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Chapter 30. 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.

      #!/bin/bash

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

      set -v
      # Exact same effect as above.
      

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

      #!/bin/bash

      set -o verbose
      # Command echoing on.
      command
      ...
      command

      set +o verbose
      # Command echoing off.
      command
      # Not echoed.


      set -v
      # Command echoing on.
      command
      ...
      command

      set +v
      # Command echoing off.
      command

      exit 0
      

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

      #!/bin/bash -x
      #
      # 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 (preceded by a single dash) or by complete name (preceded by a double dash or by -o).

Table 30-1. Bash options

AbbreviationNameEffect
-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 ...
-eerrexitAbort script at first error, when a command exits with non-zero status (except in until or while loops, if-tests, list constructs)
-fnoglobFilename expansion (globbing) disabled
-iinteractiveScript runs in interactive mode
-nnoexecRead commands in script, but do not execute them (syntax check)
-o Option-Name(none)Invoke the Option-Name option
-o posixPOSIXChange the behavior of Bash, or invoked script, to conform to POSIX standard.
-o pipefailpipe failureCauses a pipeline to return the exit status of the last command in the pipe that returned a non-zero return value.
-pprivilegedScript runs as "suid" (caution!)
-rrestrictedScript runs in restricted mode (see Chapter 21).
-sstdinRead commands from stdin
-t(none)Exit after first command
-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
-(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.

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