Skip to main content

Shell programming

Overview​

Best practices tl;dr:

  • Think hard before using bash. Something else is often better.
    • For portability, dash provides a minimal POSIX shell feature set.
    • For complicated tasks, a more modern language like Go or Python may be more reliable and easy to maintain.
  • Use shellcheck
  • Don't copy & paste code from Google or StackExchange unless you fully understand it
  • Keep your bash version up to date with your package manager (e.g. brew install bash and routinely running brew upgrade), or stick to whatever version your team has agreed to standardize on (e.g. brew pin bash). However, be aware that behavior of bash code can vary slightly between different versions of the interpreter.
  • Consolidate your bash profiles into a single ~/.bashrc file to streamline machine instructions affecting your PATH environment variables.

Debugging​

Bash will emit debugging output after set -x is executed. set +x to disable debugging output. bash -x ./brokenscript has the same effect without requiring edits to the file. It is also possible to configure the format of the output:

export PS4='+$BASH_SOURCE:$LINENO:$FUNCNAME: '

For extra verbose debugging, it may be useful to combine this option with set -v: this will also display the shell lines as they are read (i.e. before substitutions are resolved, etc).

Tools​

  • shellcheck is an excellent code linter for bash that catches many common anti-patterns. Integrations with several popular editors are available. If you only take one thing from this document, make it "use shellcheck".
  • bashdb is an interactive bash debugger inspired by gdb.

Pitfalls​

Shell programming is deceptively perilous (see Bash Pitfalls on Greg's Wiki and Beginner Mistakes on Bash-Hackers Wiki). Sites like tldp.org and StackOverflow have high pagerank for shell programming questions, but code blocks posted on these sites will not uncommonly have smells or dangerous errors. Check the Resources section of this document for some more trustworthy sources.

  • POSIX is very permissive about what can go in a file name. In fact, anything except a null byte could be in a filename. Problematic characters include (but are by no means limited to) tabs, newlines, non-breaking spaces, glob characters, and leading dashes. Many beginner mistakes are a result of failure to fully grok the implications of this; e.g. word splitting on whitespace, or globs getting expanded in surprising ways. Aggressively quoting substitutions without a specific reason not to is good practice.
  • bash is ubiquitous, but bash != bash. Not infrequently, a version of bash will change its handling of a particular case, only to revert to the old behavior in the next version. MacOS ships bash 3.2.57, but as of this writing bash 5.0 is in homebrew. Other versions may be on whatever host your script will run on, but there is no portable way to force which bash version you will get. On many systems, /bin/sh resolves to bash in POSIX compatibility mode, but you can't be guaranteed of that: prefer being explicit and using #!/usr/bin/env bash for the shebang.
  • Be aware of the unpredictable behavior of set -e / set -o errexit. This is useful when developing a script, but may not behave as you expect. The behavior is unpredictable depending on which shell version your script ultimately runs on; based on this history, consider that it may even change further in future versions of bash.
  • Avoid using cd in the main thread when possible, and always include error handling in case the command fails. Losing track of this state can lead to disastrous consequences. Try to use absolute paths, or a subshell like ( cd somedir || exit ; somecommand ).
  • Don't iterate through lines in a stream with for.
  • Prefer [[ to [: [[ is a bash extension that is a strict upgrade over [.

PATH​

  • PATH is a shortcut to a certain location on your machine, which you can view by typing echo $PATH in the terminal. To oversimplify, a PATH is an environment variable which consists of a list of directories. Adding a PATH to your bash profile will give your machine direct instructions on where to look for a certain program.

  • One of several files can affect how your machine searches for a given PATH, listed in the order a Mac searches for them:

    • ~/.bashrc (read on every new shell, so looked at far more often)
    • ~/.bash_profile (read only on login)
    • ~/.profile

  • If you have multiple files affecting your PATH, consider consolidating them into one /.bashrc and deleting the others. Next, organize each PATH directory in your ~/.bashrc from most-used to least-used to maximize efficiency.

Resources​

Notes​

The Bash CLI navigation shortcuts resource linked above refers to a keyboard shortcut for moving the cursor from word to word: meta key + f or b (under section 8.2.2 Readline Movement Commands). This shortcut does not work out of the box on a Mac. In the macOS Terminal app and iTerm2, the default keyboard shortcut for moving from word to word is option + left or right arrow. If you want to use option + f or b, there is a checkbox in Terminal app to Use Option as Meta key under Preferences -> Profiles -> Keyboard. In iTerm2, you can change key mappings under Preferences -> Profiles -> Keys.