Shell

Bash shell, Z Shell, etc.

Conditionals

You can check if a file exists with -f.

if [ -f "some_file" ]; then
    echo "file at './some_directory' exists and is not a directory!"
fi

You can test that a directory exists with -d, e.g.:

if [ -d "some_directory" ]; then
    echo "directory at './some_directory' exists!"
fi

You can reverse conditionals with !:

if [ ! -d "some_directory" ]; then
    echo "'./some_directory' does not exist!"
fi

Checking if a command exists

You can check whether a command exists by checking if command -v ${COMMAND_TO_CHECK} >/dev/null 2>/dev/null returns 0 (it exists) or non-zero (does not exist)

if [ ! command -v my_special_script >/dev/null 2>&1 ]; then
    echo "my_special_script not found"
fi

Checking if a string is a number

You can use the -eq operator to verify if something is a number: if ! [ "${some_number}" -eq "${some_number}"] 2>/dev/null; then "${some_number} is not a number"; fi

You can similarly use the -ge to determine if something is a positive number.

Traps

You can use the trap command to run code when the shell script exits (or any signal occurs), like so:

function on_end {
    echo "woohoo"
}

trap on_end exit

which will print “woohoo” to stdout when the script exits.