Shell Scripting Complete 2: Bash Variables, Quoting, Command Substitution and Parameter Expansion

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

1. An everyday example: labelled jars in the kitchen

Why do we need this?

In a kitchen every jar has a label: sugar, salt, tea. When the recipe says 'add sugar' you pick the jar by its label and use what is inside. If someone puts salt in the sugar jar, the tea tastes bad. Variables are labelled jars, and quoting is the lid that keeps the contents as they are.

What is it?

Jar label = variable name. What is inside = value. Taking from the jar = $name. Lid closed = "$name" (contents stay whole). Filling a jar from a machine = name=$(command). Sharing the jar with neighbours = export.

How to do it (step by step)

2. Create a variable: name=value

Why do we need this?

A script that repeats the same path ten times is hard to change. Put it in one variable and change it in one place.

What is it?

Write name=value with no spaces around =. Names use letters, digits and _ and cannot start with a digit. Read it with $name or ${name}. Use lower case for your own variables; UPPER case is used by the system (PATH, HOME). unset name deletes it. readonly name makes it constant.

How to do it (step by step)

1. Set and print.

city=Pune; echo "I live in $city"

Output:

I live in Pune

What it means: $city was replaced by its value.

2. Spaces break it.

city = Pune

Output:

bash: city: command not found

What it means: bash thought city was a command.

3. Braces join text.

f=report; echo "${f}_2026.csv"

Output:

report_2026.csv

What it means: Without braces bash looks for a variable called f_2026.

4. Constant.

readonly TAX=18; TAX=20

Output:

bash: TAX: readonly variable

What it means: It cannot be changed.

3. Double quotes: keep spaces, still expand

Why do we need this?

File names and user input often have spaces. Without quotes one name becomes two arguments and the command breaks.

What is it?

Inside "double quotes" bash still replaces $var, $(cmd) and $((math)), but it does not split on spaces and does not expand * or ?. Rule: always write "$var" unless you have a reason not to. Use \" for a quote inside and \$ for a literal dollar.

How to do it (step by step)

1. A name with a space.

f='Sales Q3.xlsx'; ls -l $f

Output:

ls: cannot access 'Sales': No such file or directory
ls: cannot access 'Q3.xlsx': No such file or directory

What it means: Unquoted: split into two words.

2. Quoted.

ls -l "$f"

Output:

-rw-r--r-- 1 asha asha 20480 Oct 25 11:00 Sales Q3.xlsx

What it means: One argument, it works.

3. Star stays a star.

p='*.log'; echo "$p"

Output:

*.log

What it means: Inside quotes * is not expanded.

4. Escape inside.

echo "Price: \$5 for \"chai\""

Output:

Price: $5 for "chai"

What it means: Backslash keeps $ and " as text.

4. Single quotes: everything is plain text

Why do we need this?

Sometimes you want $ and * printed exactly, for example in awk programs, regex or a message about $HOME.

What is it?

Inside 'single quotes' nothing is special: no $var, no $( ), no \. You cannot put a single quote inside single quotes; close and reopen: 'it'\''s'. Use single quotes for fixed text and double quotes when you need a variable. $'...' is a third form that understands \n and \t.

How to do it (step by step)

1. Compare.

echo "Home is $HOME"; echo 'Home is $HOME'

Output:

Home is /home/asha
Home is $HOME

What it means: Single quotes print it literally.

2. awk needs single quotes.

awk -F: '{print $1}' /etc/passwd | head -2

Output:

root
bin

What it means: $1 belongs to awk, not to bash.

3. A quote inside.

echo 'Ravi'\''s laptop'

Output:

Ravi's laptop

What it means: Close, escaped quote, reopen.

4. ANSI form.

echo $'line1\nline2'

Output:

line1
line2

What it means: \n became a new line.

5. Command substitution: $(command)

Why do we need this?

Scripts often need today's date, the server name or a count of files inside a message or a file name.

What is it?

$(command) runs the command and puts its output in its place, with the final new line removed. Old style is `command` with backticks; prefer $( ) because it nests and is easy to read. Quote it: "$(command)" keeps spaces and new lines.

