All Shell Scripting Tips

9 Mar 2018

"Again, Again!"

Doing a different thing to the same thing (or, Teletubbies mode, if you prefer!)

Computers are supposed to be good at repetitive tasks, but sometimes we end up repeating things to the computer. For example:

steve@linux:/tmp$ mkdir -p yet/another/directory
steve@linux:/tmp$ cd yet/another/directory
steve@linux:/tmp/yet/another/directory$ echo "This is yet/another/directory" > README.txt
steve@linux:/tmp/yet/another/directory$ cat README.txt
This is yet/another/directory
steve@linux:/tmp/yet/another/directory$ 

That's a lot of unnecessary typing. Do we have to keep on saying "yet/another/directory" every time we refer to it?
No, we don't, and once you get into the habit of using $_, I promise you'll never look back.

steve@linux:/tmp$ mkdir -p yet/another/directory
steve@linux:/tmp$ cd $_
steve@linux:/tmp/yet/another/directory$ echo "This is $_" > README.txt
steve@linux:/tmp/yet/another/directory$ cat README.txt 
This is yet/another/directory
steve@linux:/tmp/yet/another/directory$

We only had to type the long "yet/another/directory" text one time. $_ substituted for it all the rest of the time.
How?
Because $_ inserts the final argument of the previous command. And it keeps on going, so we can keep using $_ for "the last argument of the previous command" as many times as we like.

Other Examples: Files

Many *nix commands take a filename as the final argument, so some obvious examples are to do with filenames.

steve@linux:/tmp$ echo foo >> myfile.txt
steve@linux:/tmp$ chmod 755 $_
steve@linux:/tmp$ 

Or you might want to copy a file after editing it:

steve@linux:/tmp$ vi myfile.txt
steve@linux:/tmp$ scp $_ 192.168.3.27:
myfile.txt          100%   14     0.0KB/s   00:00    
steve@linux:/tmp$ 

Note that at this point, we have broken the spell; $_ is now 192.168.3.27: and not myfile.txt. This can be used to your advantage, it just depends on what you are trying to achieve.

More Examples: Git

Git repositories tend to be in a directory of the same name. This one is a little more involved, but follows the same principle. We want to clone a repo from the URL https://github.com/githubtraining/hellogitworld.git. This will create a "hellogitworld" directory. The basename utility will strip out the prefix, leaving "hellogitworld.git", and using the second ".git" argument, will also trim ".git" from the end. This converts "https://github.com/githubtraining/hellogitworld.git" into "hellogitworld", and we can cd hellogitworld without retyping anything:

steve@linux:/tmp$ git clone https://github.com/githubtraining/hellogitworld.git
Cloning into 'hellogitworld'...
remote: Counting objects: 306, done.
remote: Total 306 (delta 0), reused 0 (delta 0), pack-reused 306
Receiving objects: 100% (306/306), 95.31 KiB | 0 bytes/s, done.
Resolving deltas: 100% (71/71), done.
Checking connectivity... done.
steve@linux:/tmp$ cd `basename $_ .git`
steve@linux:/tmp/hellogitworld$ ls
build.gradle  fix.txt  pom.xml  README.txt  resources  runme.sh  src
steve@linux:/tmp/hellogitworld$ 

Summary

Once you get into the habit of using $_, you start finding more and more ways of using it. Give it a try for a few days, and see if you don't get hooked!