit if filename in . filename is not found.
39. 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.
40. Non-interactive shells exit if the export, readonly or unset builtin commands get an argument that is not a valid identifier, and they are not operating on shell functions. These errors force an exit because these are special builtins.
41. Assignment statements preceding POSIX special builtins persist in the shell environment after the builtin completes.
42. The command builtin does not prevent builtins that take assignment statements as arguments from expanding them as assignment statements; when not in POSIX mode, declaration commands lose their assignment statement expansion properties when preceded by command.
43. Enabling POSIX mode has the effect of setting the inherit_errexit option, so subshells spawned to execute command substitutions inherit the value of the -e option from the parent shell. When the inherit_errexit option is not enabled, Bash clears the -e option in such subshells.
44. Enabling POSIX mode has the effect of setting the shift_verbose option, so numeric arguments to shift that exceed the number of positional parameters will result in an error message.
45. Enabling POSIX mode has the effect of setting the interactive_comments option (see Comments).
46. The . and source builtins do not search the current directory for the filename argument if it is not found by searching PATH.
47. When the alias builtin displays alias definitions, it does not display them with a leading ‘alias ’ unless the -p option is supplied.
48. 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.
49. 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.
50. When the cd builtin cannot change a directory because the length of the pathname constructed from $PWD and the directory name supplied as an argument exceeds PATH_MAX when canonicalized, cd will attempt to use the supplied directory name.
51. When the xpg_echo option is enabled, Bash does not attempt to interpret any arguments to echo as options. echo displays each argument after converting escape sequences.
52. The export and readonly builtin commands display their output in the format required by POSIX.
53. When listing the history, the fc builtin does not include an indication of whether or not a history entry has been modified.
54. The default editor used by fc is ed.
55. fc treats extra arguments as an error instead of ignoring them.
56. If there are too many arguments supplied to fc -s, fc prints an error message and returns failure.
57. The output of ‘kill -l’ prints all the signal names on a single line, separated by spaces, without the ‘SIG’ prefix.
58. The kill builtin does not accept signal names with a ‘SIG’ prefix.
59. The kill builtin returns a failure status if any of the pid or job arguments are invalid or if sending the specified signal to any of them fails. In default mode, kill returns success if the signal was successfully sent to any of the specified processes.
60. The printf builtin uses double (via strtod) to convert arguments corresponding to floating point conversion specifiers, instead of long double if it’s available. The ‘L’ length modifier forces printf to use long double if it’s available.
61. 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.
62. 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.
63. When the set builtin is invoked without options, it does not display shell function names and definitions.
64. 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.
65. The test builtin compares strings using the current locale when evaluating the ‘<’ and ‘>’ binary operators.
66. The test builtin’s -t unary primary requires an argument. Historical versions of test made the argument optional in certain cases, and Bash attempts to accommodate those for backwards compatibility.
67. The trap builtin displays signal names without the leading SIG.
68. 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.
69. trap -p without arguments displays signals whose dispositions are set to SIG_DFL and those that were ignored when the shell started, not just trapped signals.
70. 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.
71. The ulimit builtin uses a block size of 512 bytes for the -c and -f options.
72. The unset builtin with the -v option specified returns a fatal error if it attempts to unset a readonly or non-unsettable variable, which causes a non-interactive shell to exit.
73. When asked to unset a variable that appears in an assignment statement preceding the command, the unset builtin attempts to unset a variable of the same name in the current or previous scope as well. This implements the required "if an assigned variable is further modified by the utility, the modifications made by the utility shall persist" behavior.
74. 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.
75. Bash removes an exited background process’s status from the list of such statuses after the wait builtin returns it.

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

1. POSIX requires that word splitting be byte-oriented. That is, each byte in the value of IFS potentially splits a word, even if that byte is part of a multibyte character in IFS or part of multibyte character in the word. Bash allows multibyte characters in the value of IFS, treating a valid multibyte character as a single delimiter, and will not split a valid multibyte character even if one of the bytes composing that character appears in IFS. This is POSIX interpretation 1560, further modified by issue 1924.
2. 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.
3. 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 Optional Features).