If you work in a terminal, you've likely used Bash's history command to save yourself the trouble of retyping a command [1] (Figure 1). However, if you're like most people, your use of history may have been confined to scrolling through the list of previously used commands. If all you are interested in are the most recently used commands, the arrow keys may be all that you need. However, the history command is capable of doing much more and in an economical way – especially if you have a good memory. You can start by adjusting history's environmental variables and then learn how to modify history entries for easier searching and for repurposing them using three types of editing options: event designators, word designators, and modifiers. The flexibility of all these options can be combined so that, with a little memorization, you can make the Bash history work for you to save time with minimal effort.

Figure 1: An excerpt from the default history command.

Environmental Variables

The history commmand has several environmental variables. All are added or modified in .profile or .bashrc in your home directory, depending on the distribution. The size of the history file is limited by HISTSIZE, which sets the number of entries in the history, and/or by HISTFILESIZE, the maximum memory to allot for the history. Both have similar structures:

HISTSIZE=NUMBER
HISTFILESIZE=NUMBER

When the maximum for either variable is reached, earlier entries are deleted and replaced by new ones. Many users' first impulse is to use a high number, such as 10,000 entries. Because the history is a text file, there should usually be no problem with how much space the history file occupies. However, too large a number can make locating entries much harder. Unless you have a clear need for such a large number, a smaller one can be more efficient. In any event, you can use Ctrl+R to cycle through entries. If you have a rough idea of where an entry might be, you can use history NUMBER to list the entries displayed, starting with the most recent, or a specific number to go directly to the entry. You can even clear your history with the command history -c, followed by history -w if a long history gets too confusing. Learning designators and modifiers will also make a longer history easier to use.

Another environmental variable for history is HISTTIMEFORMAT. As the name suggests, this variable adds a timestamp, which can help you locate entries more easily. The format is HISTTIMEFORMAT=DATE&TIME (Figure 2).

Figure 2: An excerpt from history with HISTTIMEFORMAT enabled.

The date and time are structured using the common values shown in Table 1. Their use and order is a matter of choice, but you may soon consider HISTTIMEFORMAT essential.

Values can also be assigned to HISTCONTROL. A value of ignorespace excludes any command that has a space before it, while ignoredup ignores one of the same commands when run one after the other. If you want to use both variables, use ignoreboth.

Event Designators

An event designator calls on a specific previous command and always starts with an exclamation mark (!). In its simplest form, an event designator follows ! with a command's number in the history. However, as you might expect, from a user account, you cannot run a command such as apt that can only be run as root (Figure 3). You can also call a command by how many previous commands ago it was used; for example, !-4 shows the command used four entries ago. Alternatively, you can enter a string so that !pipewire shows the last command that contains "pipewire." You can even find a string and replace it with another. For example, if you typed

cd /home/jlw/music

Figure 3: A command can be run directly from the history. However, in this example, the user has tried to run apt while not logged in as root, so the command cannot be run.

you could replace the cd command with ls with ^cd^ls and save yourself the trouble of retyping the path.

Word Designators

Word designators select the words from the most recent matching entry in the history. To use word designators, enter the new command and its options, followed by EVENT:!WORD-DESIGNATOR^. For example,

less !cat:^

will replace the most recent history entry found starting with cat with less and then run the rest of the command in the entry using less. The word designator can be a string or specify the word at an exact position, counting from the start of the event. For example, is the first word, 3 is the fourth word, and $ is the last. In addition, you can specify a range of words, such as 4-8, or every word except the first using an asterisk (*) if you recall enough to be specific.