• No Datatype in bash, everything is a string of characters

Bash Equal Sign

  • Assignment: no space between the variable name and variable value
  • Equality check: space between variable name and variable value

Bash Boolean

  • There isn’t true or false
  • There is only exit code 0 means success, anything else is failure
  • A script has a return code of its last command which can be examined using echo $?

Bash Testing Conditions

  • We need to use either test or [ ] to evaluate conditions
if [ -z "$a" ]; then
	echo "hello"
if [ -z "$a" ]; then
	echo "hello"

Bash Script Exit Code

  • By default, bash continues to execute the script if one of the command finished and didn’t exit with 0

  • The bash script will exit with 0, as long as the last command exits with 0

  • This is undesirable, because it create an illusion that all commands in the script managed to run successfully

  • The solution is to define set -e right below #!/bin/bash, so the bash script exit immediately if any command it performs fails (returns a non-zero status). Preventing it from continuing if something goes wrong

  • However, if you want the bash script to terminate when one of the commands in Pipe (管道) fails. You need to also add in set -o pipefail

Bash Debugging

  • We can place set -x to enable debugging info for the bash script
  • The shell will print each command before it’s executed, preceded by a ’+’ sign
  • This is useful for debugging scripts, as it allows you to see exactly what commands are being executed and spot any issues

Bash Variable

  • By default, undefined bash variables are empty strings. They will not terminate the bash script!
  • We can add set -u to avoid this quirk

Bash Function

  • Keyword return is for Bash Script Exit Code, not return value
  • For return value, use echo instead
  • And use $(<BASH_FUNCTION_HERE>) command substitution to capture the return value

Keep the Changes in Current Shell

  • By default, the temporary changes like setting Environment Variable or directory changes is going to be removed once the bash script finishes
  • We can run the bash script with source which executes commands from a bash script in the current shell, so temporary changes are preserved after the bash script finishes running


  • Solving the Dotfiles Problem (And Learning Bash)

  • Why shell scripts aren’t suitable for complex tasks