HISTSIZE

HISTSIZE is a shell variable that controls the number of commands kept in the interactive shell's command history for the current session. It determines how many recent commands are stored in memory and available for recall with the up-arrow key or history expansion.

In practice, as new commands are entered, they are added to the in-memory history until the total reaches HISTSIZE; older entries are discarded as needed. HISTSIZE only affects the in-memory history of the current session, not the contents of any history file by itself.

Persistent history across sessions is controlled separately by HISTFILE and HISTFILESIZE. The default history file is usually ~/.bash_history. If the shell option histappend is enabled, history lines are appended to that file on exit; otherwise the file may be overwritten. Setting HISTSIZE to a larger value is commonly paired with a larger HISTFILESIZE to retain more commands across sessions.

Usage typically involves setting these in startup files such as ~/.bashrc or /etc/profile. For example: HISTSIZE=1000; export HISTSIZE; HISTFILESIZE=2000; export HISTFILESIZE. A value of 0 disables the in-memory history. Default values vary by system and shell, and different shells may implement history handling with slight variations.

In other shells, such as Zsh, HISTSIZE serves a similar role but may be complemented by additional history controls (for example, SAVEHIST) and different default behaviors. Overall, HISTSIZE provides a convenient knob to balance memory usage against the convenience of command recall.