Shell Scripting Complete 4: if, else, test, [[ ]], (( )) and case Explained Simply

By Ravindra Bagale, trainer LinkedIn Last updated: 25 October 2026

Scripts need to make decisions: is the disk full, does the file exist, did the command work, which environment are we in? This chapter teaches if, elif and else, the test command and [ ], the safer bash [[ ]], number and string comparisons, file tests, regex matching with =~, && and || shortcuts, (( )) for numbers, and case for menus and many choices.

What you'll learn in this chapter

Write if / elif / else
Know that if checks an exit code
Compare numbers and strings correctly
Use file tests -f -d -r -s
Prefer [[ ]] and use =~
Use case for many choices

1. An everyday example: a traffic signal

Why do we need this?

At a signal you decide: if red, stop; else if yellow, slow down; else go. A traffic police officer at a big junction does the same with many roads: case road 1, case road 2, case road 3. Scripts decide in the same way, using tests that answer yes or no.

What is it?

Light colour = the value. Checking the colour = test or [[ ]]. Stop/slow/go = if, elif, else. Junction with many roads = case. Yes/no answer = exit code 0 or non-zero.

How to do it (step by step)

[ ] and [[ ]] are just commands that return 0 or 1.

2. if, then, elif, else, fi

Why do we need this?

Without decisions a script does the same thing even when something has gone wrong.

What is it?

if COMMAND; then ...; elif COMMAND; then ...; else ...; fi. If runs any command and checks its exit code: 0 is true. So you can write if grep -q x file; then. The ; before then can be a new line instead. Indent the body by two or four spaces.

How to do it (step by step)

Command as the test.

if ping -c1 -W1 8.8.8.8 &>/dev/null; then echo online; else echo offline; fi

Output:

online

What it means: ping returned 0.

Three branches.

hour=$(date +%H); if [ $hour -lt 12 ]; then echo 'Good morning'; elif [ $hour -lt 17 ]; then echo 'Good afternoon'; else echo 'Good evening'; fi

Output:

Good evening

What it means: Only the first true branch runs.

grep -q.

if grep -q '^asha:' /etc/passwd; then echo 'user exists'; fi

Output:

user exists

What it means: -q prints nothing, only the exit code matters.

Negate.

if ! systemctl is-active -q nginx; then echo 'nginx is down'; fi

Output:

nginx is down

What it means: ! flips true and false.

3. test and [ ]: numbers

Why do we need this?

Disk percent, error counts and retry numbers need comparisons.

What is it?

[ is the test command; the closing ] is required and spaces inside are required: [ $a -gt 5 ]. Number operators are words: -eq equal, -ne not equal, -lt less, -le less or equal, -gt greater, -ge greater or equal. Do not use < or > inside [ ] for numbers: they mean redirection there.

Operator	Means
-eq	equal
-ne	not equal
-lt / -le	less / less or equal
-gt / -ge	greater / greater or equal

How to do it (step by step)

Disk alert.

use=$(df --output=pcent / | tail -1 | tr -dc 0-9); if [ "$use" -ge 90 ]; then echo "ALERT $use%"; else echo "ok $use%"; fi

Output:

ok 41%

What it means: tr -dc 0-9 keeps only the digits.

Spaces matter.

[$use -ge 90]

Output:

bash: [41: command not found

What it means: [ must be a separate word.

test is the same.

test 5 -gt 3 && echo yes

Output:

yes

What it means: [ ] is just another way to write test.

Empty variable trap.

x=; [ $x -eq 1 ]

Output:

bash: [: -eq: unary operator expected

What it means: Quote it or give a default.

4. String tests: =, !=, -z, -n

Why do we need this?

Checking environment names, answers and empty values is everyday work.

What is it?

[ "$a" = "$b" ] equal, != not equal. -z "$a" true if empty, -n "$a" true if not empty. Always quote variables inside [ ] or an empty value breaks the test. Inside [[ ]] (next section) == also works and quoting is less critical.

How to do it (step by step)

Equal.

env=prod; if [ "$env" = prod ]; then echo 'careful: production'; fi

Output:

careful: production

What it means: Strings compared exactly.

Empty check.

name=''; [ -z "$name" ] && echo 'name is empty'

Output:

name is empty

What it means: -z means zero length.

Not empty.

[ -n "$HOME" ] && echo "home set: $HOME"

Output:

home set: /home/asha

What it means: -n means non-zero length.

