string-manipulation-mastery

What this skill does

## CRITICAL GUIDELINES

### Windows File Path Requirements

**MANDATORY: Always Use Backslashes on Windows for File Paths**

When using Edit or Write tools on Windows, you MUST use backslashes (`\`) in file paths, NOT forward slashes (`/`).

---

# Bash String Manipulation Mastery (2025)

## Overview

Comprehensive guide to string manipulation in bash using parameter expansion, pattern matching, regular expressions, and built-in transformations. Master these techniques to avoid spawning external processes like `sed`, `awk`, or `cut` for simple operations.

## Parameter Expansion Basics

### String Length

```bash
#!/usr/bin/env bash
set -euo pipefail

str="Hello, World!"

# Get length
echo "${#str}"  # 13

# Length of array element
arr=("short" "much longer string")
echo "${#arr[0]}"  # 5
echo "${#arr[1]}"  # 18

# Number of array elements (not string length)
echo "${#arr[@]}"  # 2
```

### Substring Extraction

```bash
#!/usr/bin/env bash
set -euo pipefail

str="Hello, World!"

# ${var:offset} - from offset to end
echo "${str:7}"     # World!

# ${var:offset:length} - from offset, length chars
echo "${str:0:5}"   # Hello
echo "${str:7:5}"   # World

# Negative offset (from end) - note the space or parentheses
echo "${str: -6}"      # World!
echo "${str:(-6)}"     # World!
echo "${str: -6:5}"    # World

# Last N characters
last_n() {
    local str="$1" n="$2"
    echo "${str: -$n}"
}
last_n "Hello" 3  # llo

# Extract between positions
between() {
    local str="$1" start="$2" end="$3"
    echo "${str:start:$((end - start))}"
}
between "0123456789" 3 7  # 3456
```

### Default Values

```bash
#!/usr/bin/env bash
set -euo pipefail

# ${var:-default} - use default if unset or empty
name="${1:-Anonymous}"
echo "Hello, $name"

# ${var:=default} - assign default if unset or empty
: "${CONFIG_FILE:=/etc/app.conf}"

# ${var:+alternate} - use alternate if var IS set
debug_flag="${DEBUG:+--verbose}"

# ${var:?error} - exit with error if unset or empty
: "${REQUIRED_VAR:?REQUIRED_VAR must be set}"

# Without colon - only checks if unset (empty is OK)
# ${var-default}, ${var=default}, ${var+alt}, ${var?err}

# Practical example: config with defaults
setup_config() {
    : "${DB_HOST:=localhost}"
    : "${DB_PORT:=5432}"
    : "${DB_NAME:=myapp}"
    : "${DB_USER:=postgres}"
}
```

### Indirect Expansion

```bash
#!/usr/bin/env bash
set -euo pipefail

# ${!var} - indirect reference
config_host="server.example.com"
config_port="8080"

key="config_host"
echo "${!key}"  # server.example.com

# Iterate over related variables
for suffix in host port; do
    var="config_$suffix"
    echo "$suffix = ${!var}"
done

# Get all variable names matching pattern
# ${!prefix@} or ${!prefix*}
for var in "${!config_@}"; do
    echo "$var = ${!var}"
done

# Array indirection
arr=(a b c d e)
idx=2
echo "${arr[$idx]}"  # c

# Indirect array reference (Bash 4.3+ nameref)
get_array_element() {
    local -n arr_ref="$1"
    local idx="$2"
    echo "${arr_ref[$idx]}"
}
get_array_element arr 3  # d
```

## Pattern Matching

### Prefix Removal

```bash
#!/usr/bin/env bash
set -euo pipefail

path="/home/user/documents/file.tar.gz"

# ${var#pattern} - remove shortest prefix match
echo "${path#*/}"     # home/user/documents/file.tar.gz

# ${var##pattern} - remove longest prefix match
echo "${path##*/}"    # file.tar.gz (basename)

# Remove extension
filename="archive.tar.gz"
echo "${filename#*.}"   # tar.gz (first . onwards)
echo "${filename##*.}"  # gz (last extension only)

# Remove prefix string
url="https://example.com/path"
echo "${url#https://}"  # example.com/path

# Practical: Get file extension
get_extension() {
    local file="$1"
    echo "${file##*.}"
}

# Practical: Strip leading zeros
strip_leading_zeros() {
    local num="$1"
    echo "${num#"${num%%[!0]*}"}"
}
strip_leading_zeros "000123"  # 123
```

