Shell Scripting Complete 8: String Manipulation and Parameter Expansion (${var#}, ${var%}, ${var//}) Explained Simply

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

Scripts work with text all the time: file names, dates, versions, URLs, log lines and user input. Bash can cut, replace, trim, change case and measure text by itself, without calling sed or awk. This is called parameter expansion. In this chapter you learn ${#var} for length, ${var:start:len} for substrings, # and % to remove prefixes and suffixes, / to replace, ^^ and ,, for case, defaults with :- := :? and :+, indirect names, and printf for clean formatting. Every command has an everyday example first and then examples from many industries.

What you'll learn in this chapter

Measure length and cut substrings
Remove prefixes and suffixes with # and %
Replace text with / and //
Change case with ^^ and ,,
Use defaults :- := :? :+
Format output with printf

1. An everyday example: a tailor and a long cloth

Why do we need this?

A tailor gets a long cloth. He measures it, cuts the first part, trims the edges, and sometimes changes a patch. He never sends the cloth to another shop for this; he has scissors and tape on his own table. Bash parameter expansion is the scissors and tape for text, right inside your script.

What is it?

Cloth = the string in a variable. Tape = ${#var} length. Cutting a piece = ${var:start:len}. Trimming the left edge = ${var#pattern}. Trimming the right edge = ${var%pattern}. Changing a patch = ${var/old/new}. Spare cloth when none given = ${var:-default}.

How to do it (step by step)

Always wrap the result in double quotes.

2. Length and substrings

Why do we need this?

You often need to check if a PIN code has 6 digits, or take the year from a date like 2026-10-25.

What is it?

${#v} gives the number of characters. ${v:start} gives everything from position start (counting from 0). ${v:start:len} gives len characters. A negative start counts from the end but needs a space: ${v: -4}. These are built into bash, so they are fast even inside big loops.

Form	Gives
${#v}	length
${v:2}	from index 2 to end
${v:0:4}	first 4 chars
${v: -3}	last 3 chars
${v:2:-1}	from 2, drop last 1

How to do it (step by step)

Length of a PIN.

pin=411038; echo ${#pin}

Output:

What it means: Six characters.

Year from a date.

d=2026-10-25; echo "${d:0:4}"

Output:

What it means: Start 0, take 4.

Month.

echo "${d:5:2}"

Output:

What it means: Start 5, take 2.

Last four of a card.

card=4111222233334444; echo "XXXX-${card: -4}"

Output:

XXXX-4444

What it means: Space before minus is needed.

Check length in an if.

mob=98220; (( ${#mob} == 10 )) || echo 'mobile must be 10 digits'

Output:

mobile must be 10 digits

What it means: Simple input validation.

3. Remove a prefix with # and ##

Why do we need this?

Paths and URLs have parts you want to drop: the folder, the https:// part, a v from v1.4.2.

What is it?

${v#pat} removes the shortest match of pat from the start. ${v##pat} removes the longest match. pat is a glob, so * means any text. ${path##*/} keeps only the file name, like basename, but without starting a new process.

How to do it (step by step)

File name from a path.

p=/var/log/nginx/access.log; echo "${p##*/}"

Output:

access.log

What it means: Longest */ removed.

Shortest vs longest.

echo "${p#*/}"

Output:

var/log/nginx/access.log

What it means: Only the first / removed.

Drop the scheme.

u=https://shop.example.in/cart; echo "${u#*://}"

Output:

shop.example.in/cart

What it means: Everything up to :// removed.

Version without v.

tag=v1.4.2; echo "${tag#v}"

Output:

1.4.2

What it means: A fixed prefix works too.

4. Remove a suffix with % and %%

Why do we need this?

Changing report.csv to report.json, or getting the folder from a path, needs the end removed.

What is it?

${v%pat} removes the shortest match from the end; ${v%%pat} removes the longest. ${f%.*} drops the last extension. ${f%%.*} drops all extensions (backup.tar.gz becomes backup). ${p%/*} gives the folder, like dirname. Easy memory trick: # is on the left of $ on the keyboard, % is on the right.

How to do it (step by step)

Change extension.

f=sales-2026.csv; echo "${f%.csv}.json"

Output:

sales-2026.json

What it means: Remove .csv, add .json.

