Sharing bash functions between scripts

I’ve spent most of the past week writing bash functions to help with automating build and deployment at work, and something I’ve never really paid much attention to is how to keep these .sh files structured. Trying to keep functions small and generic helps with reuse and saves me from having to write more code, and once I’ve tested something is working hopefully I won’t need to touch it again.

To quote the first item of the summarised Unix philosophy:

Write programs that do one thing and do it well.

It’s also made me think about another horrible mess of code that I’ve come to depend on, my .bash_aliases file.

This has truly become a dumping ground over the years to save me from having to remember almost everything I have done at one point or another. At what I feel is a whopping 1474 lines, my little black book of functions is something that I lean on a lot, but have never really taken any time to trim or polish, apart from some headings in the form of comments.

Creating some structure

So the first step in organising this mess is trying to group similar functions together. Near the very top of my .bash_aliases file I find this:

##########  Git aliases ##################
alias gs='git status'
alias ga='git add'
alias gd='git diff'
alias gc='git commit'
alias gca='git commit -a'
alias gb='git branch'
alias gbr='git branch --remote'
#########################################

So there are some aliases that I use because I’m lazy, and shaving off those extra keystrokes should save some wear and tear on my little digits. I’ve already noted in the file what this group applies to GIT so that is a great starting point.

Let’s create a new directory to store my newly structured files:
mkdir ~/scripts
There’s a new directory in my home directory called scripts. Next is to copy and paste the contents from .bash_aliases to a new file called git.sh within this folder.

Setting up the files

I could just copy and paste the contents in Emacs, but where’s the fun in that. I can use sed to grab the lines that I want and output them to this new file without having to leave the terminal.

sed -n '19,27p' ~/.bash_aliases > ~/scripts/git.sh

So here we are using sed to get the line between 19 and 27 from the file ~/.bash_aliases and outputting them to the file ~/scripts/git.sh easy. Bash files should have the following at the top ‘#!/bin/bash’, to tell the OS which interpreter should be used for the file. This way seems slightly hacky, but again, means I don’t need to leave the terminal.

sed -i '1s:^:#\!/bin/bash\n:' ~/scripts/git.sh

This sed command inserts the required text on the first command, followed by a new line, giving us what we want.

Next we need to delete the text from .bash_aliases. Again, let’s stay in the terminal, we’ve almost written the command we need already.

sed -n '19,27d' ~/.bash_aliases

So, sed lessons are over (almost), our files are set up and I’ll try and get this post back on track.

If we were to open a terminal at this point, the commands we’d receive a command not found message. This can easily be fixed, and I’m going to use sed again because why not?

sed -i '2s:^:\n\. ~/scripts/git\.sh\n\n:' ~/.bash_aliases

Here we are inserting a new line to our .bash_aliases file using the . command to source the contents of our git.sh file. Because .bash_aliases is run every time we open a bash shell, we now have contents of the ~/scripts/git.sh available at all times, whilst also keeping our git alias code separate.

I’ve got lots of functions that can be grouped together, so I envisage creating a azure-cli.sh file, docker.sh, code-generation.sh file and many others in the future.

Repeating this over the coming months should really help whip my .bash_aliases file into shape.

Conclusion

So this post devolved into a post about the Power of sed rather than what it was initially supposed to be, but the TD;DR is that:

1) massive files are bad
2) the . command will source or import code from another file
3) grouping similar functions into separate files helps make navigation of code easier
4) with the . command we can share code between files without repeating ourselves
5) we can use sed to manipulate text files succinctly, precisely with a couple of keystrokes

If you’ve made it this far, thanks for reading.