Case matters.

[ Prod = prod ] || echo different

Output:

different

What it means: Compare ${v,,} to ignore case.

5. File tests: -e -f -d -r -w -x -s

Why do we need this?

Before reading, copying or deleting you should know the file is there and is the right type.

What is it?

-e exists. -f regular file. -d directory. -L symlink. -r readable, -w writable, -x executable by you. -s exists and is not empty. a -nt b means a is newer than b; -ot older.

Test	True when
-e	exists
-f	regular file
-d	directory
-s	size > 0
-r/-w/-x	you can read/write/run
-nt	newer than

How to do it (step by step)

Config exists?

conf=/etc/ssh/sshd_config; if [ -f "$conf" ]; then echo found; else echo missing; fi

Output:

found

What it means: A regular file.

Make a folder if missing.

[ -d /tmp/reports ] || mkdir -p /tmp/reports; ls -d /tmp/reports

Output:

/tmp/reports

What it means: || runs only when the test fails.

Empty file.

: > /tmp/e.txt; [ -s /tmp/e.txt ] || echo 'file is empty'

Output:

file is empty

What it means: -s needs at least one byte.

Newer.

[ /etc/hosts -nt /etc/hostname ] && echo 'hosts changed later'

Output:

hosts changed later

What it means: Compares modification times.

6. [[ ]]: the safer bash test

Why do we need this?

[ ] has traps with empty values and special characters. [[ ]] is built into bash and avoids most of them.

What is it?

[[ ]] does not split words or expand globs on variables, so an empty $x is safe. It supports && and || inside, == with patterns ([[ $f == *.log ]]), < and > for string order, and =~ for regex. It works in bash, not in plain sh. In bash scripts, use [[ ]] for strings and files and (( )) for numbers.

Feature	[ ]	[[ ]]
empty var unquoted	breaks	safe
&& \|\| inside	no (use -a -o)	yes
pattern ==	no	yes
regex =~	no	yes
POSIX sh	yes	no

How to do it (step by step)

Pattern.

f=app-2026.log; [[ $f == *.log ]] && echo 'a log file'

Output:

a log file

What it means: * is a pattern here, not a file list.

Two conditions.

age=25; city=Pune; [[ $age -ge 18 && $city == Pune ]] && echo 'can vote in Pune'

Output:

can vote in Pune

What it means: && works inside.

Empty is safe.

x=; [[ $x == yes ]] || echo 'not yes'

Output:

not yes

What it means: No error with an empty value.

String order.

[[ apple < banana ]] && echo 'apple comes first'

Output:

apple comes first

What it means: < compares alphabetically.

7. Regex with =~ and BASH_REMATCH

Why do we need this?

Validating an email, a version number or an IP needs a pattern, not just equality.

What is it?

[[ $s =~ regex ]] is true when the regex matches. Do not quote the regex (quotes make it literal); store it in a variable if it has spaces. Groups in ( ) are saved in the BASH_REMATCH array: ${BASH_REMATCH[1]} is the first group.

How to do it (step by step)

Indian PIN code.

pin=411045; [[ $pin =~ ^[1-9][0-9]{5}$ ]] && echo 'valid PIN'

Output:

valid PIN

What it means: Six digits, not starting with 0.

Extract the version.

v='nginx/1.24.0'; [[ $v =~ ([0-9]+)\.([0-9]+) ]] && echo "major=${BASH_REMATCH[1]} minor=${BASH_REMATCH[2]}"

Output:

major=1 minor=24

What it means: Groups are saved.

Regex in a variable.

re='^[a-z]+@[a-z]+\.[a-z]+$'; [[ ravi@shop.in =~ $re ]] && echo 'looks like email'

Output:

looks like email

What it means: Unquoted $re is used as a regex.

8. && and || shortcuts, and (( )) for numbers

Why do we need this?

Short checks read better on one line, and number checks are easier with normal maths signs.

What is it?

a && b runs b only if a worked. a || b runs b only if a failed. Be careful with a && b || c: c also runs if b fails, so use a full if when b can fail. (( expr )) is true when the result is not 0, and you can use < > == != with numbers: (( count > 10 )).

How to do it (step by step)

Run only if it worked.

mkdir -p /tmp/deploy && cd /tmp/deploy && pwd

Output:

/tmp/deploy

What it means: Stops at the first failure.

