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Appendix G...

Shell Options & Shell Variables

Every shell supports a variety of ways for you to control its behavior. The Bourne Shell family (Bash, Korn Shell) uses shell options; the C-Shell family (Tcsh, C-Shell) uses both shell options and shell variables. This appendix summarizes these options and variables.

For a detailed discussion of both shell options and shell variables, see Chapter 12.

• Bourne Shell Family
• C-Shell Family

The Bourne Shell family uses a large number of options to control the behavior of the shell. In particular, interactive shells require different options than non-interactive shells. (In fact, this is what makes them interactive.)

There are two ways to specify such options. First, they may be invoked in the standard manner by specifying options on the command line when the shell is started. This is the case for non-interactive shells that are run automatically, and for interactive shells that you yourself start by entering a command. For example:

bash -vx

(By the way, these two options are particularly useful when you are testing or debugging a shell script.)

Within an interactive shell — that is, at the shell prompt — you can turn options on or off as you wish. To set (turn on) an option, use the set ‑o command. To unset (turn off) an option, use the set  +o command. When you use these commands, you specify the long name for the option, not its abbreviation. For example:

set -o verbose
set +o xtrace

To display the current state of all the options, use either command by itself:

set -o
set +o

The -o variation prints human-readable output; +o prints output suitable for a script. (Try them.)

In all, the Bourne Shell family uses a great many shell options. In most cases, you will not need to change them, because the defaults will work just fine. For reference, however, the table in Figure G-1 summarizes the shell options for Bash and the Korn Shell. Not all options are available with all shells, so be sure to look at the leftmost column, which shows you which shells support each option.

Figure G-1:  Bourne Shell family: Shell options

This table summarizes the important shell options available with Bourne family shells. The leftmost column shows which shells support each option: B = Bash; K = Korn Shell. A dot indicates that a shell does not support that option. Note that some options, such as bgnice, have a long name but not a short option name. For more information, see the man page for your particular shell.

Notes: (1) Although Bash supports the emacs and vi options, it does not use -E and -V. (2) The Korn shell uses -h and -t, but does not support the long names hashall and onecmd.

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The C-Shell family also uses options to control the behavior of the shell. These options are summarized in Figure G-2. If you compare the options in Figure G-2 with those in Figure G-1, you will notice that most of the C-Shell options are also used by the Bourne Shell family. However, there is an important difference.

In the Bourne Shell family the behavior of the shell is controlled entirely by options, of which there are three types: regular command-line options (such as -v), "long name" options used with the set  +o command, and the large number of extra options used only with the Bash shopt command.

Although the C-Shell family does use a small number of command-line options (Figure  G-2), for the most part, the behavior of a shell is controlled by shell variables. This is one of the important differences between C-Shell family and the Bourne Shell family, and is explained in detail in Chapter 12. For reference, Figure G-3 contains a summary of the shell variables used by the C-Shell and Tcsh.

Figure G-2:  C-Shell family: Shell options

This table summarizes the important shell options available with C-Shell family. The leftmost column shows which shells support each option: C = C-Shell; T = Tcsh. A dot indicates that a shell does not support that option. For more information, see the man page for your particular shell.

Note: The -v and -V options work by setting the verbose shell variable. The -x and -X options work by setting the echo shell variable.

Figure G-3:  C-Shell family: Shell variables

This table summarizes the important shell variables used with the C-Shell family to control the behavior of the shell. The leftmost column shows which shells support each option: C = C-Shell; T = Tcsh. A dot indicates that a shell does not support that option.

For more information, see the man page for your shell. For information about autocompletion, spell-correction, or command line editing, see the Tcsh man page. For a detailed discussion of shell variables, see Chapter 12.

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