Last vs all extensions.

b=backup.tar.gz; echo "${b%.*} ${b%%.*}"

Output:

backup.tar backup

What it means: % shortest, %% longest.

Folder of a path.

p=/srv/app/conf/app.yml; echo "${p%/*}"

Output:

/srv/app/conf

What it means: Like dirname.

Rename all in a loop.

for f in *.jpeg; do mv -- "$f" "${f%.jpeg}.jpg"; done; ls

Output:

a.jpg b.jpg

What it means: No sed needed.

5. Replace text with / and //

Why do we need this?

Spaces in names, a wrong city code, or a password in a log line: you need to swap text.

What is it?

${v/old/new} replaces the first match. ${v//old/new} replaces all. ${v/#old/new} only at the start, ${v/%old/new} only at the end. Leave new empty to delete: ${v// /}. old is a glob pattern, so ${v//[0-9]/#} hides all digits.

Form	Does
${v/a/b}	first a to b
${v//a/b}	every a to b
${v/#a/b}	a at start
${v/%a/b}	a at end
${v// /}	delete all spaces

How to do it (step by step)

Spaces to underscores.

n='Q3 sales report.pdf'; echo "${n// /_}"

Output:

Q3_sales_report.pdf

What it means: All spaces replaced.

First only.

echo "${n/ /_}"

Output:

Q3_sales report.pdf

What it means: Single slash = first.

Mask digits.

acct=AC-9876-5432; echo "${acct//[0-9]/*}"

Output:

AC-****-****

What it means: Pattern [0-9].

Fix at the end.

host=web1.local; echo "${host/%.local/.prod.in}"

Output:

web1.prod.in

What it means: /% anchors the end.

Remove all dashes.

ph=98-22-11-00-00; echo "${ph//-/}"

Output:

9822110000

What it means: Empty replacement deletes.

6. Change case with ^ ^^ , ,,

Why do we need this?

Users type Yes, YES or yes. Environment names should be lowercase in paths but uppercase in labels.

What is it?

${v^^} makes all uppercase, ${v,,} makes all lowercase. ${v^} uppercases only the first letter, ${v,} lowercases only the first. This needs bash 4 or newer. Compare answers after lowercasing: [[ ${ans,,} == y* ]].

How to do it (step by step)

Uppercase label.

env=prod; echo "[${env^^}]"

Output:

[PROD]

What it means: All letters up.

Lowercase input.

ans=YeS; echo "${ans,,}"

Output:

yes

What it means: Easy to compare.

Capitalise a name.

city=pune; echo "${city^}"

Output:

Pune

What it means: Only first letter.

Case-free yes check.

read -rp 'Continue? ' a; [[ ${a,,} == y* ]] && echo go

Output:

Continue? Y
go

What it means: Y, y, yes all work.

7. Defaults: :- := :? :+

Why do we need this?

Scripts must behave well when a variable or argument is missing, especially with set -u.

What is it?

${v:-x} uses x if v is empty or unset, but does not change v. ${v:=x} also sets v to x. ${v:?message} stops the script with message if v is empty. ${v:+x} gives x only if v has a value (good for optional flags). Without the colon (${v-x}) only unset counts, not empty.

Form	If v is empty
${v:-x}	gives x
${v:=x}	gives x and sets v
${v:?msg}	prints msg, exits
${v:+x}	gives nothing (x only if set)

How to do it (step by step)

Default env.

env=${1:-dev}; echo "deploying to $env"

Output:

deploying to dev

What it means: No argument given.

Set if missing.

unset PORT; : "${PORT:=8080}"; echo $PORT

Output:

What it means: : runs nothing but expands.

Required value.

unset DB_HOST; echo "${DB_HOST:?DB_HOST is required}"

Output:

bash: DB_HOST: DB_HOST is required

What it means: Script stops here.

Optional flag.

verbose=1; echo rsync -a ${verbose:+-v} src/ dst/

Output:

rsync -a -v src/ dst/

What it means: Flag only when set.

8. Indirect names and listing variables

Why do we need this?

Config like PORT_web and PORT_db is easier when you build the name from another variable.

What is it?

${!name} reads the variable whose name is stored in name. ${!PREFIX@} lists all variable names that start with PREFIX. declare -n creates a nameref, a second name for a variable, useful in functions. Prefer associative arrays for new code; indirect names are for existing env-style config.

How to do it (step by step)

Indirect read.

