Bash Control Scripts

Bash works great for "control" scripts, as part of the unix philosophy, to glue a series of steps together and play with the filesystem - try to avoid using it to implement computation directly, there are loads of gotchas that are hard to prevent or identify.

Useful snippets

Useful Snippets

What follows are a selection of snippets I find useful, some have their caveats. Feel like something important is missing? Contact me and tell me.

Work Locally

This is incredibly useful for build or run scripts, which need to execute on items relative to where the script itself is.

source_dir="$(cd $(dirname ${BASH_SOURCE[0]}) && pwd)"
cd $source_dir

At any later point you can cd $source_dir to reset yourself without having to worry about where you have managed to end up. Useful when running intermediate scripts and working inside repositories or in other 'relative path' situations.

Fail Fast

Missing errors introduces bugs. Strict mode is useful but not perfect, read the blog post where I first found it here (by Aaron Maxwell).

set -euo pipefail
$IFS=$'\n\t'

The following assumes you have at least skimmed the blog :- I use -uo pipefail at all times and will toggle -e for times when I am expecting an error code. The IFS change is not actually something I will implement until using a loop - I prefer to set it for the specific code than as a general case.

Change the input

Sometimes you want to pretend things were different:

#!/bin/bash
echo "Input: \"$@\""
set -- "Three blind mice"
echo "Out: \"$@\""

Here we manipulate the calling arguments, useful with getopt and getopts

(own page)

Less colour is more colour

Handy tip, have less output in colour:

jq -C '.' | less -R

less uses the -R switch, allowing "raw" control characters
jq uses the -C switch, forcing colouring even though it is piping

git -c color.status=always status | less -R

git uses the -c parameter before a temporary configuration change. The arguments passed here could be put into the .gitconfig file

Many programs suppress colourised output if they detect they are being redirected, if you are using the -R switch with less and the output isn't colourful then you need to enable switches earlier on in the pipe.

(own page)

.bashrc git branch

It is often useful to display your current git branch in your prompt - but not obvious how to do it.

Embedding function calls in the prompt

Example basic prompt:

export PS1="${USER}@${HOST} \w\n"

Example prompt with embedded function call:

export PS1="${USER}@${HOST} \$(date -I) \w\n"

In the example with the embedded call, notice how it is backslash escaped.

Getting the current git branch

git_branch() {
  git status --porcelain -b 2> /dev/null | grep -Po "(?<=^## )[^\.]+" 2>/dev/null
  if [ "$?" -gt 0 ]; then
    echo "--"
  fi
}

git branch() {
  # Save original directory
  local dir="`pwd`"

  # Move upwards through the directory tree until a .git directory is found
  while [ "`pwd`" != "/" ] && ! [ -e ".git" ]; do cd ..; done

  # If it has been found, echo the branch of HEAD, else echo "--"
  if [ -e ".git" ]; then grep -Eo "[^/]+$" .git/HEAD; else echo "--"; fi

  # Restore original directory
  cd "$dir"
}

X ~ N(μ, σ²)