How to do it (step by step)

1. Date in a file name.

today=$(date +%F); echo "backup-$today.tgz"

Output:

backup-2026-10-25.tgz

What it means: The date command ran first.

2. Count lines.

users=$(wc -l < /etc/passwd); echo "$users accounts"

Output:

38 accounts

What it means: < gives wc the file without printing its name.

3. Nesting.

echo "Kernel dir: $(basename $(dirname /lib/modules/$(uname -r)/kernel))"

Output:

Kernel dir: 6.1.0-18-amd64

What it means: $( ) inside $( ) is easy to read.

4. Keep new lines.

top3=$(ls /etc | head -3); echo "$top3"

Output:

adduser.conf
alternatives
apt

What it means: Quoted output keeps each name on its own line.

6. Default values and checks: ${var:-x} and friends

Why do we need this?

Scripts must behave well when a value is missing, for example when the user forgets an argument.

What is it?

${var:-default} gives default if var is empty or unset (var is not changed). ${var:=default} also stores it. ${var:?message} stops the script with the message if it is empty. ${var:+other} gives other only when var has a value. These work well with set -u.

How to do it (step by step)

1. Default port.

echo "port ${PORT:-8080}"

Output:

port 8080

What it means: PORT was not set, so 8080 was used.

2. Set and keep.

: "${ENVNAME:=dev}"; echo $ENVNAME

Output:

dev

What it means: The : command does nothing, but the expansion stores dev.

3. Required.

bash -c 'echo "${DB_HOST:?set DB_HOST first}"'

Output:

bash: line 1: DB_HOST: set DB_HOST first

What it means: The script stops with a clear message.

7. String tricks: length, cut, replace, case

Why do we need this?

Renaming files, removing extensions and cleaning input are daily jobs. Bash can do them without calling sed or awk.

What is it?

${#v} length. ${v#pat} removes the shortest match from the start, ${v##pat} the longest. ${v%pat} removes from the end, ${v%%pat} the longest. ${v/old/new} replaces the first, ${v//old/new} all. ${v:start:len} takes a part. ${v^^} upper case, ${v,,} lower case.

How to do it (step by step)

1. Length.

pin=411038; echo ${#pin}

Output:

6

What it means: Six characters.

2. Change extension.

f=invoice.docx; echo "${f%.docx}.pdf"

Output:

invoice.pdf

What it means: %.docx removed the end.

3. Replace all.

s='a b c'; echo "${s// /_}"

Output:

a_b_c

What it means: Every space became _.

4. Upper case and substring.

code=mh12ab; echo "${code^^} ${code:0:4}"

Output:

MH12AB mh12

What it means: ^^ upper, :0:4 first four.

8. Arithmetic: $(( )), (( )) and let

Why do we need this?

Counters, totals, percentages and retries all need numbers.

What is it?

$(( expr )) calculates integers: + - * / % and ** (power). Inside you can write names without $. (( count++ )) changes a variable. Division drops decimals: 7/2 is 3. For decimals use bc or awk. Numbers starting with 0 are read as octal (08 fails), so strip zeros or use 10#08.

How to do it (step by step)

1. Add.

a=40; b=2; echo $((a + b))

Output:

42

What it means: No $ needed inside.

2. Counter.

n=0; ((n++)); ((n+=5)); echo $n

Output:

6

What it means: (( )) changes the variable.

3. Percentage.

used=37; total=50; echo "$(( used * 100 / total ))%"

Output:

74%

What it means: Multiply first, then divide.

4. Decimals with bc.

echo 'scale=2; 7/3' | bc

Output:

2.33

What it means: Bash itself only does integers.

9. Environment variables and export

Why do we need this?

Child programs (python, java, another script) only see variables that you export.

What is it?

A normal variable lives only in the current shell. export name makes it an environment variable that every child process gets. env or printenv lists them. VAR=value command sets it only for that one command. To keep it for every login, add export lines to ~/.bashrc (the environment chapter has details).

