Shell Scripting Complete 9: Arithmetic, Exit Codes, set -euo pipefail and Error Handling

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

A script that keeps going after a failure is dangerous: it may delete the wrong folder or upload half a backup. In this chapter you learn bash arithmetic with $(( )) and (( )), decimals with bc and awk, what exit codes are and how to read $? and PIPESTATUS, how && and || chain steps, the safe header set -euo pipefail and its traps, and clean error handling with a die function, return codes and retries. Every topic starts with an everyday example and then shows examples from many industries.

What you'll learn in this chapter

Do integer maths with $(( )) and (( ))
Do decimal maths with bc or awk
Read exit codes with $? and PIPESTATUS
Chain steps with && and ||
Use set -euo pipefail safely
Write die, retry and clean error messages

1. An everyday example: a pressure cooker whistle

Why do we need this?

In a Pune kitchen, the cooker whistle tells you what happened. Three whistles means done. No whistle means something is wrong and you check before serving. You would never serve dal without knowing it cooked. A command's exit code is that whistle: 0 means success, any other number means a problem, and a good script listens before moving to the next step.

What is it?

Whistle = exit code. Listening = checking $? or using if. Stopping the meal if something is wrong = set -e or || die. Counting whistles = arithmetic with (( )). Trying again after the gas went off = a retry loop.

How to do it (step by step)

Never hide a failure; make it loud.

2. Integer maths with $(( )) and (( ))

Why do we need this?

Counting files, adding totals, finding percentages for a disk alert: scripts do small maths all the time.

What is it?

$(( expr )) gives the result as text. (( expr )) runs the maths and its exit code is 0 if the result is not zero, so it works as a test. Inside, you do not need $ before names. Operators: + - * / % ** and comparisons < > == !=. Division is integer only: 7/2 is 3. let and expr are older ways; prefer (( )).

Form	Use
$((a+b))	result as text
((n++))	increase n
((a>b))	test, exit 0 if true
$((7%3))	remainder 1
$((2**10))	power 1024

How to do it (step by step)

Add totals.

a=1200; b=850; echo $((a+b))

Output:

What it means: No $ needed inside.

Counter.

n=0; for f in *.log; do ((n++)); done; echo $n

Output:

What it means: Counts files.

Percentage.

used=45; total=60; echo $((used*100/total))%

Output:

75%

What it means: Multiply first, then divide.

Compare.

disk=91; (( disk > 85 )) && echo 'disk alert'

Output:

disk alert

What it means: (( )) as a test.

Even or odd.

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

Output:

odd

What it means: Remainder check.

3. Decimals with bc and awk

Why do we need this?

Prices with paise, GST at 18 percent, average response time in ms: bash alone cannot do decimals.

What is it?

bc is a calculator: echo 'scale=2; 10/3' | bc gives 3.33. scale sets decimal places. awk can also do it: awk 'BEGIN{printf "%.2f\n", 10/3}'. To compare decimals, let bc or awk return 1 or 0 and test that. Watch out: bc is not installed on some minimal images; awk almost always is.

How to do it (step by step)

Divide with decimals.

echo 'scale=2; 10/3' | bc

Output:

3.33

What it means: scale=2.

GST.

price=499.50; echo "scale=2; $price*1.18" | bc

Output:

589.41

What it means: 18 percent added.

awk average.

printf '120\n95\n310\n' | awk '{s+=$1} END{printf "%.1f\n", s/NR}'

Output:

175.0

What it means: Average response ms.

Compare decimals.

load=3.75; (( $(echo "$load > 2.5" | bc) )) && echo high

Output:

high

What it means: bc prints 1 for true.

4. Exit codes, $? and PIPESTATUS

Why do we need this?

Cron, CI systems and other scripts decide success or failure only from your exit code.

What is it?

Every command ends with a number from 0 to 255. 0 means success. 1 is a general error, 2 is misuse, 126 is not executable, 127 is command not found, 130 is stopped by Ctrl+C, 137 is killed (often out of memory). $? holds the last code; read it right away because the next command replaces it. In a pipe, $? is the last command's code; ${PIPESTATUS[@]} has all of them. exit N ends a script, return N ends a function.

Code	Meaning
0	success
1	general error
2	bad usage
126	not executable
127	command not found
130	Ctrl+C
137	killed by signal 9

How to do it (step by step)

Success code.

ls /etc/hostname; echo $?

Output:

/etc/hostname
0

What it means: 0 = OK.

Failure code.

ls /nope; echo $?

Output:

ls: cannot access '/nope': No such file or directory
2

What it means: Not 0.

Not found.

nosuchcmd; echo $?

Output:

bash: nosuchcmd: command not found
127

What it means: 127.

Pipe codes.

false | true; echo "$? ${PIPESTATUS[@]}"

Output:

0 1 0

What it means: Last was true, first failed.

Save it first.

grep -q ERROR app.log; rc=$?; echo "grep said $rc"

Output:

grep said 1

What it means: 1 means no match.

5. Chain steps with && and ||

Why do we need this?

Build then deploy only if the build worked; send an alert only if the check failed.

