Shell Scripting Complete 6: Bash Functions, local, return and Reusable Script Libraries

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

Functions give a name to a block of commands so you can reuse it, test it and read the script like a story: check_disk, backup_db, send_alert. This chapter covers defining and calling functions, arguments inside functions, return codes versus output, local variables, returning values with echo and nameref, and building a shared library file that many scripts source. You will also learn a main function, traps for cleanup, and a simple logging library.

What you'll learn in this chapter

Define and call functions
Use $1, $@ inside functions
Know return vs echo
Keep variables local
Build and source a library
Use main and trap for clean scripts

1. An everyday example: a pressure cooker recipe card

Why do we need this?

Every Pune home has a few fixed steps: 'make tadka' is the same for dal, sabzi and poha. Mother does not explain it each time; she just says 'tadka kar'. A function is that named step. A recipe file shared by the whole family is a library, and each recipe card can just say 'use the tadka step'.

What is it?

Named step = function. Ingredients you pass = arguments $1 $2. 'Done well' or 'burnt' = return code. The cooked dish = output from echo. Your own bowl = local variable. Family recipe file = library you source.

How to do it (step by step)

Define a function before you call it.

2. Define and call a function

Why do we need this?

Repeated blocks make scripts long and bugs repeat too. One function means one place to fix.

What is it?

Write name() { commands; } and call it like a command: name. The keyword form function name { } also works in bash. Names use letters, digits and _. The function must be defined before the line that calls it runs. declare -f name shows its code; type name tells you it is a function.

How to do it (step by step)

Define and call.

greet() { echo "Namaskar from $(hostname)"; }; greet

Output:

Namaskar from web01

What it means: Called like any command.

See it.

type greet

Output:

greet is a function
greet () 
{ 
    echo "Namaskar from $(hostname)"
}

What it means: Bash stored the code.

Called too early.

bash -c 'early; early(){ echo hi; }'

Output:

bash: line 1: early: command not found

What it means: Define first.

One-line body needs ;

bad(){ echo x }

Output:

bash: syntax error near unexpected token `}'

What it means: Put ; before }.

3. Arguments inside functions

Why do we need this?

A function that can take a server name or a path works for every server and path.

What is it?

Inside a function $1, $2, $#, $@ are the function's own arguments, not the script's. To use the script's arguments, pass them in: main "$@". $0 is still the script name; FUNCNAME[0] is the function name. Check required arguments with ${1:?usage} or $#.

How to do it (step by step)

Two arguments.

add_user(){ echo "adding $1 to group $2"; }; add_user meera devs

Output:

adding meera to group devs

What it means: $1 and $2 belong to the call.

All arguments.

each(){ for x in "$@"; do echo "- $x"; done; }; each 'Pune Camp' Kothrud

Output:

- Pune Camp
- Kothrud

What it means: "$@" keeps spaces.

Required.

need(){ local f=${1:?need a file}; echo "using $f"; }; need

Output:

bash: 1: need a file

What it means: Stops with a message.

Its own name.

whoami_fn(){ echo "in ${FUNCNAME[0]} with $# args"; }; whoami_fn a b

Output:

in whoami_fn with 2 args

What it means: Useful in log lines.

4. return codes vs output

Why do we need this?

Beginners try to return text with return. return only gives a number 0-255 meaning success or failure.

What is it?

return N ends the function with exit code N (0 success). Without return, the code is that of the last command. To give back data, print it with echo or printf and capture it: x=$(fn). Use the code for yes/no and the output for values, so a function can be used both in if and in $( ).

Need	Use
yes/no	return 0 or 1
a value	echo it, x=$(fn)
many values	array via nameref
an error message	echo ... >&2

How to do it (step by step)

Yes/no.

is_root(){ [[ $EUID -eq 0 ]]; }; if is_root; then echo root; else echo 'not root'; fi

Output:

not root

What it means: The last test is the return code.

Return a value by echo.

disk_pct(){ df --output=pcent "${1:-/}" | tail -1 | tr -dc 0-9; }; p=$(disk_pct /); echo "root at $p%"

Output:

root at 41%

What it means: Captured with $( ).

return with text fails.

bad(){ return hello; }; bad

Output:

bash: return: hello: numeric argument required

What it means: return takes numbers only.

