6.11 Bash and POSIX =================== 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 1003.1-2017, 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 . 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 to the standard more closely. 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. When a command in the hash table no longer exists, Bash will re-search '$PATH' to find the new location. This is also available with 'shopt -s checkhash'. 3. Bash will not insert a command without the execute bit set into the command hash table, even if it returns it as a (last-ditch) result from a '$PATH' search. 4. The message printed by the job control code and builtins when a job exits with a non-zero status is 'Done(status)'. 5. The message printed by the job control code and builtins when a job is stopped is 'Stopped(SIGNAME)', where SIGNAME is, for example, 'SIGTSTP'. 6. Alias expansion is always enabled, even in non-interactive shells. 7. Reserved words appearing in a context where reserved words are recognized do not undergo alias expansion. 8. Alias expansion is performed when initially parsing a command substitution. The default 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). 9. 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. 10. The POSIX startup files are executed ('$ENV') rather than the normal Bash files. 11. Tilde expansion is only performed on assignments preceding a command name, rather than on all assignment statements on the line. 12. The default history file is '~/.sh_history' (this is the default value of '$HISTFILE'). 13. Redirection operators do not perform filename expansion on the word in the redirection unless the shell is interactive. 14. Redirection operators do not perform word splitting on the word in the redirection. 15. Function names must be valid shell 'name's. 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. 16. Function names may not be the same as one of the POSIX special builtins. 17. 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. 18. POSIX special builtins are found before shell functions during command lookup. 19. When printing shell function definitions (e.g., by 'type'), Bash does not print the 'function' keyword. 20. Literal tildes that appear as the first character in elements of the 'PATH' variable are not expanded as described above under *note Tilde Expansion::. 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. The '!' character does not introduce history expansion within a double-quoted string, even if the 'histexpand' option is enabled. 25. 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. 26. The 'unset' builtin with the '-v' option specified returns a fatal error if it attempts to unset a 'readonly' or 'non-unsettable' variable, or encounters a variable name argument that is an invalid identifier, which causes a non-interactive shell to exit. 27. 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. 28. 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"). 29. 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. 30. Non-interactive shells exit if FILENAME in '.' FILENAME is not found. 31. Non-interactive shells exit if a syntax error in an arithmetic expansion results in an invalid expression. 32. Non-interactive shells exit if a parameter expansion error occurs. 33. 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. 34. While variable indirection is available, it may not be applied to the '#' and '?' special parameters. 35. Expanding the '*' special parameter in a pattern context where the expansion is double-quoted does not treat the '$*' as if it were double-quoted. 36. Assignment statements preceding POSIX special builtins persist in the shell environment after the builtin completes. 37. 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'. 38. 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. 39. The output of 'kill -l' prints all the signal names on a single line, separated by spaces, without the 'SIG' prefix. 40. The 'kill' builtin does not accept signal names with a 'SIG' prefix. 41. The 'export' and 'readonly' builtin commands display their output in the format required by POSIX. 42. The 'trap' builtin displays signal names without the leading 'SIG'. 43. 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. 44. '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. 45. The '.' and 'source' builtins do not search the current directory for the filename argument if it is not found by searching 'PATH'. 46. 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. 47. 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. 48. When the 'alias' builtin displays alias definitions, it does not display them with a leading 'alias ' unless the '-p' option is supplied. 49. When the 'set' builtin is invoked without options, it does not display shell function names and definitions. 50. 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. 51. When the 'cd' builtin is invoked in logical mode, and the pathname constructed from '$PWD' and the directory name supplied as an argument does not refer to an existing directory, 'cd' will fail instead of falling back to physical mode. 52. When the 'cd' builtin cannot change a directory because the length of the pathname constructed from '$PWD' and the directory name supplied as an argument exceeds 'PATH_MAX' when all symbolic links are expanded, 'cd' will fail instead of attempting to use only the supplied directory name. 53. 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. 54. When listing the history, the 'fc' builtin does not include an indication of whether or not a history entry has been modified. 55. The default editor used by 'fc' is 'ed'. 56. If there are too many arguments supplied to 'fc -s', 'fc' prints an error message and returns failure. 57. 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 '$PATH'. 58. The 'vi' editing mode will invoke the 'vi' editor directly when the 'v' command is run, instead of checking '$VISUAL' and '$EDITOR'. 59. When the 'xpg_echo' option is enabled, Bash does not attempt to interpret any arguments to 'echo' as options. Each argument is displayed, after escape characters are converted. 60. The 'ulimit' builtin uses a block size of 512 bytes for the '-c' and '-f' options. 61. The arrival of 'SIGCHLD' when a trap is set on 'SIGCHLD' does not interrupt the 'wait' builtin and cause it to return immediately. The trap command is run once for each child that exits. 62. The 'read' builtin may be interrupted by a signal for which a trap has been set. If Bash receives a trapped signal while executing 'read', the trap handler executes and 'read' returns an exit status greater than 128. 63. The 'printf' builtin uses 'double' (via 'strtod') to convert arguments corresponding to floating point conversion specifiers, instead of 'long double' if it's available. The 'L' length modifier forces 'printf' to use 'long double' if it's available. 64. Bash removes an exited background process's status from the list of such statuses after the 'wait' builtin is used to obtain it. 65. A double quote character ('"') is treated specially when it appears in a backquoted command substitution in the body of a here-document that undergoes expansion. That means, for example, that a backslash preceding a double quote character will escape it and the backslash will be removed. 66. Command substitutions don't set the '?' special parameter. The exit status of a simple command without a command word is still the exit status of the last command substitution that occurred while evaluating the variable assignments and redirections in that command, but that does not happen until after all of the assignments and redirections. There is other POSIX behavior that Bash does not implement by default even when in POSIX mode. Specifically: 1. The 'fc' builtin checks '$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. A non-interactive shell does not exit if a variable assignment preceding the 'command' builtin or another non-special builtin fails. 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 (*note Optional Features::).