Writing robust scripts#

We should use the following options in our bash script for better quality:

Meaning of these option:

• -e: Exit immediately if a command fails.
• -u: If a variable is not set, this is considered an error¹.
• -x: Expand variables in a command and print it before executing the command (better for debugging if command fails).
• -o pipefail: If any command in a pipe fails, this pipe will fail.

For more options, see bash set command doc.

common check conditions for files#

Some commonly used options for checking file conditions:

Note that to check if the opposite is true, use [ ! condition ].

Use [ ] or [[ ]] for condition check?#

Both [ ] and [[ ]] can be used to check some conditions, but [ ] is POSIX compatible, and [[ ]] is not. Both bash, zsh support [[ ]].

[[ ]] is also more versatile and powerful than [ ]. For example, it supports &&, || and grouping command using () etc.

Compare string equality#

We can use = and != to compare the equality of strings. Unlike common programming languages, use of == is non-standard (see here).

How to set boolean variable?#

In bash, there is not really a boolean type. We can just use literal string true to set a bool variable and check if variable is equal to string true. For example:

use_python=true
if [[ $use_python = true ]]; then
    # do something
else:
    # do another thing
fi

How to check if a command exists?#

We can use command -v YOUCOMMAND to check if YOURCOMMAND exists. For example:

if [[ ! "$(command -v rg)" ]]; then
    # install ripgrep
else
    echo "ripgrep already exists. No action needed."
fi

According to here, which command is not a reliable way to do this, since it may return an exit code of 0 even if no command is found.

References#

• Introduction to bash if.
• More advanced if usage.
• Difference between [ and [[.
• Check if a command exists in bash script.
• How to declare boolean variable.