histexpand
Execute commands from history using event designators
TLDR
Run the previous command as root (!! is replaced by the previous command)
Run a command with the last argument of the previous command
Run a command with the first argument of the previous command
Run the Nth command of the history
Run the command n lines back in the history
Run the most recent command containing string
Run the previous command, replacing string1 with string2
Perform a history expansion, but print the command that would be run instead of actually running it
SYNOPSIS
shopt [-s | -u] histexpand
PARAMETERS
-s
Enable (set) the histexpand option
-u
Disable (unset) the histexpand option
-p
Print shopt commands to recreate current settings
-q
Quiet mode: exit status indicates if option is set
-o
Fail if optionname invalid (restricted shells)
DESCRIPTION
The histexpand is a Bash shell option that controls history expansion, allowing interpretation of special characters like ! and ^ on input lines to recall and modify previous commands from the history list.
When enabled, Bash scans each line for history expansion before executing it. Common expansions include:
• !!: repeats the previous command.
• !n: runs history line n.
• !-n: runs nth line backward.
• !string: executes most recent command starting with string.
• ?string?: most recent containing string.
• ^old^new^: quick substitution in last command.
Enable with shopt -s histexpand; disable via shopt -u histexpand. Default is on for interactive non-login shells. Useful for efficient command-line workflows but risky in scripts, as unescaped ! in strings or regex can trigger unintended expansion.
Expansions can be quoted to prevent, e.g., '!cmd'. Related options: histverify (edits before execute), cmdhist (multi-line as one). Enhances productivity in interactive sessions.
CAVEATS
Can break scripts with literal ! (e.g., in echo '!'; use single quotes). Conflicts with some regex/awk patterns. Disabled by default in login shells.
COMMON EXPANSIONS
!!: last cmd
!$:p: last arg (physical)
!^: first arg of last
!$: last arg of last
> !ls: word after ls
TOGGLING
shopt | grep histexpand (check status)
set +H (disable, old syntax)
set -H (enable)
HISTORY
Introduced in Bash 2.0 (1996), inspired by csh(1) history substitution. Evolved with shopt in Bash 3.0+ for granular control. Widely used in interactive shells; POSIX excludes history expansion.
