String Manipulation¶

Bash has powerful built-in string manipulation through parameter expansion. Understanding these techniques reduces the need for external tools like sed and awk.

String Length¶

str="Hello, World!"
echo "${#str}"    # 13

Substring Extraction¶

str="Hello, World!"

echo "${str:0:5}"     # Hello (from 0, length 5)
echo "${str:7}"       # World! (from 7 to end)
echo "${str:7:5}"     # World (from 7, length 5)
echo "${str: -6}"     # World! (last 6, note space)
echo "${str: -6:5}"   # World (last 6, then length 5)

Negative Offset

Space before negative number is required to distinguish from :- default value syntax:

${str: -6}    # Substring from end
${str:-6}     # Default value if unset

Pattern Removal¶

Remove from Beginning¶

file="/path/to/file.txt"

echo "${file#*/}"      # path/to/file.txt (shortest match)
echo "${file##*/}"     # file.txt (longest match - basename)

Remove from End¶

file="/path/to/file.txt"

echo "${file%/*}"      # /path/to (shortest match - dirname)
echo "${file%%/*}"     # (empty - longest match)
echo "${file%.txt}"    # /path/to/file (remove extension)
echo "${file%.*}"      # /path/to/file (remove any extension)

Summary¶

Syntax	Direction	Match
`${var#pattern}`	From start	Shortest
`${var##pattern}`	From start	Longest
`${var%pattern}`	From end	Shortest
`${var%%pattern}`	From end	Longest

Pattern Substitution¶

Replace First Match¶

str="hello hello hello"
echo "${str/hello/hi}"    # hi hello hello

Replace All Matches¶

str="hello hello hello"
echo "${str//hello/hi}"   # hi hi hi

Replace at Beginning¶

str="hello world"
echo "${str/#hello/hi}"   # hi world
echo "${str/#world/hi}"   # hello world (no match at start)

Replace at End¶

str="hello world"
echo "${str/%world/earth}"  # hello earth
echo "${str/%hello/hi}"     # hello world (no match at end)

Delete Pattern¶

str="hello world"
echo "${str/o/}"      # hell world (delete first o)
echo "${str//o/}"     # hell wrld (delete all o)

Case Conversion¶

Requires Bash 4.0+

str="Hello World"

echo "${str,,}"       # hello world (all lowercase)
echo "${str^^}"       # HELLO WORLD (all uppercase)
echo "${str,}"        # hello World (first char lowercase)
echo "${str^}"        # Hello World (first char uppercase)

With pattern:

str="Hello World"
echo "${str,,[AEIOU]}"    # hEllO wOrld (lowercase vowels only)

Default Values¶

unset var
echo "${var:-default}"     # default (use if unset/empty)
echo "${var-default}"      # default (use if unset only)

var=""
echo "${var:-default}"     # default (empty counts as unset)
echo "${var-default}"      # (empty - var is set)

Set and Use Default¶

unset var
echo "${var:=default}"     # default (and sets var)
echo "$var"                # default

Error if Unset¶

unset var
echo "${var:?Variable not set}"
# bash: var: Variable not set

Use Alternative Value¶

var="hello"
echo "${var:+alternative}"  # alternative (var is set)

unset var
echo "${var:+alternative}"  # (empty - var is unset)

String Splitting¶

Using IFS¶

str="one,two,three"
IFS=',' read -ra arr <<< "$str"
echo "${arr[1]}"    # two

Using Parameter Expansion¶

path="/usr/local/bin"

# Split on /
IFS='/' read -ra parts <<< "$path"
for part in "${parts[@]}"; do
    [[ -n "$part" ]] && echo "$part"
done

String Joining¶

arr=("one" "two" "three")

# Join with delimiter
IFS=','
echo "${arr[*]}"    # one,two,three
IFS=' '

# Using printf
printf -v joined '%s,' "${arr[@]}"
echo "${joined%,}"  # one,two,three

Pattern Matching¶

Glob Patterns in [[ ]]¶

str="hello.txt"

[[ "$str" == *.txt ]] && echo "Text file"
[[ "$str" == h* ]] && echo "Starts with h"
[[ "$str" == *ll* ]] && echo "Contains ll"

Regex Matching¶

str="user@example.com"

if [[ "$str" =~ ^([^@]+)@(.+)$ ]]; then
    echo "User: ${BASH_REMATCH[1]}"    # user
    echo "Domain: ${BASH_REMATCH[2]}"  # example.com
fi

