published priority 14 30 min. 50 XP.

Basic Shell Scripting. The 80% That Gets Things Done

You will write small bash scripts during the competition. backup-and-restore, health checks, config-diff alerts. This lesson covers the 80% of bash you need: shebangs, quoting, conditionals, loops, exit codes, and the pipe model.

DCWF roles: IT-451 System Administrator SE-621 Software Developer SE-627 DevSecOps Specialist

Objectives

Write a script that starts with `#!/bin/bash` and has +x permission
Use `$1`, `$2`, `$@`, `$#` to take command-line args
Quote correctly: single quotes (literal), double quotes (expand), backticks/$()
Use `if`, `elif`, `else`, `case` for branching
Use `for` and `while` loops
Interpret exit codes and chain with `&&` and `||`
Redirect output: `>`, `>>`, `2>`, `2>&1`, `&> file`

Quick reference

Command	Purpose
#!/bin/bash	Shebang line. tells kernel to use bash
chmod +x script.sh	Make executable
set -euo pipefail	Fail fast: exit on error, undefined var, pipe failure
"$var" vs '$var'	Double = expand, single = literal
$(cmd) vs `cmd`	Command substitution; $() is preferred (nestable)
[[ "$a" == "$b" ]]	String equality (bash test)
[[ -f /path/file ]]	File exists and is regular
[[ -d /path/dir ]]	Path is a directory
echo $?	Exit code of last command (0 = success)
cmd1 && cmd2	Run cmd2 only if cmd1 succeeded
cmd1 \|\| cmd2	Run cmd2 only if cmd1 FAILED
cmd &> log	Redirect stdout AND stderr to file
for i in *.conf; do echo $i; done	Glob loop
while read line; do echo $line; done < file	Read file line-by-line
trap 'echo interrupted' INT	Catch Ctrl-C

Common pitfalls

Forgetting to `chmod +x`. bash says 'Permission denied'
Single-quoting a variable: `echo '$USER'` prints `$USER`, not the value
Spaces around `=` in assignments: `x = 5` is wrong, `x=5` is right
`if [ $var = 'x' ]` blows up when $var is empty. use `if [[ $var = 'x' ]]` (bash) or quote: `if [ "$var" = 'x' ]`
Piping to `while read` creates a subshell. variables set inside are lost outside. Workaround: process substitution `while read . ; done < <(cmd)`
Running a script with `bash script.sh` vs `./script.sh`. the first ignores the shebang

How it works (walkthrough)

#!/bin/bash
# restore-loop.sh. keep critical configs pristine
set -euo pipefail
GOLDEN=/root/golden-configs.tar.gz
LOG=/root/restore.log

while true; do
    if [[ -f "$GOLDEN" ]]; then
        tar -xzf "$GOLDEN" -C / 2>/dev/null
        for svc in sshd smbd named apache2 postgresql; do
            if systemctl is-active --quiet "$svc"; then
                systemctl reload-or-restart "$svc" 2>/dev/null || true
            fi
        done
        echo "$(date '+%F %T') restore cycle" >> "$LOG"
    fi
    sleep 60
done

Skill drills

1. What's a shebang line?

#!/path/to/interpreter on line 1. tells kernel what runs the script
2. Diff between $() and backticks?

$() is nestable and preferred; both do command substitution
3. What does `set -e` do in a script?

Exit immediately on any command failure
4. Exit code of a successful command?

0
5. What's the difference between `>` and `>>`?

> overwrites the file; >> appends
6. How do you redirect stderr to stdout?

2>&1
7. Shell operator that runs next command only if previous succeeded?

&&