Admittedly, this TOTW is one day late, so this week there will be 2xTOTW! In any case, the tip I want to bring up here is very much related to last week's TOTW on Reverse search in bash. Sometimes, reverse searching just doesn't work out. You may not be quite sure what you are looking for, or there are just too many recent commands that look samey. In such cases, using the history command is a good alternative.

history

The history command will display the last commands that you have entered, and looks something like this:

$ history
 1009  fg
 1010  git status
 1011  git commit -a -m 'Add module docstring to github_api module'
 [***OUTPUT TRUNCATED***]
 2007  history

Each command is called an event, and the output is formatted as <event_nr> <event>. Precisely how many commands are returned by the history is determined by the HISTSIZE and HISTFILESIZE environment variables. Setting these to something like 5000 and 10000, respectively, should be manageable even for the weakest of computers. You can also limit the output of history by providing an integer argument, so e.g. history 5 will display the last 5 commands. Now, the real power of history becomes apparent when using it with history expansion.

History expansion

History expansion can be used to expand an event number into the whole command it corresponds to. To expand an event, one simply types !<event_nr>. For example, looking at the history output above I can see that event number 1010 corresponds to git status. I can execute the command again with history expansion like so:

$ !1010
git status         # Command is echoed
On branch master   # Output from executing the command
[***REST OF OUTPUT OMITTED***]

The command is first echoed, and then executed. There are a few other ways to specify the event number.

• !: Execute the last event.
  • I.e. type !! in the terminal.
  • Can be useful to re-execute a command that you realized you needed sudo for with sudo !!.
• -n: Execute the nth previous event.
  • E.g. type !-1 to execute the last event, !-2 to execute the one before that, and so on.
  • I personally don't find this very useful.

There is one more very useful feature that I often use, and that is the ability to only print the command. This can be achieved by appending :p to the history expansion command. Here is an example:

$ !1011:p
git commit -a -m 'Add module docstring to github_api module'

The command can then be accessed by pressing UP-arrow or ctrl-p, which is very useful if you need to do minor modifications to it. There are tons of more ways to use history expansion, and I strongly recommend reading the man-page on it. Type man bash and then search for HISTORY EXPANSION, or do the same in this online bash man page.

Filtering history

A final tip on using history expansion is to filter the output with grep. For example, if I only want to find commands that include the word git, I can filter the output of history by piping to grep with the | character.

$ history | grep git
 1010  git status
 1011  git commit -a -m 'Add module docstring to github_api module'

I will most likely do another TOTW on piping, but the basic principle is that | takes the output from the command on the left and feeds it as input to the command on the right. That's it for this TOTW, stay tuned for the next one coming on Sunday the 28th of April!