Practical Examples¶

Get File Extension¶

file="document.backup.txt"

# Last extension
ext="${file##*.}"
echo "$ext"    # txt

# Without extension
base="${file%.*}"
echo "$base"   # document.backup

Get Filename from Path¶

path="/home/user/documents/file.txt"

# Basename
filename="${path##*/}"
echo "$filename"    # file.txt

# Directory
dirname="${path%/*}"
echo "$dirname"     # /home/user/documents

Change File Extension¶

file="document.txt"
newfile="${file%.txt}.md"
echo "$newfile"    # document.md

Trim Whitespace¶

str="   hello world   "

# Trim leading
trimmed="${str#"${str%%[![:space:]]*}"}"
echo "[$trimmed]"    # [hello world   ]

# Trim trailing
trimmed="${str%"${str##*[![:space:]]}"}"
echo "[$trimmed]"    # [   hello world]

# Trim both (function)
trim() {
    local s="$1"
    s="${s#"${s%%[![:space:]]*}"}"
    s="${s%"${s##*[![:space:]]}"}"
    echo "$s"
}
echo "[$(trim "$str")]"    # [hello world]

Check String Contains Substring¶

str="hello world"

# Using pattern matching
[[ "$str" == *world* ]] && echo "Contains 'world'"

# Using =~
[[ "$str" =~ world ]] && echo "Contains 'world'"

Check String Starts/Ends With¶

str="hello world"

# Starts with
[[ "$str" == hello* ]] && echo "Starts with 'hello'"

# Ends with
[[ "$str" == *world ]] && echo "Ends with 'world'"

Repeat String¶

repeat() {
    local str="$1"
    local n="$2"
    local result=""
    for ((i=0; i<n; i++)); do
        result+="$str"
    done
    echo "$result"
}

echo "$(repeat "ab" 3)"    # ababab

Using printf:

printf '=%.0s' {1..20}    # ====================
echo

Pad String¶

# Left pad with zeros
num="42"
printf "%05d\n" "$num"    # 00042

# Right pad with spaces
str="hello"
printf "%-10s|\n" "$str"  # hello     |

Split Path Components¶

path="/home/user/documents/file.txt"

dir="${path%/*}"
base="${path##*/}"
name="${base%.*}"
ext="${base##*.}"

echo "Dir: $dir"      # /home/user/documents
echo "Base: $base"    # file.txt
echo "Name: $name"    # file
echo "Ext: $ext"      # txt

URL Parsing¶

url="https://user:pass@example.com:8080/path?query=1"

protocol="${url%%://*}"           # https
without_proto="${url#*://}"       # user:pass@example.com:8080/path?query=1
user_pass="${without_proto%%@*}"  # user:pass
host_path="${without_proto#*@}"   # example.com:8080/path?query=1
host_port="${host_path%%/*}"      # example.com:8080
path_query="${host_path#*/}"      # path?query=1

echo "Protocol: $protocol"
echo "User:Pass: $user_pass"
echo "Host:Port: $host_port"
echo "Path: $path_query"

Try It¶

Substring operations:

str="Hello, World!"
echo "Length: ${#str}"
echo "First 5: ${str:0:5}"
echo "Last 6: ${str: -6}"

Pattern removal:

path="/home/user/file.txt"
echo "Basename: ${path##*/}"
echo "Dirname: ${path%/*}"
echo "No ext: ${path%.txt}"

Substitution:

str="one two one three one"
echo "First: ${str/one/1}"
echo "All: ${str//one/1}"

Case conversion:

str="Hello World"
echo "Lower: ${str,,}"
echo "Upper: ${str^^}"

Summary¶

Operation	Syntax
Length	`${#var}`
Substring	`${var:offset:length}`
Remove prefix (shortest)	`${var#pattern}`
Remove prefix (longest)	`${var##pattern}`
Remove suffix (shortest)	`${var%pattern}`
Remove suffix (longest)	`${var%%pattern}`
Replace first	`${var/old/new}`
Replace all	`${var//old/new}`
Replace start	`${var/#old/new}`
Replace end	`${var/%old/new}`
Lowercase all	`${var,,}`
Uppercase all	`${var^^}`
Default if unset	`${var:-default}`
Set default	`${var:=default}`
Error if unset	`${var:?message}`