String Manipulation in Linux Shell Scripting - Substrings, Length & Regex

Linux Bash supports a powerful set of built-in string manipulation operations. While older Bourne shell scripts frequently invoked external tools like expr or awk, modern Bash provides instant Parameter Expansion with the ${} syntax that executes directly within the shell memory without launching child subshells.

1. Finding String Length

The fastest and cleanest way to calculate the length (character count) of a string variable is ${#string}:

#!/bin/bash
# Note: Never put spaces around '=' during variable assignment!
str="Veewom Linux Programming"

# Pure Bash parameter expansion (Recommended - very fast)
len=${#str}
echo "Length using \${#str}: $len"

# Legacy UNIX method using expr (Requires quotes if string contains spaces)
len_expr=$(expr length "$str")
echo "Length using expr: $len_expr"

2. Substring Extraction (Slicing)

Extracting a specific slice of a string is done using ${string:offset:length}, where offset is the 0-based starting index:

3. Substring Search and Replace

Bash allows pattern-based text replacement without running sed:

#!/bin/bash
msg="I love unix. unix is fast and unix is robust."

# Replace first match: ${var/pattern/replacement}
echo "${msg/unix/Linux}"
# Output: I love Linux. unix is fast and unix is robust.

# Replace ALL matches globally: ${var//pattern/replacement}
echo "${msg//unix/Linux}"
# Output: I love Linux. Linux is fast and Linux is robust.

# Replace only if match occurs at the BEGINNING: ${var/#pattern/replacement}
path="/var/log/syslog"
echo "${path/#\/var/\/tmp}"
# Output: /tmp/log/syslog

# Replace only if match occurs at the END: ${var/%pattern/replacement}
file="report.txt"
echo "${file/%.txt/.pdf}"
# Output: report.pdf

4. Stripping Prefixes and Suffixes (Path Trimming)

System administrators and shell programmers constantly extract filenames or directories from path strings using prefix/suffix removal:

5. Changing Case (Uppercase & Lowercase in Bash 4+)

#!/bin/bash
company="Veewom Tech"

# Convert to all UPPERCASE with ^^
echo "${company^^}"    # VEEWOM TECH

# Convert to all lowercase with ,,
echo "${company,,}"    # veewom tech

# Capitalize only the first character with ^
lang="bash"
echo "${lang^}"        # Bash

6. String Comparison & Validation Operators

#!/bin/bash
# palindrome.sh: Check if string is palindrome
read -p "Enter a word or string: " input

len=${#input}
reverse=""

# Loop backwards through characters
for (( i=$len-1; i>=0; i-- )); do
    reverse="$reverse${input:$i:1}"
done

echo "Original string : $input"
echo "Reversed string : $reverse"

if [ "$input" = "$reverse" ]; then
    echo "Result: '$input' IS a Palindrome!"
else
    echo "Result: '$input' is NOT a Palindrome."
fi

#!/bin/bash
# path_parser.sh
full_path="/var/www/html/veewom/index.htm"

filename="${full_path##*/}"         # index.htm
basename="${filename%.*}"           # index
extension="${full_path##*.}"        # htm
directory="${full_path%/*}"         # /var/www/html/veewom

echo "Full Path : $full_path"
echo "Directory : $directory"
echo "File Name : $filename"
echo "Base Name : $basename"
echo "Extension : $extension"

Frequently Asked Questions (FAQs)