Shell by Example: String Manipulation POSIX 

#!/bin/sh

Shell provides powerful string manipulation through parameter expansion. These techniques work without calling external commands.

String length with ${#var}:

 

str="Hello, World!"
echo "String: '$str'"
echo "Length: ${#str}"
Bash

Substring extraction with ${var:start:length}: Note: This is bash-specific, not POSIX sh

 
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 -

POSIX alternative for substrings uses expr or cut:

 

echo "POSIX substring with expr:"
text="Hello, World!"
echo "  First 5: $(expr "$text" : '\(.....\)')"
echo "  Using cut: $(echo "$text" | cut -c1-5)"

Remove prefix with \({var#pattern} and \){var##pattern}:

 

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##*/}"

Remove suffix with \({var%pattern} and \){var%%pattern}:

 

# Remove shortest match from end
echo "Remove extension (%.*): ${filepath%.*}"

# Remove filename from end (get directory)
echo "Directory (%/*): ${filepath%/*}"

More practical examples:

 

filename="document.backup.tar.gz"
echo "Filename: $filename"
echo "Remove .gz: ${filename%.gz}"
echo "Remove all extensions: ${filename%%.*}"
echo "Get extension: ${filename##*.}"

Extract directory and filename:

 

path="/usr/local/bin/script.sh"
echo "Path: $path"
echo "Directory: ${path%/*}"
echo "Filename: ${path##*/}"
echo "Basename without ext: $(basename "${path%.*}")"
Bash

Substitution with ${var/pattern/replacement}:

 
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}"

POSIX substitution alternatives:

 

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')"
Bash 4+

Case conversion (bash 4+):

 
word="Hello World"
echo "Uppercase: ${word^^}"
echo "Lowercase: ${word,,}"
echo "First char upper: ${word^}"
echo "First char lower: ${word,}"

POSIX case conversion:

 

word="Hello World"
echo "POSIX case conversion with tr:"
echo "  Uppercase: $(echo "$word" | tr '[:lower:]' '[:upper:]')"
echo "  Lowercase: $(echo "$word" | tr '[:upper:]' '[:lower:]')"

String comparison:

 

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

Check if string is empty or not:

 

empty=""
nonempty="hello"

[ -z "$empty" ] && echo "empty is zero-length"
[ -n "$nonempty" ] && echo "nonempty has content"

Check if string contains substring:

 

haystack="The quick brown fox"
needle="quick"

case "$haystack" in
  *"$needle"*) echo "'$haystack' contains '$needle'" ;;
  *) echo "'$haystack' does not contain '$needle'" ;;
esac
Bash

Using [[ ]] for pattern matching:

 
if [[ "$haystack" == *quick* ]]; then
    echo "Found quick"
fi

Using =~ for regex:

 
if [[ "$haystack" =~ ^The ]]; then
    echo "Starts with 'The'"
fi

Split string into parts:

 

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
Bash

Join array elements:

 
fruits=("apple" "banana" "cherry")
IFS=','
echo "Joined: ${fruits[*]}"
unset IFS

Trim whitespace:

 

padded="   hello world   "
echo "Padded: '$padded'"

# Using sed:
trimmed=$(echo "$padded" | sed 's/^[[:space:]]*//;s/[[:space:]]*$//')
echo "Trimmed: '$trimmed'"
Bash

Using parameter expansion (bash):

 
trimmed="${padded#"${padded%%[![:space:]]*}"}"
trimmed="${trimmed%"${trimmed##*[![:space:]]}"}"

Repeat a string:

 

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)"

String formatting with printf:

 

printf "Padded: |%10s|\n" "hello"
printf "Left:   |%-10s|\n" "hello"

echo "String manipulation examples complete"

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by Jose Diaz-Gonzalez | inspired by Go by Example | source