9.12. Bash Options
9.12.1. Shell Invocation Options
When the shell is started using the bash command, it can take options to modify its behavior. There are two types of options: single-character options and multicharacter options. The single-character options consist of a single leading dash followed by a single character. The multicharacter options consist of two leading dashes and any number of characters. Multicharacter options must appear before single-character options. An interactive login shell normally starts up with -i (start an interactive shell), -s (read from standard input), and -m (enable job control). See Table 9.8 on page 475.
| -c string | Commands are read from string. Any arguments after string are assigned to positional parameters, starting at $0. |
| -D | A list of double quoted strings, preceded by a $, are printed to standard output. These strings are subject to language translation when the current locale is not C or POSIX. The -n option is implied; no commands will be executed. |
| -i | Shell is in the interactive mode. TERM, QUIT, and INTERRUPT are ignored. |
| -s | Commands are read from standard input and allows the setting of positional parameters. |
| -r | Starts a restricted shell. |
| - | Signals the end of options and disables further option processing. Any arguments after -- or - are treated as filenames and arguments. |
| -login | Causes bash to be invoked as a login shell. |
| -nobraceexpansion | Curly brace expansion is turned off. |
| -nolineediting | When bash is running interactively, does not use the Readline library. |
| -noprofile | When starting up, bash does not read the initialization files: /etc/profile, ~/.bash_profile, ~/.bash_login, or ~/.profile. |
| -posix | Changes the behavior of bash to match the POSIX standard, if otherwise, it wouldn't. |
| -quiet | Displays no information at shell startup, the default. |
| -rcfile file | If bash is interactive, uses this intialization file instead of ~/.bashrc. |
| -verbose | Turns on verbose; same as -v. |
| -version | Displays version information about this bash shell and exit. |
9.12.2. The set Command and Options
The set command can be used to turn shell options on and off, as well as for handling command line arguments. To turn an option on, the dash (-) is prepended to the option; to turn an option off, the plus sign (+) is prepended to the option. See Table 9.10 on page 477 for a list of set options.
Example 9.78.
1 $ set -f 2 $ echo ** 3 $ echo ?? ?? 4 $ set +f |
Explanation
The f option is turned on, disabling filename expansion.
The asterisk is not expanded.
The question marks are not expanded.
The f is turned off; filename expansion is enabled.
| Name of Option | Shortcut Switch | What It Does |
|---|---|---|
| allexport | -a | Automatically marks new or modified variables for export from the time the option is set, until unset. |
| [1]braceexpand | -B | Enables brace expansion, and is a default setting. |
| emacs | For command line editing, uses the emacs built-in editor, and is a default setting | |
| errexit | -e | If a command returns a nonzero exit status (fails), exits. Not set when reading initialization files. |
| [a]histexpand | -H | Enables ! and !! when performing history substitution, and is a default setting. |
| [a]history | Enables command line history; on by default. | |
| ignoreeof | Disables EOF (Control-D) from exiting a shell; must type exit. Same as setting shell variable, IGNOREEOF=10. | |
| [a]keyword | -k | Places keyword arguments in the environment for a command. |
| interactive-comments | For interactive shells, a leading # is used to comment out any text remaining on the line. | |
| monitor | -m | Allows job control. |
| noclobber | -C | Protects files from being overwritten when redirection is used. |
| noexec | -n | Reads commands, but does not execute them. Used to check the syntax of scripts. Not on when running interactively. |
| noglob | -d | Disables pathname expansion; i.e., turns off wildcards. |
| notify | -b | Notifies user when background job finishes |
| nounset | -u | Displays an error when expanding a variable that has not been set. |
| [a]onecmd | -t | Exits after reading and executing one command. |
| physical | -P | If set, does not follow symbolic links when typing cd or pwd. The physical directory is used instead. |
| posix | Shell behavior is changed if the default operation doesn't match the POSIX standard. | |
| privileged | -p | When set, the shell does not read the .profile or ENV file and shell functions are not inherited from the environment; automatically set for setuid scripts. |
| posix | Changes the default behavior to POSIX 1003.2 | |
| verbose | -v | Turns on the verbose mode for debugging. |
| vi | For command line editing, uses the vi built-in editor. | |
| xtrace | -x | Turns on the echo mode for debugging. |
[1] Asterisk indicates option applies only to versions of bash 2.x.
9.12.3. The shopt Command and Options
The shopt (bash 2.x) command can also be used to turn shell options on and off.
| Option | Meaning |
|---|---|
| cdable_vars | If an argument to the cd built-in command is not a directory, it is assumed to be the name of a variable whose value is the directory to change to. |
| cdspell | Corrects minor errors in the spelling of a directory name in a cd command. The errors checked for are transposed characters, a missing character, and a character too many. If a correction is found, the corrected path is printed, and the command proceeds. Only used by interactive shells. |
| checkhash | Bash checks that a command found in the hash table exists before trying to execute it. If a hashed command no longer exists, a normal path search is performed. |
| checkwinsize | Bash checks the window size after each command and, if necessary, updates the values of LINES and COLUMNS. |
| cmdhist | Bash attempts to save all lines of a multiple-line command in the same history entry. This allows easy re-editing of multiline commands. |
| dotglob | Bash includes filenames beginning with a "." in the results of filename expansion. |
| execfail | A noninteractive shell will not exit if it cannot execute the file specified as an argument to the exec built-in command. An interactive shell does not exit if exec fails. |
| expand_aliases | Aliases are expanded. Enabled by default. |
| extglob | The extended pattern matching features (regular expression metacharacters derived from Korn shell for filename expansion) are enabled. |
| histappend | The history list is appended to the file named by the value of the HISTFILE variable when the shell exits, rather than overwriting the file. |
| histreedit | If Readline is being used, a user is given the opportunity to re-edit a failed history substitution. |
| histverify | If set, and Readline is being used, the results of history substitution are not immediately passed to the shell parser. Instead, the resulting line is loaded into the Readline editing buffer, allowing further modification. |
| hostcomplete | If set, and Readline is being used, bash will attempt to perform hostname completion when a word containing an "@" is being completed Enabled by default. |
| huponexit | If set, bash will send SIGHUP (hangup signal) to all jobs when an interactive login shell exits. |
| interactive_comments | Allows a word beginning with "#" to cause that word and all remaining characters on that line to be ignored in an interactive shell. Enabled by default. |
| lithist | If enabled, and the cmdhist option is enabled, multiline commands are saved to the history with embedded newlines rather than using semicolon separatorswhere possible. |
| mailwarn | If set, and a file that bash is checking for mail has been accessed since the last time it was checked, the message The mail in mailfile has been read is displayed. |
| nocaseglob | If set, bash matches filenames in a case-insensitive fashion when performing filename expansion. |
| nullglob | If set, bash allows filename patterns which match no files to expand to a null string, rather than themselves. |
| promptvars | If set, prompt strings undergo variable and parameter expansion after being expanded. Enabled by default. |
| restricted_shell | The shell sets this option if it is started in restricted mode. The value may not be changed. This is not reset when the startup files are executed, allowing the startup files to discover whether or not a shell is restricted. |
| shift_verbose | If this is set, the shift built-in prints an error message when the shift count exceeds the number of positional parameters. |
| sourcepath | If set, the source built-in uses the value of PATH to find the directory containing the file supplied as an argument. Enabled by default. |
| source | A synonym for . (a dot) |
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