Shell by Example: String Manipulation

Shell by Example: String Manipulation POSIX + Bash

Shell provides powerful string manipulation through parameter expansion. These techniques work without calling external commands. String length with `${#var}`:	Edit `#!/bin/sh str="Hello, World!" echo "String: '$str'" echo "Length: ${#str}"`
	Output: String: 'Hello, World!' Length: 13

Bash Substring extraction with `${var:start:length}`.	Edit `#!/bin/bash text="Hello, World!" echo "First 5 chars: ${text:0:5}" echo "From position 7: ${text:7}" echo "Last 6 chars: ${text: -6}" # Note the space before -`
	Output: First 5 chars: Hello From position 7: World! Last 6 chars: World!

POSIX alternative for substrings uses `expr` or `cut`.	Edit `#!/bin/sh echo "POSIX substring with expr:" text="Hello, World!" echo " First 5: $(expr "$text" : '$.....$')" echo " Using cut: $(echo "$text" \| cut -c1-5)"`
	Output: POSIX substring with expr: First 5: Hello Using cut: Hello

Remove prefix with `${var#pattern}` and `${var##pattern}`.	Edit `#!/bin/sh filepath="/home/user/documents/file.txt" echo "Path: $filepath" # Remove shortest match from beginning echo "Remove leading /: ${filepath#/}" # Remove longest match from beginning echo "Remove path (##/): ${filepath##/}"`
	Output: Path: /home/user/documents/file.txt Remove leading /: home/user/documents/file.txt Remove path (##*/): file.txt

Remove suffix with `${var%pattern}` and `${var%%pattern}`.	Edit `#!/bin/sh # Remove shortest match from end filepath="/home/user/documents/file.txt" echo "Remove extension (%.): ${filepath%.}" # Remove filename from end (get directory) echo "Directory (%/): ${filepath%/}"`
	Output: Remove extension (%.): /home/user/documents/file Directory (%/): /home/user/documents

More practical examples using parameter expansion.	Edit `#!/bin/sh filename="document.backup.tar.gz" echo "Filename: $filename" echo "Remove .gz: ${filename%.gz}" echo "Remove all extensions: ${filename%%.}" echo "Get extension: ${filename##.}"`
	Output: Filename: document.backup.tar.gz Remove .gz: document.backup.tar Remove all extensions: document Get extension: gz

Extract directory and filename using parameter expansion.	Edit `#!/bin/sh path="/usr/local/bin/script.sh" echo "Path: $path" echo "Directory: ${path%/}" echo "Filename: ${path##/}" echo "Basename without ext: $(basename "${path%.*}")"`
	Output: Path: /usr/local/bin/script.sh Directory: /usr/local/bin Filename: script.sh Basename without ext: script

Bash Substitution with `${var/pattern/replacement}`.	Edit `#!/bin/bash msg="hello world world" echo "Replace first: ${msg/world/universe}" echo "Replace all: ${msg//world/universe}" echo "Replace at start: ${msg/#hello/hi}" echo "Replace at end: ${msg/%world/planet}"`
	Output: Replace first: hello universe world Replace all: hello universe universe Replace at start: hi world world Replace at end: hello world planet

POSIX substitution alternatives:	Edit `#!/bin/sh msg="hello world world" echo "POSIX substitution with sed:" echo " Replace first: $(echo "$msg" \| sed 's/world/universe/')" echo " Replace all: $(echo "$msg" \| sed 's/world/universe/g')"`
	Output: POSIX substitution with sed: Replace first: hello universe world Replace all: hello universe universe

Bash Case conversion:	Edit `#!/bin/bash word="Hello World" echo "Uppercase: ${word^^}" echo "Lowercase: ${word,,}" echo "First char upper: ${word^}" echo "First char lower: ${word,}"`
	Output: Uppercase: HELLO WORLD Lowercase: hello world First char upper: Hello World First char lower: hello World

POSIX case conversion:	Edit `#!/bin/sh word="Hello World" echo "POSIX case conversion with tr:" echo " Uppercase: $(echo "$word" \| tr '[:lower:]' '[:upper:]')" echo " Lowercase: $(echo "$word" \| tr '[:upper:]' '[:lower:]')"`
	Output: POSIX case conversion with tr: Uppercase: HELLO WORLD Lowercase: hello world

Bash String comparison can be done with `=` and `!=` operators. These work in both sh and bash. The < syntax only works in bash.	Edit `#!/bin/bash s1="apple" s2="banana" if [ "$s1" = "$s2" ]; then echo "$s1 equals $s2" else echo "$s1 does not equal $s2" fi # Lexicographic comparison if [ "$s1" \< "$s2" ]; then echo "$s1 comes before $s2" fi`
	Output: apple does not equal banana apple comes before banana

Check if string is empty or not:	Edit `#!/bin/sh empty="" nonempty="hello" [ -z "$empty" ] && echo "empty is zero-length" [ -n "$nonempty" ] && echo "nonempty has content"`
	Output: empty is zero-length nonempty has content

Check if string contains substring:	Edit `#!/bin/sh haystack="The quick brown fox" needle="quick" case "$haystack" in "$needle") echo "'$haystack' contains '$needle'" ;; *) echo "'$haystack' does not contain '$needle'" ;; esac`
	Output: 'The quick brown fox' contains 'quick'

Bash Using `[[ ]]` for pattern matching.	Edit `#!/bin/bash haystack="The quick brown fox" if [[ "$haystack" == quick ]]; then echo "Found quick" else echo "Not found" fi`
	Output: Found quick

Bash Using `=~` for regex.	Edit `#!/bin/bash haystack="The quick brown fox" if [[ "$haystack" =~ ^The ]]; then echo "Starts with 'The'" fi`
	Output: Starts with 'The'

Split string into parts:	Edit `#!/bin/sh csv="apple,banana,cherry" echo "Splitting '$csv':" # Using IFS IFS=',' set -- $csv echo " Part 1: $1" echo " Part 2: $2" echo " Part 3: $3" unset IFS`
	Output: Splitting 'apple,banana,cherry': Part 1: apple Part 2: banana Part 3: cherry

Bash Join array elements:	Edit `#!/bin/bash fruits=("apple" "banana" "cherry") IFS=',' echo "Joined: ${fruits[*]}" unset IFS`
	Output: Joined: apple,banana,cherry

Trim whitespace:	Edit `#!/bin/sh padded=" hello world " echo "Padded: '$padded'" # Using sed: trimmed=$(echo "$padded" \| sed 's/^[[:space:]]//;s/[[:space:]]$//') echo "Trimmed: '$trimmed'"`
	Output: Padded: ' hello world ' Trimmed: 'hello world'

Bash Using parameter expansion:	Edit `#!/bin/bash padded=" hello world " trimmed="${padded#"${padded%%[![:space:]]}"}" trimmed="${trimmed%"${trimmed##[![:space:]]}"}" echo "Padded: '$padded'" echo "Trimmed: '$trimmed'"`
	Output: Padded: ' hello world ' Trimmed: 'hello world'

Repeat a string:	Edit `#!/bin/sh repeat_char() { char="$1" count="$2" result="" i=0 while [ "$i" -lt "$count" ]; do result="$result$char" i=$((i + 1)) done echo "$result" } echo "Repeat '-' 20 times: $(repeat_char '-' 20)"`
	Output: Repeat '-' 20 times: --------------------

String formatting with `printf`.	Edit `#!/bin/sh printf "Padded: \|%10s\|\n" "hello" printf "Left: \|%-10s\|\n" "hello" echo "String manipulation examples complete"`
	Output: Padded: \| hello\| Left: \|hello \| String manipulation examples complete

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