Over 255 wraps.

f(){ return 300; }; f; echo $?

Output:

What it means: 300 mod 256 = 44.

5. local variables and scope

Why do we need this?

A function that changes a global variable called i or file can break the caller in strange ways.

What is it?

By default all variables in bash are global, even those set inside functions. local name=value makes a variable live only inside the function and the functions it calls. Make every helper variable local. Note: local x=$(cmd) hides cmd's exit code; write local x; x=$(cmd) when you need it.

How to do it (step by step)

Global leak.

count=10; f(){ count=0; }; f; echo $count

Output:

What it means: The caller's value changed.

local.

count=10; g(){ local count=0; }; g; echo $count

Output:

What it means: Caller is safe.

Hidden exit code.

h(){ local out=$(false); echo "code $?"; }; h

Output:

code 0

What it means: local itself succeeded.

Correct form.

k(){ local out; out=$(false); echo "code $?"; }; k

Output:

code 1

What it means: Now you see the real code.

6. Return several values: nameref and globals

Why do we need this?

Sometimes a function must fill an array or set two values, and echo is clumsy for that.

What is it?

local -n ref=$1 (bash 4.3+) makes ref another name for the caller's variable whose name you pass. Assigning to ref changes the caller's variable. Use a clear name (not the same as the caller's) to avoid loops. Older style: set a documented global like RESULT.

How to do it (step by step)

Fill a variable.

get_os(){ local -n _out=$1; _out=$(. /etc/os-release; echo $ID); }; get_os os; echo $os

Output:

rhel

What it means: The caller's os was set.

Fill an array.

list_users(){ local -n _arr=$1; mapfile -t _arr < <(cut -d: -f1 /etc/passwd | head -3); }; list_users u; echo "${#u[@]}: ${u[*]}"

Output:

3: root bin daemon

What it means: Arrays are covered in chapter 7.

Two values by echo and read.

mem(){ free -m | awk '/Mem/{print $2, $3}'; }; read -r total used < <(mem); echo "$used of $total MB"

Output:

3100 of 7820 MB

What it means: read splits the output.

7. A main function and script structure

Why do we need this?

Big scripts are easier to read when the top is settings, the middle is functions and the bottom is one call to main.

What is it?

Order: shebang, set -euo pipefail, constants, functions, then main "$@" as the last line. Because bash reads the whole file before main runs, functions can be in any order. The guard [[ ${BASH_SOURCE[0]} == $0 ]] && main "$@" runs main only when the file is executed, not when it is sourced (useful for testing).

How to do it (step by step)

Skeleton.

cat deploy.sh

Output:

#!/usr/bin/env bash
set -euo pipefail
APP=shop
log(){ echo "[$(date +%T)] $*"; }
build(){ log "building $APP"; }
ship(){ log "shipping to $1"; }
main(){ build; ship "${1:-stage}"; }
main "$@"

What it means: Settings, functions, main.

Run it.

./deploy.sh prod

Output:

[10:15:02] building shop
[10:15:02] shipping to prod

What it means: main got the script's arguments.

Source guard.

printf 'hi(){ echo hi; }\n[[ ${BASH_SOURCE[0]} == $0 ]] && hi\n' | tee g.sh; source g.sh; bash g.sh

Output:

hi

What it means: Sourcing did not run it; executing did.

8. Libraries: source a shared file

Why do we need this?

Logging, alerts and checks are needed by many scripts. Keep them in one file and load it everywhere.

What is it?

source file (or . file) runs the file in the current shell, so its functions and variables become available. A library contains only functions and constants, no actions at the top level. Find it relative to the script: source "$(dirname "${BASH_SOURCE[0]}")/lib/common.sh". Prefix function names (log_info, net_check) to avoid clashes. Keep it in git and in /usr/local/lib for servers.

