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Variables

Check if variable is unset or empty

Also works with [ .

Terminal window
if [[ -n "${var}" ]]; then
  echo "var is set to '${var}'"
else
  echo "var is unset or empty"
fi

Check if variable is unset

Also works with [ .

Terminal window
if [[ -n "${var+x}" ]]; then
  echo "var is set to '${var}'"
else
  echo "var is unset or empty"
fi

Parameter substitution

Excerpts from an advanced parameter substitution guide.

Almost all substitution operators work with and without :. If : is used then the default value is substituted both when variable is unset or empty. If : is not used, then the default is substituted only when the variable is unset.

Terminal window
# If ABC is unset or empty use a default
XYZ="${ABC:-default}"


# If ABC is unset or empty write message to stderr and exit with error
XYZ="${ABC:?error message}"


# If ABC is unset or empty assign a default to it
# colon (:) is required to not interpret the rest as a command
: "${ABC:=default}"
# this can also be used in other places
XYZ="${ABC:=/tmp}/path/to/somewhere"
# After that ABC is set to '/tmp' if it was unset or empty before


# Substitute variable with a name which is taken from another variable
VAR_NAME="ABC"
XYZ="${!VAR_NAME}"

Functions

Report fatal errors

Terminal window
oops() {
    echo "$0:" "$@" >&2
    exit 1
}


# Oneliner:
oops() { echo "$0:" "$@" >&2; exit 1; }


# Inline usage
false || oops "you do not have '$1' installed, which I need to $2"


# Usage
if [ -z "${MYVAR}" ]; then
  # ...doing more commands...
  oops "Something went wrong, please check ${ARGS}"
fi

Common

Use env in shebang to locate shell without hardcoding a path to it

This will allow the script to find the shell even if it is installed into an non-common location by the user.

#!/usr/bin/env sh
#!/usr/bin/env bash
#!/usr/bin/env zsh

Run command in another directory using sub-shell

Terminal window
ABC=abc
# parentheses create a sub-shell which sees all vars from the current shell
# but `cd` does not affect current shell
(cd path/to/another/dir; echo "${ABC}"; ls)

Create a temporary directory and remove it when script finishes in any way

Terminal window
# Note: specifying absolute prefix for tmp file may fail on MacOS
tmpDir="$(mktemp -d -t mytmp.XXXXXXXXXX || oops "Can't create temporary directory")"
cleanup() {
  rm -rf "$tmpDir"
}
trap cleanup EXIT INT QUIT TERM


# Oneliner clean-up: trap "rm -rf $tmpDir" EXIT INT QUIT TERM

Exit script if any of the commands returned non-zero code

Terminal window
set -e


# Still possible to handle errors for commands in subshells
# XYZ=$(false || echo some_default)


# use `set +e` to disable this behaviour

Dot operator is the same as source

Terminal window
# Dot operator is the same as `source` command,
# which executes or sources given script in the current shell
. path/to/my/script.sh

Tricks

Prevent execution of non-complete scripts

If the last line of the file is not copied, then the shell won’t be able to parse the script because of the missing curly bracket, so the partial script will never run.

#!/usr/bin/env sh


# Wrap the whole script into {}
{


echo Hello!


} # end of wrapper

Run command without saving it into history

First, make sure variable HISTCONTROL is set either to ignorespace or ignoreboth.

Then any command that starts with an extra space will not be recorded to history.

Terminal window
$ echo blah # is saved to history
$  echo wow # is NOT saved to history

Get script directory to access any files relative to the script’s location

Terminal window
SCRIPT_DIR="$(dirname "$0")"