Command-Line Options

bash has several command-line options that change the behavior of and pass information to the shell. The options fall into two sets: single character options, like we've seen in previous chapters of this book, and multicharacter options, which are a relatively recent improvement to UNIX utilities.[5]Table 10-1 lists all of the options.[6]

Table 10-1. bash command-line options

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Option

Meaning

-c string

Commands are read from string, if present. Any arguments after string are interpreted as positional parameters, starting with $0.

-D

A list of all double-quoted strings preceded by $ is printed on the standard ouput. These are the strings that are subject to language translation when the current locale is not C or POSIX. This also turns on the -n option.

-i

Interactive shell. Ignores signals TERM, INT, and QUIT. With job control in effect, TTIN, TTOU, and TSTP are also ignored.

-l

Makes bash act as if invoked as a login shell.

-o option

Takes the same arguments as set -o.

-O, +O shopt-option

shopt-option is one of the shell options accepted by the shopt builtin. If shopt-option is present, -O sets the value of that option; +O unsets it. If shopt-option is not supplied, the names and values of the shell options accepted by shopt are printed on the standard output. If the invocation option is +O, the output is displayed in a format that may be reused as input.

-s

Reads commands from the standard input. If an argument is given to bash, this flag takes precedence (i.e., the argument won't be treated as a script name and standard input will be read).

-r

Restricted shell. See the Section 10.3.1 later in this chapter.

-v

Prints shell input lines as they're read.

-

Signals the end of options and disables further option processing. Any options after this are treated as filenames and arguments. — is synonymous with -.

—debugger

Arranges for the debugger profile to be executed before the shell starts. Turns on extended debugging mode and shell function tracing.[7]

—dump-strings

Does the same as -D.

—dump-po-strings

Does the same as -D but the output is in the GNU gettext po (portable object) file format.

—help

Displays a usage message and exits.

—login

Makes bash act as if invoked as a login shell. Same as -l.

—noediting

Does not use the GNU readline library to read command lines if interactive.

—noprofile

Does not read the startup file /etc/profile or any of the personal initialization files.

—norc

Does not read the initialization file ~/.bashrc if the shell is interactive. This is on by default if the shell is invoked as sh.

—posix

Changes the behavior of bash to follow the POSIX guidelines more closely where the default operation of bash is different.

—quiet

Shows no information on shell startup. This is the default.

—rcfile file, —init-file file

Executes commands read from file instead of from the initialization file ~/.bashrc if the shell is interactive.

—verbose

Equivalent to -v.

—version

Shows the version number of this instance of bash and then exits.

[7] Only available in bash version 3.0 and later.

The multicharacter options have to appear on the command line before the single-character options. In addition to these, any set option can be used on the command line. Like shell built-ins, using a + instead of - turns an option off.

Of these options, the most useful are -i (interactive), -r (restricted), -s (read from standard input), -p (privileged), and -m (enable job control). Login shells are usually run with the -i, -s, and -m flags. We'll look at restricted and privileged modes later in this chapter.

 

[1] A good source of information on system administration is Essential System Administration by Æleen Frisch (O'Reilly ).

[2] The exact message varies from system to system; make sure that yours prints this message when given the name of a shell script. If not, just substitute the message your file command prints for "shell script" in the following code.

[3] The inclusion of /dev/null in the grep command is a kludge that forces grep to print the names of files that contain a match, even if there is only one such file in a given directory.

[4] bash also enters POSIX mode when started as sh. Versions of bash prior to 2.0 don't—POSIX mode has to be explicitly set with the —posix command-line option.

[5] Multicharacter options are far more readable and easier to remember than the old, and usually cryptic, single character options. All of the GNU utilities have multicharacter options, but many applications and utilities (certainly those on old UNIX systems) allow only single-character options.

[6] See Appendix B for a list of options for versions of bash prior to 2.0.