Bash scripts: variables, quoting, arguments and exit codes

Why scripts?

If you type the same ten commands every week, put them in a script. It runs the same way every time, anyone on the team can run it, and cron or a timer can run it at night. Bash scripts are the glue of Linux administration.

Your first script

$ cat > ~/bin/hello.sh << 'EOF'
#!/bin/bash
# hello.sh: greet the user and show the date
echo "Namaste, $USER"
echo "Today is $(date +%A)"
EOF
$ chmod +x ~/bin/hello.sh
$ ~/bin/hello.sh
Namaste, asha
Today is Monday

Variables

name="Asha Rao"          # NO spaces around =
count=3
echo "$name has $count tickets"
today=$(date +%F)        # command substitution: the output of date
backup="/backup/etc-$today.tar.gz"
echo "Writing $backup"
echo "${name}_report"     # braces separate the name from text after it

name = "x" (with spaces) is an error: bash tries to run a command called name. Variables are text by default. For whole-number maths use $(( )): total=$(( count * 250 )).

Quoting rules

Arguments

#!/bin/bash
# greet.sh NAME [CITY]
echo "Script: $0"
echo "First argument: $1"
echo "Second argument: ${2:-Pune}"   # use Pune if $2 is empty
echo "Number of arguments: $#"
echo "All arguments: $@"
$ ./greet.sh Ravi Chennai
Script: ./greet.sh
First argument: Ravi
Second argument: Chennai
Number of arguments: 2
All arguments: Ravi Chennai

read asks the user for input: read -p "Username: " user. For scripts run by cron, prefer arguments or a config file; nobody is there to type.

Exit codes

Every command ends with an exit code (exit status), a number from 0 to 255. 0 means success; anything else means some kind of failure. $? holds the code of the last command. Your script should end with exit 0 on success and a non-zero code on failure, so cron, systemd and other scripts can tell what happened.

$ grep -q asha /etc/passwd; echo $?
0
$ grep -q nobodyhere /etc/passwd; echo $?
1
$ ls /nope; echo $?
ls: cannot access '/nope': No such file or directory
2
$ mkdir /backup && echo "created"       # && runs the next one only on success
$ ping -c1 db01 || echo "db01 is down"  # || runs the next one only on failure

Common mistakes

• Spaces around = in an assignment.
• Forgetting chmod +x, then getting "Permission denied". Or running a script written on Windows: "bad interpreter: /bin/bash^M" means Windows line endings; fix with sed -i "s/\r$//" script.sh.
• Using unquoted variables.
• Not setting an exit code, so a failed backup looks successful to cron.