What is it?

a && b runs b only if a succeeded. a || b runs b only if a failed. if cmd; then ... fi tests a command's exit code directly, so you rarely need $?. ! cmd flips the result. Be careful with a && b || c: c also runs if b fails, so it is not a true if-else; use if for important logic.

How to do it (step by step)

Only on success.

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

Output:

/tmp/rel

What it means: Stops at the first failure.

Only on failure.

ping -c1 -W1 10.9.9.9 || echo 'gateway down'

Output:

gateway down

What it means: Alert path.

if on a command.

if grep -q 'Out of memory' /var/log/syslog; then echo 'OOM seen'; fi

Output:

OOM seen

What it means: No $? needed.

Negate.

if ! id deploy; then echo 'no deploy user'; fi

Output:

id: 'deploy': no such user
no deploy user

What it means: ! flips.

6. The safe header: set -euo pipefail

Why do we need this?

By default bash ignores failures and unset variables. One typo like rm -rf "$DIR/" with empty DIR can be a disaster.

What is it?

set -e exits when a command fails. set -u treats unset variables as errors. set -o pipefail makes a pipe fail if any part fails. Together: set -euo pipefail. Know the limits: -e does not fire inside if conditions, in parts of && or || lists, or for commands like ((n++)) when n was 0 (that returns 1 and kills the script). Use n=$((n+1)) or ((n++)) || true. grep with no match returns 1, so add || true when that is fine.

Option	Effect
-e	exit on error
-u	unset variable is error
-o pipefail	pipe fails if any part fails
-x	trace each command

How to do it (step by step)

Header.

#!/usr/bin/env bash
set -euo pipefail

What it means: First lines of a script.

-u catches typos.

set -u; echo "$DIRR"

Output:

bash: DIRR: unbound variable

What it means: Stops before damage.

pipefail.

set -o pipefail; false | sort; echo $?

Output:

What it means: Without it you get 0.

Counter trap.

set -e; n=0; ((n++)); echo never

What it means: Script exits; use n=$((n+1)).

Allowed failure.

count=$(grep -c WARN app.log || true); echo $count

Output:

What it means: No match is OK here.

7. Clear errors: stderr, die and usage

Why do we need this?

Error messages mixed into normal output break pipes and confuse the person on call at 3 a.m.

What is it?

Send errors to stderr with echo 'msg' >&2. Write a small die function that prints the message and exits with a code. Print a usage line and exit 2 when arguments are wrong. Include the script name with ${0##*/} and what failed, so the message is useful in logs.

How to do it (step by step)

To stderr.

echo 'config missing' >&2

Output:

config missing

What it means: Not in stdout.

die function.

die(){ echo "${0##*/}: $*" >&2; exit "${2:-1}"; }
[[ -r app.conf ]] || die 'cannot read app.conf'

Output:

backup.sh: cannot read app.conf

What it means: Clear and exits 1.

Usage.

(( $# == 1 )) || { echo "usage: ${0##*/} ENV" >&2; exit 2; }

Output:

usage: deploy.sh ENV

What it means: Exit 2 = bad usage.

Return from a function.

is_up(){ curl -fs -o /dev/null "$1"; }; is_up https://example.com && echo up

Output:

up

What it means: Function returns curl's code.

8. Retries and timeouts

Why do we need this?

Networks blink. A deploy should try a health check a few times before calling it a failure, and should not hang forever.

What is it?

A retry loop runs a command up to N times with a sleep between tries and returns failure at the end. Double the sleep each time (backoff) to be kind to a busy server. timeout 10 cmd kills cmd after 10 seconds and returns 124. Combine both for safe network steps.

How to do it (step by step)

Retry function.

retry(){ local n=$1 d=1 i; shift; for ((i=1;i<=n;i++)); do "$@" && return 0; echo "try $i failed" >&2; sleep $d; d=$((d*2)); done; return 1; }

What it means: Backoff 1, 2, 4 seconds.

Use it.

retry 3 curl -fsS http://localhost:8080/health

Output:

try 1 failed
OK

What it means: Second try worked.

Timeout.

timeout 5 sleep 30; echo $?

Output:

What it means: 124 = timed out.

Both.

retry 3 timeout 10 pg_isready -h db1 || die 'database not ready'

Output:

db1:5432 - accepting connections

What it means: Safe wait for a database.

9. 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
Nikhil, logistics, Pune	Stop the route import if the CSV download fails	`set -euo pipefail`
Grace, hospital, Nairobi	Alert when lab-results sync fails	`rsync -a src/ dst/ \|\| alert`
Hiro, manufacturing, Osaka	Average sensor temp with decimals	`awk '{s+=$1} END{print s/NR}'`
Olivia, retail, Sydney	Retry the payment health check	`retry 5 curl -fs "$URL"`
Ahmed, telecom, Dubai	Disk percent alert	`(( used*100/total > 90 ))`
Sneha, edtech, Bengaluru	Usage error for bad arguments	`exit 2`

