Bash is a language that is quite useful for automation no matter what language you write in. Bash can do so many powerful system level tasks. Even if you are on windows these days you are likely to come accross bash inside a cloud VM, Continuous Integration, or even inside of docker. I have three techniques that help me write more composable bash scripts. FunctionsError Handlingif __name__ == "__main__" functions Functions in bash are quite simple. They are something that I wish I would have started using long ago. They make your code much more reusable. I often use them in my aliases as well since they can simplify the process and allow more flexibility. syntax () { } #!/bin/bash # hello_world.sh hello_world echo "hello world" Source the file to load the function and run it from the terminal. run it hello_world.sh hello_world source output hello world Arguments Arguments and options are quite a bit more complex in bash. For now we will focus on the basics which are not all that bad. positional arguments Positional arguments can be pulled out quite easily using $1 for the first one, $2 for the second and so on. $0 is the command that was called. You will see this often used to find the command called to open up your current shell. note syntax () { } #!/bin/bash # hello.sh hello echo "hello " $1 run it hello.sh hello waylon source Now we have a function that accepts positional arguments and we can call it by passing things into it. outputs hello waylon More than one argument would be ignoreed since we are only looking at $1. run it hello.sh hello waylon walker source outputs hello waylon All Arguments Bash has another special variable $@ that stores in one. all arguments syntax () { } #!/bin/bash # hello.sh hello echo "hello " $@ Just the same as before. run it hello.sh hello waylon walker source Now the function will output all arguments that are passed into it, since we are using the $@ variable. outputs hello waylon walker Error Handling The easiest and most common way to handle an error in bash is through the use of the logical operators && (and) and || (or). Here I have a concrete example from earlier today. I was creating a bash script to run a python script from cron. The bash script is there to make sure that we have the python environment, activate it, and run. If it doesn't have it, it should create it. () { conda create -n python=3.8 conda activate pip install -r requirements.txt } () { conda info --envs | awk | tail -n +3 | grep -w > /dev/null } () { env_exists && || create_env } create_if my_env create_env $1 $1 env_exists '{print $1}' $1 create_if $1 echo "environment exists" $1 If we look at the create_if function, it will check if the environment exists if there is a passing status code 0, then it will run echo "environment exists" otherwise it will run create_env $1. Inside of env_exists grep will look for whole words if there is a match it will give a status code 0 if it finds a match and not 0 if there is no match. note main script _if __name__ == _ "__main__" The last thing I want to discuss is making a bash script both runnable and sourceable. This makes it so that you can source filename.sh and run each function individually, or bash filename.sh to run the script. This is a similar concept to if __name__ == "__main__" from python. I did try this from bash and zsh with success. The following is an example that would pass all arguments into a main function. syntax [[ == ]]; main if " " ${BASH_SOURCE[0]} " " ${0} then " " $@ fi Extending the example above that created a python example if necessary we can create the env if necessary, activate the environment, and run the script. syntax [[ == ]]; create_if my_env conda activate my_env python script.py if " " ${BASH_SOURCE[0]} " " ${0} then fi Using this syntax to run our "main" functions will allow us to both run the script or source the script to utilize the functions that we created. 👀see an issue, edit this post on GitHub Previously published at https://waylonwalker.com/blog/reusable-bash/
