David Peach

Being a Linux user for just over 10 years now, I can’t imagine my life with my aliases.

Aliases help with removing the repetition of commonly-used commands on a system.

For example, here’s some of my own that I use with the Laravel framework:

alias a="php artisan"
alias sail='[ -f sail ] && bash sail || bash vendor/bin/sail'
alias stan="./vendor/bin/phpstan analyse"

You can set these in your ~/.bashrc file. See mine in my dotfiles as a fuller example.

However, I recently came to want greater control over my development workflow. And so, with the help of videos by rwxrob, I came to embrace the idea of learning bash, and writing my own little scripts to help in various places in my workflow.

A custom bash script

For the example here, I’ll use the action of wanting to “exec” on to a local docker container.

Sometimes you’ll want to get into a shell within a local docker container to test / debug things.

I found I was repeating the same steps to do this and so I made a little script.

Here is the script in full:

#!/bin/bash

docker container ls | fzf | awk '{print $1}' | \
xargs -o -I % docker exec -it % bash

Breaking it down

In order to better understand this script I’ll assume no prior knowledge and explain some bash concepts along the way.

Sh-bang line.

the first line is the “sh-bang”. It basically tells your shell which binary should execute this script when ran.

For example you could write a valid php script and add #!/usr/bin/php at the top, which would tell the shell to use your php binary to interpret the script.

So #!/usr/bash means we are writing a bash script.

Pipes

The pipe symbol: |.

In brief, a “pipe” in bash is a way to pass the output of the left hand command to the input of the right hand command.

So the order of the commands to be ran in the script is in this order:

1. docker container ls
2. fzf
3. awk ‘{print $1}’
4. xargs -o -I % docker exec -it % bash

docker container ls

This gives us the list of currently-running containers on our system. The output is the list like so (I’ve used an image as the formatting gets messed up when pasting into a post as text) :

fzf

So the output of the docker container ls command above is the table in the image above, which is several rows of text.

fzf is a “fuzzy finder” tool, which can be passed a list of pretty much anything, which can then be searched over by “fuzzy searching” the list.

In this case the list is each row of that output (header row included)

When you select (press enter) on your chosen row, that row of text is returned as the output of the command.

In this image example you can see I’ve typed in “app” to search for, and it has highlighted the closest matching row.

awk ‘{print $1}’

awk is an extremely powerful tool, built into linux distributions, that allows you to parse structured text and return specific parts of that text.

'{print $1}' is saying “take whatever input I’m given, split it up based on a delimeter, and return the item that is 1st ($1).

The default delimeter is a space. So looking at that previous image example, the first piece of text in the docker image rows is the image ID: `”df96280be3ad” in the app image chosen just above.

So pressing enter for that row from fzf, wil pass it to awk, which will then split that row up by spaces and return you the first element from that internal array of text items.

xargs -o -I % docker exec -it % bash

xargs is another powerful tool, which enables you to pass what ever is given as input, into another command. I’ll break it down further to explain the flow:

The beginning of the xargs command is as so:

xargs -o -I %

-o is needed when running an “interactive application”. Since our goal is to “exec” on to the docker container we choose, interactive is what we need. -o means to “open stdin (standard in) as /dev/tty in the child process before executing the command we specify.

Next, -I % is us telling xargs, “when you next see the ‘%’ character, replace it with what we give you as input. Which in this case will be that docker container ID returned from the awk command previously.

So when you replace the % character in the command that we are giving xargs, it will read as such:

docker exec -it df96280be3ad bash

This is will “exec” on to that docker container and immediately run “bash” in that container.

Goal complete.

Put it in a script file

So all that’s needed now, is to have that full set of piped commands in an executable script:

#!/bin/bash

docker container ls | fzf | awk '{print $1}' | xargs -o -I % docker exec -it % bash

My own version of this script is in a file called d8exec, which after saving it I ran:

chmod +x ./d8exec

Call the script

In order to be able to call your script from anywhere in your terminal, you just need to add the script to a directory that is in your $PATH. I keep mine at ~/.local/bin/, which is pretty standard for a user’s own scripts in Linux.

You can see how I set my own in my .bashrc file here. The section that reads $HOME/.local/bin is the relevant piece. Each folder that is added to the $PATH is separated by the : character.

Feel free to explore further

You can look over all of my own little scripts in my bin folder for more inspiration for your own bash adventures.

Have fun. And don’t put anything into your scripts that you wouldn’t want others seeing (api keys / secrets etc)