Chapter 31. Options

Options are settings that change shell and/or script behavior.

The set command enables options within a script. At the point in the script where you want the options to take effect, use set -o option-name or, in short form, set -option-abbrev. These two forms are equivalent.

   1       #!/bin/bash
   3       set -o verbose
   4       # Echoes all commands before executing.

   1       #!/bin/bash
   3       set -v
   4       # Exact same effect as above.


To disable an option within a script, use set +o option-name or set +option-abbrev.

   1       #!/bin/bash
   3       set -o verbose
   4       # Command echoing on.
   5       command
   6       ...
   7       command
   9       set +o verbose
  10       # Command echoing off.
  11       command
  12       # Not echoed.
  15       set -v
  16       # Command echoing on.
  17       command
  18       ...
  19       command
  21       set +v
  22       # Command echoing off.
  23       command
  25       exit 0

An alternate method of enabling options in a script is to specify them immediately following the #! script header.

   1       #!/bin/bash -x
   2       #
   3       # Body of script follows.

It is also possible to enable script options from the command line. Some options that will not work with set are available this way. Among these are -i, force script to run interactive.

bash -v script-name

bash -o verbose script-name

The following is a listing of some useful options. They may be specified in either abbreviated form or by complete name.

Table 31-1. bash options

-CnoclobberPrevent overwriting of files by redirection (may be overridden by >|)
-D(none)List double-quoted strings prefixed by $, but do not execute commands in script
-aallexportExport all defined variables
-bnotifyNotify when jobs running in background terminate (not of much use in a script)
-c ...(none)Read commands from ...
-fnoglobFilename expansion (globbing) disabled
-iinteractiveScript runs in interactive mode
-pprivilegedScript runs as "suid" (caution!)
-rrestrictedScript runs in restricted mode (see Chapter 21).
-unounsetAttempt to use undefined variable outputs error message, and forces an exit
-vverbosePrint each command to stdout before executing it
-xxtraceSimilar to -v, but expands commands
-eerrexitAbort script at first error (when a command exits with non-zero status)
-nnoexecRead commands in script, but do not execute them (syntax check)
-sstdinRead commands from stdin
-t(none)Exit after first command
-(none)End of options flag. All other arguments are positional parameters.
--(none)Unset positional parameters. If arguments given (-- arg1 arg2), positional parameters set to arguments.