Shell scripts are a powerful way to automate tasks and manage your work. However, they can be difficult to format correctly. Here are three tips to help you format your shell scripts better:
- Use proper indentation. Indentation is important for making your code easier to read and understand. Use a strong, consistent style when indenting your code. For example, use: my $script = “./script.sh”
- Use whitespace wisely. White space is important for clarity and organization. Use spaces between words, not between lines of code. For example, use: #!/bin/bash my $script = “./script2.sh” # This line should be indented 3 spaces my $output = “./output2.” # This line should be indented 1 space
- Use comments sparingly. Comments help explain the code and make it more understandable. But don’t overuse them – use them only when needed and avoid making the code difficult to read or understand later on. For example, do not put comments at the beginning of every line of code or at the end of every file:
Yes, formatting is necessary and helps tremendously with understanding complex code. However, for those who write shell scripts often, double checking formatting can become a tedious task. This article will show you how to shortcut the work using shfmt!
What Is shfmt?
Developed by Dustin Krysak, shfmt is a Shell formatter, parser and interpreter. The project itself is hosted on GitHub and has a clear README and cleanly presented repository. The tool was developed in Go and supports the POSIX, Bash and mksh shells. This makes shfmt a truly universal program instead of being restricted to Bash only.
Installing shfmt
To install shfmt on your snap-enabled Linux distribution (Like Ubuntu and Mint), execute the following command in your terminal:
To install shfmt on your RedHat/Yum based Linux distribution (Like RHEL, Centos and Fedora), execute the following commands in your terminal:
Note: you will have to reboot your machine (or logout and re-login) after executing the first command and before executing the next commands.
On RHEL and Centos you may also have to install the EPEL repository first.
Using shfmt
One the snap package is installed, you can start using shfmt.
Let us define a very poorly formatted and written script as test.sh as follows:
There are several issues with this script, the most prominent being the formatting thereof. But there is also an error/bug in the script: The function call func on the last line is followed by more brackets. A function call in Bash (rather than a function definition) should only have the name, not the brackets. It is a bit higher that the proper function definition happened.
Let’s see what shfmt thinks of this.
Whereas the output looks a little cryptic, note that the term foo (used here) and bar (not used here now) are often used in IT circles to indicate/represent any like idiom or element. foo here really refers to func.
Even then, the message remains a little cryptic until we realize, looking at the last line, that what is really happening is the start of a function definition (and not a function call) because the two brackets were included. This then explains why the message is telling us that something more is expected; must be followed by a statement. shmft is looking here for something like func(){ some_command[s]; }.
Bingo! This increases the functionality of shfmt to being a shell script validation/checker tool, though likely nowhere near a comprehensive one as the one we wrote about in using shellcheck to find and fix scripting bugs. Still, very handy indeed!
We fix our bug and now the input script test.sh reads as follows:
We again execute shfmt against the code and receive a much more suitable, and well formatted, output:
Great. We can now take this one level further and indicate to shfmt that we would like to use an indent/tab width of two spaces instead of a full tab. I always write code using two spaces as an indent/tab width, and use an additional space where a command on the next line closely relates to the previous one, like a continued command etc. though this does not happen often. Over more than 10 years, I have found the two spaces to be the personal and shared projects ideal.
Everyone and every project has to find their own ideal syntax, but note that if you use a large tab (8 spaces) like the formatting presented by shfmt in the example above, your code may become harder to read easily.
We will set the indent/tab width to two spaces using the -i option (which the –help defines as indent: 0 for tabs (default), >0 for number of spaces): shfmt -i 2 test.sh which will render the script as follows:
Great! However, we note that shfmt did not pickup on our deliberate error: #!/bin/bash__ is not correct, and should read #!/bin/bash instead.
There is thus still a use case for using shellcheck to find script bugs besides using shfmt to format scripts better. However, interestingly, in this particular case, even shellcheck failed to notice the issue. This shortcoming was reported to the shellcheck team so in due time this may be fixed.
If you are interested in learning more about Linux, you can review the Bash Automation & Scripting Basics Series, as well as the Bash Loops: for, while, and until Bash Loops: for, while, and until and Exporting Variables in Bash: the Why and How articles.
Wrapping up
Being able to write clean, well-formatted, and bug-free scripts becomes an easier job when you use a shell formatting tool like shfmt and a bug/error checker like shellcheck. Even then, as we saw, some things may go unnoticed even up to the moment where you run the script for the first time. shfmt is a small but effective utility which will help you format your scripts and code in accordance with your selected indents. Enjoy!