How to do it (step by step)

Write a library.

mkdir -p lib; printf 'log_info(){ printf "%%s INFO %%s\\n" "$(date +%%F_%%T)" "$*"; }\nlog_err(){ printf "%%s ERROR %%s\\n" "$(date +%%F_%%T)" "$*" >&2; }\n' | tee lib/common.sh

What it means: Only functions, no actions at the top.

Use it.

source lib/common.sh; log_info 'backup started'; log_err 'disk full'

Output:

2026-10-25_10:20:01 INFO backup started
2026-10-25_10:20:01 ERROR disk full

What it means: Both functions loaded.

Relative path.

here=$(dirname "${BASH_SOURCE[0]}"); echo "$here/lib/common.sh"

Output:

./lib/common.sh

What it means: Works from any folder.

List loaded functions.

declare -F | grep log_

Output:

declare -f log_err
declare -f log_info

What it means: declare -F lists names only.

9. trap: cleanup and error handlers

Why do we need this?

Temp files and lock files must be removed even when the script fails or the user presses Ctrl+C.

What is it?

trap 'commands' SIGNALS runs commands when the signal arrives. EXIT runs at every end (normal or error). ERR runs when a command fails (with set -E it works inside functions too). INT is Ctrl+C, TERM is kill. Put cleanup in a function: trap cleanup EXIT. $LINENO and $BASH_COMMAND help in error messages.

How to do it (step by step)

Cleanup.

bash -c 't=$(mktemp); cleanup(){ rm -f "$t"; echo "removed $t"; }; trap cleanup EXIT; echo work; false; echo never'

Output:

work
removed /tmp/tmp.Q8x1

What it means: Cleanup ran even though false ended nothing here; it runs at every exit.

Error line.

bash -c 'set -eE; trap "echo failed at line \$LINENO: \$BASH_COMMAND" ERR; ls /nope'

Output:

ls: cannot access '/nope': No such file or directory
failed at line 1: ls /nope

What it means: Shows which command failed.

Ctrl+C.

trap 'echo " stopping nicely"; exit 130' INT; sleep 100

Output:

^C stopping nicely

What it means: Exit 130 = 128 + signal 2.

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
Rutuja, bank ops, Pune	Shared log library for 40 scripts	`source /usr/local/lib/rb/log.sh`
James, SaaS, Austin	main "$@" structure	`main "$@"`
Sakura, retail, Osaka	trap cleanup for temp files	`trap cleanup EXIT`
Karim, oil and gas, Abu Dhabi	is_up function used in if	`if is_up db1; then`
Elena, healthcare, Milan	local everywhere in helpers	`local f=$1`
Ganesh, telecom, Hyderabad	Nameref to return arrays	`local -n out=$1`

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
`command not found for my function`	Called before defined	Define first or use main at the end. `main "$@"`
syntax error near unexpected token `}'	Missing ; before } on one line	Add ;. `f(){ echo x; }`
`numeric argument required`	return with text	echo the value instead. `echo "$val"`
`Caller's variable changed`	Not local	Use local. `local i`
`$1 is empty in the function`	Script args not passed	Call with "$@". `main "$@"`
`No such file when sourcing library`	Path relative to current folder	Use BASH_SOURCE. `source "$(dirname "${BASH_SOURCE[0]}")/lib.sh"`
`Error code lost`	local x=$(cmd)	Split declare and assign. `local x; x=$(cmd)`
`circular name reference`	nameref same name as caller var	Use a unique ref name. `local -n _ref=$1`
`Temp files left after Ctrl+C`	No trap	Add trap cleanup EXIT. `trap cleanup EXIT`

12. Troubleshooting step by step

Why do we need this?

See which functions exist, trace calls and check return codes.

What is it?

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

How to do it (step by step)

List functions.

declare -F

Show one.

declare -f log_info

Trace with function names.

PS4='+${FUNCNAME[0]:-main}:$LINENO: ' bash -x s.sh

Call stack.

echo "${FUNCNAME[*]}"

Return code.

myfn; echo $?

Lint.

shellcheck s.sh lib/*.sh

13. Lab 1: Build a small library

Why do we need this?

Create lib/ops.sh with log, die and need_cmd functions, then use it from two scripts.