PORT_web=80; PORT_db=5432; s=db; k=PORT_$s; echo "${!k}"

Output:

What it means: Name built from s.

List by prefix.

echo ${!PORT_@}

Output:

PORT_db PORT_web

What it means: All names starting PORT_.

Nameref.

set_out(){ declare -n r=$1; r=done; }; set_out status; echo $status

Output:

done

What it means: Function wrote into status.

9. Trim spaces and clean input

Why do we need this?

Values from CSV files and forms often have extra spaces or Windows line endings that break comparisons.

What is it?

Remove a Windows carriage return with ${v%$'\r'}. Trim leading spaces with ${v#"${v%%[![:space:]]*}"} and trailing with ${v%"${v##*[![:space:]]}"}. That looks hard, so many people wrap it in a trim function once. read with default IFS also trims spaces at both ends.

How to do it (step by step)

Drop Windows CR.

line=$'pune\r'; line=${line%$'\r'}; [[ $line == pune ]] && echo clean

Output:

clean

What it means: Comparison now works.

Trim function.

trim(){ local s=$1; s=${s#"${s%%[![:space:]]*}"}; s=${s%"${s##*[![:space:]]}"}; printf '%s' "$s"; }; echo "[$(trim '   Asha Patil  ')]"

Output:

[Asha Patil]

What it means: Both ends cleaned, middle space kept.

read trims too.

read -r x <<< '   nashik   '; echo "[$x]"

Output:

[nashik]

What it means: Default IFS trims.

10. printf for clean output

Why do we need this?

echo behaves differently on different systems. Reports need aligned columns and fixed decimals.

What is it?

printf FORMAT args. %s is a string, %d an integer, %5.2f a number with 2 decimals, %-10s a left-aligned 10-wide column, \n a new line. printf repeats the format for extra arguments. printf -v var stores the result in a variable instead of printing. printf '%(%F %T)T' prints the time without calling date.

How to do it (step by step)

Aligned table.

printf '%-8s %6s\n' City Orders Pune 1200 Nagpur 85

Output:

City     Orders
Pune       1200
Nagpur       85

What it means: Format repeats per pair.

Two decimals.

printf 'Total: %.2f\n' 1499.5

Output:

Total: 1499.50

What it means: Money format.

Zero padding.

printf 'INV-%05d\n' 42

Output:

INV-00042

What it means: Invoice numbers.

Into a variable.

printf -v stamp '%(%Y%m%d)T' -1; echo "backup-$stamp.tgz"

Output:

backup-20261025.tgz

What it means: -1 means now.

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)

Team and city	How they use it	Command
Aditi, e-commerce, Pune	Rename product images .jpeg to .jpg	`mv "$f" "${f%.jpeg}.jpg"`
Ben, broadcasting, London	File name from upload path	`echo "${p##*/}"`
Min-jun, gaming, Seoul	Strip v from release tags	`echo "${tag#v}"`
Layla, airline, Cairo	Mask passport numbers in logs	`echo "${id//[0-9]/*}"`
Mateo, fintech, Buenos Aires	Invoice IDs with zero padding	`printf 'INV-%05d' "$n"`
Swapnil, insurance, Nagpur	Default region when not passed	`region=${1:-ap-south-1}`

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)

Error you see	Why it happens	How to fix
`bad substitution`	Typo or sh instead of bash	Run with bash; check braces. `bash script.sh`
`${v: -3} gives the whole string`	Wrote ${v:-3} without space	Add a space before minus. `echo "${v: -3}"`
`^^ prints bad substitution on Mac`	bash 3.2	Use bash 4+ or tr. `tr '[:lower:]' '[:upper:]'`
`Comparison fails though text looks same`	Hidden \r from Windows file	Strip CR. `v=${v%$'\r'}`
`Removed too much of the path`	Used ## instead of #	Pick shortest or longest correctly. `echo "${p#*/}"`
`parameter null or not set`	${v:?} triggered	Pass the value or set it. `export DB_HOST=db1`
`printf: invalid number`	Text passed to %d	Pass a number or use %s. `printf '%s\n' "$x"`
`echo -e prints -e`	sh echo differs	Use printf. `printf 'a\tb\n'`
`Glob in pattern matched files`	Unquoted result	Quote the expansion. `echo "${v//x/*}"`

13. Troubleshooting step by step

Why do we need this?