10. 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
`unbound variable`	set -u and var not set	Set it or use a default. `echo "${VAR:-}"`
`Script exits silently after ((n++))`	n was 0, result 0, exit 1 under -e	Use n=$((n+1)). `n=$((n+1))`
`value too great for base`	Leading zero like 08 read as octal	Force base 10. `echo $((10#08))`
`syntax error: invalid arithmetic operator`	Decimal in $(( ))	Use bc or awk. `echo '1.5+2' \| bc`
`division by 0`	Divisor empty or zero	Check before dividing. `(( total > 0 )) && echo $((u*100/total))`
`Pipe failure not detected`	No pipefail	Enable it. `set -o pipefail`
`Script stops when grep finds nothing`	grep returns 1 under -e	Allow it. `grep x f \|\| true`
`bc: command not found`	Minimal image	Install bc or use awk. `sudo dnf install -y bc`
`Exit code 141`	SIGPIPE: reader closed early with pipefail	Avoid head on long pipes or ignore 141. `yes \| head -1 \|\| true`

11. Troubleshooting step by step

Why do we need this?

When a script stops or does not stop, find which command returned what code.

What is it?

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

How to do it (step by step)

Print last code.

echo $?

Pipe codes.

echo "${PIPESTATUS[@]}"

Trace.

bash -x script.sh

Show options.

echo $-; set -o | grep -E 'errexit|nounset|pipefail'

Find the failing line.

trap 'echo "failed at line $LINENO" >&2' ERR

Lint.

shellcheck script.sh

12. Lab 1: Disk usage alert with maths and exit codes

Why do we need this?

Write diskcheck.sh that reads disk use for / and exits 1 with a message when it is above a limit, so cron or a monitor can act on it.

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)

Create script.

cat > ~/diskcheck.sh <<'X'
#!/usr/bin/env bash
set -euo pipefail
limit=${1:-85}
used=$(df --output=pcent / | tail -1 | tr -dc '0-9')
if (( used > limit )); then echo "disk / at ${used}% (limit ${limit}%)" >&2; exit 1; fi
echo "disk / ok at ${used}%"
X
chmod +x ~/diskcheck.sh

Run.

~/diskcheck.sh; echo "rc=$?"

Output:

disk / ok at 41%
rc=0

Low limit.

~/diskcheck.sh 10; echo "rc=$?"

Output:

disk / at 41% (limit 10%)
rc=1

Use in a chain.

~/diskcheck.sh 10 || echo 'send alert now'

Output:

disk / at 41% (limit 10%)
send alert now

Lint.

shellcheck ~/diskcheck.sh && echo clean

Output:

clean

13. Lab 2: See set -e and pipefail in action

Why do we need this?

Run the same small script with and without the safe header to see how it changes failures.

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)

Unsafe script.

printf 'cat /nope | sort\necho "after: still running"\n' > ~/t.sh; bash ~/t.sh

Output:

cat: /nope: No such file or directory
after: still running

Safe header.

sed -i '1i set -euo pipefail' ~/t.sh; bash ~/t.sh; echo "rc=$?"

Output:

cat: /nope: No such file or directory
rc=1

Unset var.

bash -uc 'echo "$NOPE"'; echo "rc=$?"

Output:

bash: line 1: NOPE: unbound variable
rc=1

Counter trap.

bash -ec 'n=0; ((n++)); echo reached'; echo "rc=$?"

Output:

rc=1

Fixed counter.

bash -ec 'n=0; n=$((n+1)); echo reached $n'

Output:

reached 1

Clean.

rm ~/t.sh

Ravindra Bagale's Tip

Start every new script with set -euo pipefail, send errors to stderr, and exit with a non-zero code on failure. Then learn the few places -e surprises you (counters, grep with no match, if conditions) so the safe header helps you instead of confusing you.

14. Frequently asked questions

Q. Why is 0 success?

There is one way to succeed and many to fail.

Q. Max exit code?

255; larger wraps around.

Q. Is set -e enough?

No; also check important steps clearly.

Q. Floating point in bash?

No; use bc or awk.

Q. let vs (( ))?

Same; (( )) is clearer.

Q. Exit code of a function?

Its last command or return N.

Q. Why 124?

timeout killed the command.

Q. Ignore one failure?

cmd || true.

15. Interview questions and answers

Beginner level

Q. What is exit code 0?

Success.

Q. Where is the last code?

$?.

Q. Add two numbers?

$((a+b)).

Q. Run b only if a works?

a && b.

Mid level level

Q. What does pipefail do?

Pipe fails if any part fails.

Q. Decimal maths?

bc or awk.

Q. Code 127?

Command not found.

Q. Errors where?

stderr with >&2.

Senior level

Q. Where does set -e not trigger?

In if tests, && || lists, and negated commands.

Q. Why does ((n++)) kill scripts?

Result 0 gives exit 1 under -e.

Q. Design retries?

Limited tries, backoff, timeout, clear final error.

Q. Exit code 137?

Killed by signal 9, often OOM.

Try at home

Write a GST calculator script that takes a price and rate and prints the tax and total with 2 decimals.
Add a retry function to any script that pings a website 3 times before saying it is down.

Practice Tasks

About this scenario

Vidarbha Cotton Mills is a textiles business in Nagpur. Ganesh Wankhede 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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