Simple shell conditional.

# To echo a different thing depending on a command's success:
<command> && echo "success" || echo "failure"

# To full if syntax:
if <condition>; then echo "true"; else echo "false"; fi

# To list available if conditions:
help test

# To test if a given variable is empty:
if [[ -z $GIT_BRANCH ]]; then echo "true"; else echo "false"; fi

# To test if a file exists:
if [[ -e <filename> ]]; then echo "true"; else echo "false"; fi

# To if directory not exists:
if [[ ! -d <path/to/directory> ]]; then echo "true"; else echo "false"; fi