Bash String Manipulation: Substring, Replace, Length, and More

When you write Bash scripts, many small text tasks do not need sed, awk, or another external command. Parameter expansion can extract part of a string, replace text, strip a prefix, or change case directly inside ${}.

This guide covers the most useful string operations with practical examples for each one.

String Length #

Get the number of characters in a variable using ${#variable}:

filename="backup-2026-04-14.tar.gz"
echo "${#filename}"

24

This returns the character count. For strings that contain only ASCII characters, the character count and byte count are the same.

Extracting a Substring #

Use ${variable:offset:length} to extract part of a string:

str="Hello, World"
echo "${str:7:5}"

World

The offset is zero-based, so 7 starts at the eighth character. 5 is how many characters to return.

Omit the length to extract from the offset to the end of the string:

echo "${str:7}"

World

Negative offsets count from the end of the string. Note the space before the minus sign; it is required to avoid being interpreted as the :- default value syntax:

echo "${str: -5}"

World

Replacing a Substring #

Use ${variable/pattern/replacement} to replace the first match of a pattern:

path="/var/log/app/error.log"
echo "${path/log/logs}"

/var/logs/app/error.log

Only the first occurrence is replaced. To replace every occurrence, double the forward slash:

echo "${path//log/logs}"

/var/logs/app/error.logs

Leave the replacement empty to delete a substring:

version="v1.2.3"
echo "${version/v/}"

1.2.3

The pattern supports glob characters: * matches any sequence of characters, ? matches a single character, and [...] matches a character class.

Removing a Prefix #

Use # to remove a prefix from the beginning of a string. The shortest matching prefix is removed:

file="photo.backup.tar.gz"
echo "${file#*.}"

backup.tar.gz

Use ## to remove the longest possible matching prefix:

echo "${file##*.}"

gz

A common use is stripping a directory path to get just the filename:

filepath="/home/user/documents/report.pdf"
echo "${filepath##*/}"

report.pdf

Removing a Suffix #

Use % to strip a suffix from the end of a string. The shortest match is removed:

file="photo.backup.tar.gz"
echo "${file%.*}"

photo.backup.tar

Use %% for the longest match:

echo "${file%%.*}"

photo

Combining prefix and suffix removal is a clean way to extract a bare filename from a path:

filepath="/home/user/documents/report.pdf"
base="${filepath##*/}"
name="${base%.*}"
echo "$name"

report

Changing Case #

Convert a string to uppercase with ^^:

greeting="hello, world"
echo "${greeting^^}"

HELLO, WORLD

Convert to lowercase with ,,:

input="Hello World"
echo "${input,,}"

hello world

To capitalize only the first character, use a single ^:

word="linux"
echo "${word^}"

Linux

To lowercase only the first character, use a single ,:

word="LINUX"
echo "${word,}"

lINUX

Case conversion requires Bash 4.0 or newer. The default shell on older macOS systems ships with Bash 3.2 and does not support these operators. As a portable alternative, use tr:

echo "$greeting" | tr '[:lower:]' '[:upper:]'

Checking if a String Contains a Substring #

Use a glob pattern inside [[ ]] to test whether a string contains another string:

str="Running on Linux kernel 6.8"
if [[ "$str" == *"Linux"* ]]; then
    echo "Linux detected"
fi

Linux detected

The * wildcards match anything before and after the search pattern. The comparison is case-sensitive. To check case-insensitively, convert both sides to the same case first using ,, before comparing.

For a more detailed walkthrough of substring detection, including case-insensitive matching and edge cases, see How to Check if a String Contains a Substring in Bash . For more on [[ ]] conditionals and string comparison operators, see the guide on Bash comparison operators .

Quick Reference #

For a printable quick reference, see the Bash cheatsheet .

Conclusion #

Bash parameter expansion handles most day-to-day string operations without extra commands, which keeps scripts shorter and faster. For more on working with strings, see the guides on Bash string concatenation and splitting strings by delimiter .

About the authors

Dejan Panovski

Dejan Panovski is the founder of Linuxize, an RHCSA-certified Linux system administrator and DevOps engineer based in Skopje, Macedonia. Author of 800+ Linux tutorials with 20+ years of experience turning complex Linux tasks into clear, reliable guides.

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