Print the value inside brackets before and after each step so you can see hidden spaces and characters.

What is it?

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

How to do it (step by step)

Show with brackets.

printf '[%s]\n' "$v"

Show hidden characters.

printf '%s' "$v" | od -c

Length.

echo ${#v}

Bash version.

echo $BASH_VERSION

Trace the step.

set -x; out=${v%.*}; set +x

Lint.

shellcheck s.sh

14. Lab 1: Bulk rename and clean file names

Why do we need this?

Create messy file names, then clean them with only parameter expansion: spaces to underscores, lowercase, and .jpeg to .jpg.

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 and files.

mkdir -p ~/lab44 && cd ~/lab44 && touch 'Diwali Sale.JPEG' 'Shop Front.jpeg' 'logo.jpeg'

See them.

ls

Output:

Diwali Sale.JPEG  Shop Front.jpeg  logo.jpeg

Dry run.

for f in *; do n=${f,,}; n=${n// /_}; n=${n%.jpeg}.jpg; echo "$f -> $n"; done

Output:

Diwali Sale.JPEG -> diwali_sale.jpg
Shop Front.jpeg -> shop_front.jpg
logo.jpeg -> logo.jpg

Rename.

for f in *; do n=${f,,}; n=${n// /_}; n=${n%.jpeg}.jpg; [[ $f == "$n" ]] || mv -n -- "$f" "$n"; done; ls

Output:

diwali_sale.jpg  logo.jpg  shop_front.jpg

Clean up.

cd ~ && rm -r ~/lab44

15. Lab 2: Parse a URL and a version string

Why do we need this?

Split a URL into scheme, host and path, and split a version into major, minor and patch, all without sed or cut.

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)

Set values.

u='https://api.shop.in/v2/orders?id=7'; ver=v3.12.5

Scheme.

echo "${u%%://*}"

Output:

https

Host.

rest=${u#*://}; echo "${rest%%/*}"

Output:

api.shop.in

Path without query.

path=/${rest#*/}; echo "${path%%\?*}"

Output:

/v2/orders

Version parts.

v=${ver#v}; major=${v%%.*}; minor=${v#*.}; minor=${minor%.*}; patch=${v##*.}; echo "$major $minor $patch"

Output:

3 12 5

Bump patch.

echo "v$major.$minor.$((patch+1))"

Output:

v3.12.6

Ravindra Bagale's Tip

Learn # for the start and % for the end, one sign for the shortest match and two for the longest. With that rule plus ${v//old/new} and ${v:-default}, you can replace most small sed and basename calls in your scripts.

16. Frequently asked questions

Q. Is parameter expansion faster than sed?

Yes for small strings, no new process.

Q. Does it work in sh?

# % :- yes; / ^^ substrings are bash only.

Q. Regex in patterns?

No, globs only; use [[ =~ ]] for regex.

Q. Count characters with spaces?

${#v} counts them.

Q. Unicode length?

Correct in a UTF-8 locale.

Q. Difference :- and -?

Colon also treats empty as missing.

Q. Why printf over echo?

Same output everywhere.

Q. Split on a comma?

IFS=, read -ra parts <<< "$v".

17. Interview questions and answers

Beginner level

Q. Length of a variable?

${#v}.

Q. First 3 characters?

${v:0:3}.

Q. Default value?

${v:-x}.

Q. All uppercase?

${v^^}.

Mid level level

Q. File name without folder?

${p##*/}.

Q. Change extension?

${f%.*}.new.

Q. Replace all spaces?

${v// /_}.

Q. Fail if missing?

${v:?msg}.

Senior level

Q. Why avoid basename in loops?

Each call starts a process; expansion is built in.

Q. When use sed instead?

Regex or multi-line edits on files.

Q. Nameref use?

Return values into a caller's variable.

Q. Handle CRLF input?

Strip \r with ${v%$'\r'} or dos2unix.

Try at home

Write a script that takes a full name like 'asha patil' and prints 'Asha Patil' and the initials 'AP'.
Make a printf table of 5 cities and their PIN codes with aligned columns.

Practice Tasks

About this scenario

Maple Leaf Books is a bookselling business in Toronto. Emily Tremblay looks after its IT and cloud work. They count money in CAD and run their AWS resources in the ca-central-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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