Master Bash Scripting: Essential Basics, Quirks, and Safety Tips

Yesterday I realized that even after ten years of using Bash, there are still basic details I don’t fully understand, so I wrote this article.

Author: oschina Source: https://www.oschina.net/translate/bash-scripting-quirks-safety-tips

We will cover:

Some Bash basics (e.g., how to write a for loop)

Miscellaneous quirks (e.g., always quote your Bash variables)

Bash script safety tips (e.g., always use set -u )

If you write shell scripts, you should know about the linter shellcheck to improve your scripts.

Variable Assignment

In Bash you assign variables without spaces: VARIABLE=2 Reference them with $VARIABLE. Do not put spaces around the = operator; otherwise you may unintentionally run a command named 2 and set the variable to an empty string.

Variables are usually uppercase, but not required. Most are strings; Bash also has array variables.

Referencing Variables with ${}

To avoid ambiguity when appending text, use ${MYVAR} instead of $MYVAR:

mv $MYVAR ${MYVAR}__bak   # correct

Global, Local, and Environment Variables

Bash has three kinds of variables. Environment variables are visible to all child processes: export MYVAR=2 Global variables are simple assignments: MYVAR=2 Local variables exist only inside functions and are rarely used in Bash scripts.

For Loop

A typical loop that prints numbers 1 to 10:

for i in `seq 1 10`; do
  echo "$i"
 done

You can write it in one line: for i in `seq 1 10`; do echo $i; done You can also iterate over command output, e.g., using seq, or use $(command) syntax.

If Statement

Bash if statements require spaces around brackets. Both [ ] and [[ ]] work, but [[ ]] is preferred:

if [[ "vulture" = "panda" ]]; then
  echo expression evaluated as true
else
  echo expression evaluated as false
fi

You can test file existence with [[ -e /tmp/awesome.txt ]] or the test command.

Functions Are Simple

Define and call a function without parameters:

my_function () {
  echo "This is a function"
}
my_function

Always Quote Your Variables

Never use an unquoted variable in tests:

X="i am awesome"
Y="i are awesome"
if [ "$X" = "$Y" ]; then
  echo awesome
fi

Return Codes, &&, and ||

Every Unix program returns a code from 0 (success) to 127 (failure). You can chain commands based on success: create_user && make_home_directory Or run the second command only on failure:

create_user || make_home_directory

Background Processes

Start a job in the background with & and bring it to the foreground with fg. List jobs with jobs. Use wait to wait for all background jobs.