Fallback.

cd /no/such/dir 2>/dev/null || echo 'could not cd, stopping'

Output:

could not cd, stopping

What it means: || handles the failure.

Numbers the easy way.

errors=12; if (( errors > 10 )); then echo 'too many errors'; fi

Output:

too many errors

What it means: No -gt needed.

Even or odd.

n=7; (( n % 2 == 0 )) && echo even || echo odd

Output:

odd

What it means: % is remainder.

9. case: many choices in a clean way

Why do we need this?

A chain of ten elifs comparing the same variable is hard to read. case is cleaner and supports patterns.

What is it?

case $x in pattern1) ...;; pattern2|pattern3) ...;; *) default;; esac. Patterns are globs: *.jpg, [yY]*, start|begin. ;; ends each branch. The first match wins, so put *) last. Very common for start/stop/status menus and for file types.

How to do it (step by step)

Service menu.

act=restart; case $act in start) echo starting;; stop) echo stopping;; restart|reload) echo 'stop then start';; *) echo "usage: start|stop|restart";; esac

Output:

stop then start

What it means: | gives two names to one branch.

By extension.

f=photo.JPG; case ${f,,} in *.jpg|*.png) echo image;; *.pdf) echo document;; *) echo other;; esac

Output:

image

What it means: ${f,,} makes it lower case first.

Yes/no.

ans=Yes; case $ans in [yY]*) echo ok;; *) echo no;; esac

Output:

ok

What it means: [yY]* matches y, Y, yes, Yes.

Weekday.

case $(date +%u) in 6|7) echo weekend;; *) echo 'work day';; esac

Output:

weekend

What it means: date +%u gives 1-7.

10. Real examples from many industries

Why do we need this?

The same Linux skill shows up in every kind of company. Seeing it in many places helps you remember it.

What is it?

Each row is a made-up team. It shows who uses this and how.

How to do it (step by step)

Team and city	How they use it	Command
Sneha, hospital, Pune	Alert when disk >= 90	`(( use >= 90 ))`
Liam, bank, Sydney	Stop if not on prod host	`[[ $(hostname) == prod-* ]]`
Yuki, gaming, Tokyo	Version check with =~	`[[ $v =~ ^2\. ]]`
Fatima, telecom, Dubai	start/stop menu with case	`case $1 in start)`
Lucas, logistics, São Paulo	Skip empty CSV files	`[ -s "$f" ] \|\| continue`
Anil, manufacturing, Aurangabad	Restart only if down	`systemctl is-active -q x \|\| systemctl restart x`

11. Common errors and how to fix them

Why do we need this?

Everyone hits these errors. Knowing the cause saves hours.

What is it?

Read the exact message first. Then match it to a row below.

How to do it (step by step)

Error you see	Why it happens	How to fix
[: missing `]'	No space before ]	Add spaces inside brackets. `[ "$a" = b ]`
`[41: command not found`	No space after [	Write [ $x ... `[ "$x" -gt 5 ]`
`unary operator expected`	Empty unquoted variable	Quote or use [[ ]]. `[[ $x -eq 1 ]]`
`integer expression expected`	Text used with -gt	Check it is a number. `[[ $x =~ ^[0-9]+$ ]]`
`A file named 5 appeared`	Used > in [ ] for numbers	Use -gt or (( )). `(( a > 5 ))`
syntax error near unexpected token `fi'	Missing then	Add then. `if x; then y; fi`
syntax error near unexpected token `)' in case	Missing ;; or in	End each branch with ;;. `case $a in x) y;; esac`
`Regex never matches`	Regex was quoted	Remove quotes or use a variable. `[[ $s =~ $re ]]`
`[[: not found`	Script run with sh	Use bash or #!/bin/bash. `bash s.sh`

12. Troubleshooting step by step

Why do we need this?

Print the values being compared, then run the test alone and look at $?.

What is it?

Go top to bottom. Stop at the first step that shows the problem.

How to do it (step by step)

See the values.

echo "[$a] [$b]"

Test alone.

[[ $a == $b ]]; echo $?

Trace.

bash -x s.sh

Lint.

shellcheck s.sh

Which shell?

head -1 s.sh

Hidden \r from Windows.

cat -A s.sh | grep -m3 '\^M'

13. Lab 1: A health check with if and file tests

Why do we need this?

Write health.sh that checks disk, a config file and a service, and prints OK/WARN/FAIL lines.