### Suffix Removal

```bash
#!/usr/bin/env bash
set -euo pipefail

path="/home/user/documents/file.tar.gz"

# ${var%pattern} - remove shortest suffix match
echo "${path%/*}"     # /home/user/documents (dirname)

# ${var%%pattern} - remove longest suffix match
echo "${path%%/*}"    # (empty - removes everything)

# Remove extension
filename="archive.tar.gz"
echo "${filename%.*}"   # archive.tar
echo "${filename%%.*}"  # archive

# Practical: Get directory
dirname="${path%/*}"

# Practical: Remove file extension
basename="${path##*/}"
name_without_ext="${basename%.*}"

# Combined: Change extension
change_extension() {
    local file="$1" new_ext="$2"
    echo "${file%.*}.$new_ext"
}
change_extension "doc.txt" "md"  # doc.md
```

### Pattern Substitution

```bash
#!/usr/bin/env bash
set -euo pipefail

str="hello hello hello"

# ${var/pattern/replacement} - replace first match
echo "${str/hello/hi}"     # hi hello hello

# ${var//pattern/replacement} - replace all matches
echo "${str//hello/hi}"    # hi hi hi

# ${var/#pattern/replacement} - replace if at start
echo "${str/#hello/hi}"    # hi hello hello

# ${var/%pattern/replacement} - replace if at end
echo "${str/%hello/goodbye}"  # hello hello goodbye

# Delete pattern (empty replacement)
echo "${str//hello/}"      # "   " (just spaces)

# Practical: Sanitize filename
sanitize_filename() {
    local name="$1"
    # Replace spaces with underscores
    name="${name// /_}"
    # Remove special characters
    name="${name//[^a-zA-Z0-9._-]/}"
    echo "$name"
}
sanitize_filename "My File (2024).txt"  # My_File_2024.txt

# Practical: Path manipulation
normalize_path() {
    local path="$1"
    # Remove double slashes
    while [[ "$path" == *//* ]]; do
        path="${path//\/\//\/}"
    done
    # Remove trailing slash
    echo "${path%/}"
}
```

## Case Transformation

### Basic Case Changes

```bash
#!/usr/bin/env bash
set -euo pipefail

str="Hello World"

# Lowercase first character
echo "${str,}"      # hello World

# Lowercase all
echo "${str,,}"     # hello world

# Uppercase first character
echo "${str^}"      # Hello World (already uppercase)

str2="hello world"
echo "${str2^}"     # Hello world

# Uppercase all
echo "${str,,}"     # hello world
echo "${str2^^}"    # HELLO WORLD

# Toggle case (Bash 4.4+)
echo "${str~~}"     # hELLO wORLD
```

### Pattern-Based Case Changes

```bash
#!/usr/bin/env bash
set -euo pipefail

str="hello world"

# Uppercase only matching pattern
echo "${str^^[aeiou]}"  # hEllO wOrld

# Lowercase only matching pattern
str2="HELLO WORLD"
echo "${str2,,[AEIOU]}"  # HeLLo WoRLD

# Practical: Title case
title_case() {
    local str="$1"
    local result=""
    local capitalize=true

    for ((i=0; i<${#str}; i++)); do
        local char="${str:$i:1}"
        if [[ "$char" == " " ]]; then
            result+="$char"
            capitalize=true
        elif $capitalize; then
            result+="${char^}"
            capitalize=false
        else
            result+="${char,}"
        fi
    done

    echo "$result"
}
title_case "hello WORLD from BASH"  # Hello World From Bash
```

## Regular Expressions

### Bash Regex Matching

```bash
#!/usr/bin/env bash
set -euo pipefail

# =~ operator for regex matching
str="Hello World 123"

if [[ "$str" =~ ^Hello ]]; then
    echo "Starts with Hello"
fi

if [[ "$str" =~ [0-9]+ ]]; then
    echo "Contains numbers"
fi

# Capture groups with BASH_REMATCH
email="[email protected]"
if [[ "$email" =~ ^([^@]+)@(.+)$ ]]; then
    echo "User: ${BASH_REMATCH[1]}"    # user
    echo "Domain: ${BASH_REMATCH[2]}"  # example.com
    echo "Full match: ${BASH_REMATCH[0]}"  # [email protected]
fi

# Store regex in variable (avoids quoting issues)
pattern='^[0-9]{4}-[0-9]{2}-[0-9]{2}$'
date="2024-03-15"
if [[ "$date" =~ $pattern ]]; then
    echo "Valid date format"
fi

# Multiple capture groups
log_line='2024-03-15 10:30:45 ERROR Connection failed'
pattern='^([0-9-]+) ([0-9:]+) ([A-Z]+) (.+)$'
if [[ "$log_line" =~ $pattern ]]; then
    date="${BASH_REMATCH[1]}"
    time="${BASH_REMATCH[2]}"
    level="${BASH_REMATCH[3]}"
    message="${BASH_REMATCH[4]}"
fi
```

### Practical Rege