HISTCONTROL

HISTCONTROL is an environment variable used by some Unix-like shells, most notably Bash, to control how commands are added to the shell's command history. It influences which commands are saved and whether duplicate commands are ignored or preserved. The value of HISTCONTROL is a colon-separated list of keywords.

Common keywords include ignoredups, which prevents duplicate consecutive commands from being saved. For example, if you type a command, press up arrow, and type the exact same command again, only the first instance will be saved to the history. Another keyword is ignorespace, which means commands starting with a space character will not be saved to the history. This is often used for commands that might contain sensitive information which you don't want to accidentally recall. The keyword erasesame allows duplicate commands to be saved, but it removes all previous occurrences of that command before saving the new one.

If HISTCONTROL is not set, or is unset, then all commands are typically saved to the history, subject to other history-related settings. Setting HISTCONTROL effectively modifies the default behavior of the shell's command history mechanism. For instance, setting it to 'ignoredups:ignorespace' would enable both of those behaviors. This variable allows users to customize their command history to better suit their workflow, reducing clutter and preventing accidental logging of sensitive commands.