What is it?

A full hands-on lab. Run every step on a test Linux machine (an EC2 instance, a VM, or WSL).

How to do it (step by step)

Folder.

mkdir -p ~/lab40 && cd ~/lab40

Disk part.

use=$(df --output=pcent / | tail -1 | tr -dc 0-9); if (( use >= 90 )); then echo "FAIL disk $use%"; elif (( use >= 75 )); then echo "WARN disk $use%"; else echo "OK disk $use%"; fi

Output:

OK disk 41%

Config part.

f=/etc/ssh/sshd_config; [[ -r $f ]] && echo "OK $f readable" || echo "FAIL $f"

Output:

OK /etc/ssh/sshd_config readable

Service part.

if systemctl is-active -q sshd; then echo 'OK sshd'; else echo 'FAIL sshd'; fi

Output:

OK sshd

Empty log check.

touch app.log; [[ -s app.log ]] || echo 'WARN app.log is empty'

Output:

WARN app.log is empty

Exit code.

bash -c 'exit 0'; echo $?

Output:

14. Lab 2: A case-based service tool

Why do we need this?

Build svc.sh start|stop|status|-h for a fake service, with pattern matching and a usage message.

What is it?

A full hands-on lab. Run every step on a test Linux machine (an EC2 instance, a VM, or WSL).

How to do it (step by step)

Write.

printf '#!/bin/bash\ncase ${1:-} in\n  start) touch /tmp/svc.pid; echo started;;\n  stop) rm -f /tmp/svc.pid; echo stopped;;\n  status) [[ -e /tmp/svc.pid ]] && echo running || echo stopped;;\n  -h|--help|"") echo "usage: $0 start|stop|status";;\n  *) echo "unknown: $1" >&2; exit 2;;\nesac\n' | tee svc.sh && chmod +x svc.sh

Start.

./svc.sh start; ./svc.sh status

Output:

started
running

Stop.

./svc.sh stop; ./svc.sh status

Output:

stopped
stopped

Help.

./svc.sh

Output:

usage: ./svc.sh start|stop|status

Bad input.

./svc.sh boom; echo $?

Output:

unknown: boom
2

Lint it.

shellcheck svc.sh && echo clean

Output:

clean

Ravindra Bagale's Tip

In bash scripts use [[ ]] for strings and files, (( )) for numbers, and case when you compare one value against many choices. Remember: if only looks at an exit code, so any command can be a test.

15. Frequently asked questions

Q. [ ] or [[ ]]?

[[ ]] in bash; [ ] for POSIX sh.

Q. Why spaces inside brackets?

[ is a command; its args need spaces.

Q. = or ==?

Both in [[ ]]; = in [ ].

Q. How to test 'not'?

Put ! before the test.

Q. Can case use regex?

No, only glob patterns.

Q. Is (( 0 )) true?

No; 0 is false there.

Q. Compare decimals?

Use awk or bc.

Q. What is ;;& in case?

Continues testing next patterns (bash 4).

16. Interview questions and answers

Beginner level

Q. Number greater than?

-gt or (( > )).

Q. File exists?

-e or -f.

Q. String empty?

-z.

Q. End an if?

fi.

Mid level level

Q. [ ] vs [[ ]]?

[[ ]] is safer, supports patterns, regex, && ||.

Q. When case over if?

Many choices on one value.

Q. Danger of a && b || c?

c runs if b fails too.

Q. Get regex groups?

BASH_REMATCH.

Senior level

Q. Why does if grep work?

if checks exit codes; grep returns 0 on match.

Q. Write portable tests?

Use [ ] with quoted vars and = only.

Q. Validate an IPv4 roughly?

=~ ^([0-9]{1,3}\.){3}[0-9]{1,3}$ plus range checks.

Q. Guard against running on prod?

[[ $(hostname) == prod* ]] && read confirmation.

Try at home

Write grade.sh that takes marks (0-100) and prints A/B/C/Fail with if/elif. Reject bad input.
Write ftype.sh that uses case to say image, video, document or other for each argument.

Practice Tasks

About this scenario

Amravati Agro Market is a agriculture business in Amravati. Anjali Thakre looks after its IT and cloud work. They count money in INR and run their AWS resources in the ap-south-1 Region. In the tasks below you do real jobs for them using what this lesson teaches. All names, account IDs, IP addresses and numbers are made up for practice.

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