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 ~/lab42/lib && cd ~/lab42

Library.

printf 'log(){ echo "[$(date +%%T)] $*"; }\ndie(){ echo "FATAL: $*" >&2; exit 1; }\nneed_cmd(){ command -v "$1" &>/dev/null || die "$1 not installed"; }\n' | tee lib/ops.sh

Script A.

printf '#!/bin/bash\nsource "$(dirname "$0")/lib/ops.sh"\nneed_cmd tar\nlog "tar is ready"\n' | tee a.sh && chmod +x a.sh && ./a.sh

Output:

[10:30:11] tar is ready

Script B fails nicely.

printf '#!/bin/bash\nsource "$(dirname "$0")/lib/ops.sh"\nneed_cmd nosuchtool\n' | tee b.sh && chmod +x b.sh && ./b.sh; echo $?

Output:

FATAL: nosuchtool not installed
1

Run from elsewhere.

cd /tmp && ~/lab42/a.sh

Output:

[10:30:20] tar is ready

List functions.

bash -c 'source ~/lab42/lib/ops.sh; declare -F'

Output:

declare -f die
declare -f log
declare -f need_cmd

14. Lab 2: main, local and trap together

Why do we need this?

Write report.sh with a main function, local variables, a value-returning function and a trap that removes the temp file.

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.

cd ~/lab42 && printf '#!/usr/bin/env bash\nset -euo pipefail\ntmp=$(mktemp)\ncleanup(){ rm -f "$tmp"; echo "cleaned"; }\ntrap cleanup EXIT\ncount_files(){ local d=$1; find "$d" -maxdepth 1 -type f | wc -l; }\nmain(){ local dir=${1:-/etc}; local n; n=$(count_files "$dir"); echo "$dir has $n files" | tee "$tmp"; }\nmain "$@"\n' | tee report.sh && chmod +x report.sh

Run.

./report.sh /etc

Output:

/etc has 94 files
cleaned

Default.

./report.sh

Output:

/etc has 94 files
cleaned

Failure still cleans.

./report.sh /nope; echo $?

Output:

find: '/nope': No such file or directory
cleaned
1

Temp gone.

ls /tmp/tmp.* 2>/dev/null | wc -l

Output:

dir stayed local.

bash -c 'source <(sed "\$d" report.sh); main /tmp; echo "[${dir:-}]"'

Output:

/tmp has 3 files
[]
cleaned

Ravindra Bagale's Tip

Make every variable inside a function local, use return only for success or failure, and echo the data you want to give back. Put shared functions in a library and source it with a path built from BASH_SOURCE.

15. Frequently asked questions

Q. name() or function name?

Both in bash; name() is portable.

Q. Can functions call each other?

Yes, and themselves (recursion).

Q. How to return a string?

echo it and use $( ).

Q. Does $( ) slow things down?

It forks a subshell; fine for most scripts.

Q. source vs running a script?

source runs in the current shell.

Q. Export a function to child scripts?

export -f name.

Q. Unset a function?

unset -f name.

Q. Where to keep libraries?

Repo lib/ or /usr/local/lib.

16. Interview questions and answers

Beginner level

Q. Define a function?

name() { ...; }.

Q. First argument in a function?

$1.

Q. Load a library?

source file.

Q. Function exit code?

return N or last command.

Mid level level

Q. return vs echo?

Code vs data.

Q. Why local?

Avoid changing globals.

Q. What does trap EXIT do?

Runs cleanup on any exit.

Q. Pass script args to main?

main "$@".

Senior level

Q. Design a shared bash library?

Prefixed functions, no top-level actions, source guard, versioned, shellcheck in CI.

Q. local x=$(cmd) issue?

Masks exit code; split it.

Q. Return arrays?

nameref local -n or print lines with mapfile.

Q. Make scripts testable?

Functions + source guard; test with bats.

Try at home

Write a function is_port_open HOST PORT that returns 0 or 1 and use it in an if.
Move it into lib/net.sh and source it from two different scripts.

Practice Tasks

About this scenario

Ahmednagar Sugar Co-op is a sugar co-operative business in Ahilyanagar. Balasaheb Pawar 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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