Bash: Backup and restore settings

Problem

How do I backup and restore Bash settings?

Solution

There are different bash settings:

• 'set' options
• 'shopt' options
• traps

'set' options

First, you don't need to backup 'set' options if the script executes in a subshell. The changed settings vanish as soon as the subshell exits. For example:

$> cat foo.sh              # The script
#!/bin/bash
set -o errexit
set -o | grep errexit
$> set -o | grep errexit
errexit         off        # Setting is off in shell
$> ./foo.sh                # Execute script
errexit         on         # Script turns setting on
$> set -o | grep errexit
errexit         off        # Setting is off again when subshell exits
$> 

You do need to backup 'set' options when the script is sourced (included) in the current shell. For example:

$> cat foo.sh              # The script
#!/bin/bash
set -o errexit
set -o | grep errexit
$> set -o | grep errexit
errexit         off        # Setting is off in shell
$> . foo.sh                # Source script
errexit         on         # Script turns setting on
$> set -o | grep errexit
errexit         on         # Setting is on after script has finished
$> 

All

oldSetOptions=$(set +o)             # Backup
eval "$oldSetOptions" 2> /dev/null  # Restore

Example output of set +o:

set +o allexport
set -o braceexpand
set +o emacs
set +o errexit
set +o errtrace
set +o functrace
set -o hashall
set -o histexpand
set -o history
set +o ignoreeof
set -o interactive-comments
set +o keyword
set -o monitor
set +o noclobber
set +o noexec
set +o noglob
set +o nolog
set +o notify
set +o nounset
set +o onecmd
set +o physical
set +o pipefail
set +o posix
set +o privileged
set +o verbose
set -o vi
set +o xtrace

Single

Invoking a subshell ($() or ``) is relative heavy, so if you want to backup/restore a single `set' option, you can use this, e.g. for `set -f':

$ [[ $- = *f* ]]; oldf=$?  # Save -f state
$ set -f                   # Disable globbing
$ (( $oldf )) && set +f    # Reset -f if necessary

'shopt' options

Code to backup and restore 'shopt' options:

oldShoptOptions=$(shopt -p)           # Backup
eval "$oldShoptOptions" 2> /dev/null  # Restore

Example output of shopt -p:

shopt -u cdable_vars
shopt -u cdspell
shopt -u checkhash
shopt -s checkwinsize
shopt -s cmdhist
shopt -u dotglob
shopt -u execfail
shopt -s expand_aliases
shopt -u extdebug
shopt -u extglob
shopt -s extquote
shopt -u failglob
shopt -s force_fignore
shopt -u gnu_errfmt
shopt -u histreedit
shopt -u histappend
shopt -u histverify
shopt -s hostcomplete
shopt -u huponexit
shopt -s interactive_comments
shopt -u lithist
shopt -u login_shell
shopt -u mailwarn
shopt -u no_empty_cmd_completion
shopt -u nocaseglob
shopt -u nullglob
shopt -s progcomp
shopt -s promptvars
shopt -u restricted_shell
shopt -u shift_verbose
shopt -s sourcepath
shopt -u xpg_echo

traps

Code to set and restore trap:

trap 'echo Error; exit' ERR  # Set ERR trap
trap - SIGNAL                # Restore ERR trap

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