bash Cookbook

Playing It Safe

Problem

It is so easy to type the wrong character by mistrake (see!). Even for simple bash commands this can be quite serious—you could move or remove the wrong files. When pattern matching is added to the mix, the results can be even more exciting, as a typo in the pattern can lead to wildly different-than-intended consequences. What’s a conscientious person to do?

Solution

You can use these history features and keyboard shortcuts to repeat arguments without retyping them, thereby reducing the typos. If you need a tricky pattern match for files, try it out with echo to see that it works, and then when you’ve got it right use !$ to use it for real. For example:

$ ls
ab1.txt  ac1.txt  jb1.txt  wc3.txt
$ echo *1.txt
ab1.txt ac1.txt jb1.txt
$ echo [aj]?1.txt
ab1.txt ac1.txt jb1.txt
$ echo ?b1.txt
ab1.txt jb1.txt
$ rm !$
rm ?b1.txt
$

Discussion

The echo is a way to see the results of your pattern match. Once you’re convinced it gives you what you want, then you can use it for your intended command. Here we remove the named files—not something that one wants to get wrong.

Also, when you’re using the history commands, you can add a :p modifier and it will cause bash to print but not execute the command—another handy way to see if you got your history substitutions right. From the Solution’s example, we add:

$ echo ?b1.txt
ab1.txt jb1.txt

$ rm !$:p
rm ?b1.txt
$

The :p modifier caused bash to print but not execute the command—but notice that the argument is ?b1.txt and not expanded ...