182 lines
7.6 KiB
Text
182 lines
7.6 KiB
Text
6.11 Bash POSIX Mode
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====================
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Starting Bash with the `--posix' command-line option or executing `set
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-o posix' while Bash is running will cause Bash to conform more closely
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to the POSIX standard by changing the behavior to match that specified
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by POSIX in areas where the Bash default differs.
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When invoked as `sh', Bash enters POSIX mode after reading the startup
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files.
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The following list is what's changed when `POSIX mode' is in effect:
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1. When a command in the hash table no longer exists, Bash will
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re-search `$PATH' to find the new location. This is also
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available with `shopt -s checkhash'.
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2. The message printed by the job control code and builtins when a job
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exits with a non-zero status is `Done(status)'.
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3. The message printed by the job control code and builtins when a job
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is stopped is `Stopped(SIGNAME)', where SIGNAME is, for example,
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`SIGTSTP'.
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4. The `bg' builtin uses the required format to describe each job
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placed in the background, which does not include an indication of
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whether the job is the current or previous job.
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5. Reserved words appearing in a context where reserved words are
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recognized do not undergo alias expansion.
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6. The POSIX `PS1' and `PS2' expansions of `!' to the history number
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and `!!' to `!' are enabled, and parameter expansion is performed
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on the values of `PS1' and `PS2' regardless of the setting of the
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`promptvars' option.
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7. The POSIX startup files are executed (`$ENV') rather than the
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normal Bash files.
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8. Tilde expansion is only performed on assignments preceding a
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command name, rather than on all assignment statements on the line.
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9. The default history file is `~/.sh_history' (this is the default
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value of `$HISTFILE').
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10. The output of `kill -l' prints all the signal names on a single
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line, separated by spaces, without the `SIG' prefix.
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11. The `kill' builtin does not accept signal names with a `SIG'
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prefix.
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12. Non-interactive shells exit if FILENAME in `.' FILENAME is not
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found.
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13. Non-interactive shells exit if a syntax error in an arithmetic
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expansion results in an invalid expression.
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14. Redirection operators do not perform filename expansion on the word
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in the redirection unless the shell is interactive.
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15. Redirection operators do not perform word splitting on the word in
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the redirection.
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16. Function names must be valid shell `name's. That is, they may not
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contain characters other than letters, digits, and underscores, and
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may not start with a digit. Declaring a function with an invalid
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name causes a fatal syntax error in non-interactive shells.
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17. POSIX special builtins are found before shell functions during
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command lookup.
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18. If a POSIX special builtin returns an error status, a
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non-interactive shell exits. The fatal errors are those listed in
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the POSIX standard, and include things like passing incorrect
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options, redirection errors, variable assignment errors for
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assignments preceding the command name, and so on.
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19. If `CDPATH' is set, the `cd' builtin will not implicitly append
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the current directory to it. This means that `cd' will fail if no
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valid directory name can be constructed from any of the entries in
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`$CDPATH', even if the a directory with the same name as the name
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given as an argument to `cd' exists in the current directory.
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20. A non-interactive shell exits with an error status if a variable
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assignment error occurs when no command name follows the assignment
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statements. A variable assignment error occurs, for example, when
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trying to assign a value to a readonly variable.
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21. A non-interactive shell exits with an error status if the iteration
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variable in a `for' statement or the selection variable in a
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`select' statement is a readonly variable.
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22. Process substitution is not available.
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23. Assignment statements preceding POSIX special builtins persist in
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the shell environment after the builtin completes.
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24. Assignment statements preceding shell function calls persist in the
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shell environment after the function returns, as if a POSIX
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special builtin command had been executed.
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25. The `export' and `readonly' builtin commands display their output
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in the format required by POSIX.
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26. The `trap' builtin displays signal names without the leading `SIG'.
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27. The `trap' builtin doesn't check the first argument for a possible
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signal specification and revert the signal handling to the original
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disposition if it is, unless that argument consists solely of
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digits and is a valid signal number. If users want to reset the
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handler for a given signal to the original disposition, they
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should use `-' as the first argument.
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28. The `.' and `source' builtins do not search the current directory
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for the filename argument if it is not found by searching `PATH'.
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29. Subshells spawned to execute command substitutions inherit the
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value of the `-e' option from the parent shell. When not in POSIX
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mode, Bash clears the `-e' option in such subshells.
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30. Alias expansion is always enabled, even in non-interactive shells.
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31. When the `alias' builtin displays alias definitions, it does not
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display them with a leading `alias ' unless the `-p' option is
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supplied.
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32. When the `set' builtin is invoked without options, it does not
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display shell function names and definitions.
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33. When the `set' builtin is invoked without options, it displays
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variable values without quotes, unless they contain shell
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metacharacters, even if the result contains nonprinting characters.
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34. When the `cd' builtin is invoked in LOGICAL mode, and the pathname
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constructed from `$PWD' and the directory name supplied as an
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argument does not refer to an existing directory, `cd' will fail
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instead of falling back to PHYSICAL mode.
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35. When the `pwd' builtin is supplied the `-P' option, it resets
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`$PWD' to a pathname containing no symlinks.
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36. The `pwd' builtin verifies that the value it prints is the same as
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the current directory, even if it is not asked to check the file
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system with the `-P' option.
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37. When listing the history, the `fc' builtin does not include an
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indication of whether or not a history entry has been modified.
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38. The default editor used by `fc' is `ed'.
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39. The `type' and `command' builtins will not report a non-executable
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file as having been found, though the shell will attempt to
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execute such a file if it is the only so-named file found in
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`$PATH'.
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40. The `vi' editing mode will invoke the `vi' editor directly when
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the `v' command is run, instead of checking `$VISUAL' and
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`$EDITOR'.
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41. When the `xpg_echo' option is enabled, Bash does not attempt to
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interpret any arguments to `echo' as options. Each argument is
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displayed, after escape characters are converted.
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42. The `ulimit' builtin uses a block size of 512 bytes for the `-c'
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and `-f' options.
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There is other POSIX behavior that Bash does not implement by default
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even when in POSIX mode. Specifically:
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1. The `fc' builtin checks `$EDITOR' as a program to edit history
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entries if `FCEDIT' is unset, rather than defaulting directly to
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`ed'. `fc' uses `ed' if `EDITOR' is unset.
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2. As noted above, Bash requires the `xpg_echo' option to be enabled
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for the `echo' builtin to be fully conformant.
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Bash can be configured to be POSIX-conformant by default, by specifying
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the `--enable-strict-posix-default' to `configure' when building (*note
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Optional Features::).
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