How to do it (step by step)

1. Not exported.

color=blue; bash -c 'echo "child sees: $color"'

Output:

child sees:

What it means: The child shell got nothing.

2. Exported.

export color; bash -c 'echo "child sees: $color"'

Output:

child sees: blue

What it means: Now it is in the environment.

3. Only for one command.

TZ=Asia/Tokyo date +%H:%M; date +%H:%M

Output:

14:35
11:05

What it means: TZ changed only for the first date.

4. Find one.

printenv | grep ^LANG

Output:

LANG=en_IN.UTF-8

What it means: printenv shows exported variables.

10. Special variables: $0, $$, $!, $?, $RANDOM, $LINENO

Why do we need this?

Bash fills some variables for you. They are useful for logs, temp files and error messages.

What is it?

$0 script name. $$ PID of the current shell (good for unique temp names). $! PID of the last background job. $? last exit code. $RANDOM a number 0-32767. $LINENO current line number. $SECONDS seconds since the script started. $1, $#, $@ are arguments; chapter 3 covers them.

How to do it (step by step)

1. Unique temp file.

echo "/tmp/report.$$"

Output:

/tmp/report.48213

What it means: Different every run.

2. Background PID.

sleep 30 & echo "started $!"

Output:

[1] 48250
started 48250

What it means: Save $! to wait for or kill it later.

3. Timer.

bash -c 'sleep 2; echo "took $SECONDS s"'

Output:

took 2 s

What it means: Easy timing for scripts.

4. Safer temp file.

t=$(mktemp); echo "$t"

Output:

/tmp/tmp.Xk29aQ

What it means: mktemp makes a random, private file.

11. 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)

12. 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)

13. Troubleshooting step by step

Why do we need this?

Print the variable with markers so you can see spaces and empty values.

What is it?

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

How to do it (step by step)

1. See the exact value.

echo "[$name]"

2. Show type and attributes.

declare -p name

3. Is it exported?

printenv name

4. Trace expansion.

bash -x s.sh

5. Find unquoted use.

shellcheck s.sh

6. Check hidden characters.

printf '%q\n' "$name"

14. Lab 1: A backup name builder

Why do we need this?

Write a script that builds a safe backup file name from the host, date and a folder, using variables, $( ) and string tricks.

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)

1. Folder.

mkdir -p ~/lab38 && cd ~/lab38

2. Variables.

src="/var/www/my site"; host=$(hostname -s); day=$(date +%F)

3. Build the name.

base=${src##*/}; name="${host}-${base// /_}-${day}.tgz"; echo "$name"

Output:

web01-my_site-2026-10-25.tgz

4. Length check.

echo ${#name}

Output:

28

5. Upper case tag.

echo "${host^^}"

Output:

WEB01

6. Default target folder.

echo "${DEST:-/backup}/$name"

Output:

/backup/web01-my_site-2026-10-25.tgz

15. Lab 2: Quoting experiments and export

Why do we need this?

See with your own eyes how quotes change the result, and how export passes values to child 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)

1. Make tricky files.

touch 'a b.txt' c.txt && ls

Output:

a b.txt  c.txt

2. Unquoted loop.

for f in $(ls); do echo "[$f]"; done

Output:

[a]
[b.txt]
[c.txt]

3. Glob instead.

for f in *.txt; do echo "[$f]"; done

Output:

[a b.txt]
[c.txt]

4. Child script.

printf '#!/bin/bash\necho "mode=$MODE"\n' | tee child.sh && chmod +x child.sh

5. Without export.

MODE=prod; ./child.sh

Output:

mode=

6. With export.

export MODE; ./child.sh

Output:

mode=prod

7. Maths.

files=$(ls | wc -l); echo "$(( files * 2 )) after copy"

Output:

6 after copy

16. Frequently asked questions

17. Interview questions and answers

Beginner level

Mid level level

Senior level

18. Related chapters

• Linux Ops Refresh for DevOps
• Bash Scripting for Automation
• Linux grep and find Commands with Examples
• How to Set Up Passwordless SSH Between Linux Servers

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