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Common Shell Scripting Patterns

This reference contains frequently-used patterns and solutions for shell scripting challenges.

File and Directory Operations

Check if file/directory exists

if [[ -f "$file" ]]; then
    echo "File exists"
fi

if [[ -d "$dir" ]]; then
    echo "Directory exists"
fi

# Check if file exists and is not empty
if [[ -s "$file" ]]; then
    echo "File exists and has content"
fi

Create directory if it doesn't exist

mkdir -p "$dir"

Find and process files safely

# Method 1: Using find with null delimiter (safest)
while IFS= read -r -d '' file; do
    echo "Processing: $file"
done < <(find . -type f -name "*.txt" -print0)

# Method 2: Using find with -exec
find . -type f -name "*.txt" -exec process_function {} \;

# Method 3: Simple glob (when no subdirectories)
shopt -s nullglob  # Bash only
for file in *.txt; do
    echo "Processing: $file"
done

Read file line by line

# Correct way
while IFS= read -r line; do
    echo "$line"
done < file.txt

# Or with process substitution
while IFS= read -r line; do
    echo "$line"
done < <(command)

Get file information

# File size
size=$(stat -f%z "$file")  # macOS
size=$(stat -c%s "$file")  # Linux

# File modification time
mtime=$(stat -f%m "$file")  # macOS
mtime=$(stat -c%Y "$file")  # Linux

# Portable basename and dirname
filename="${path##*/}"
dirname="${path%/*}"

# File extension
extension="${filename##*.}"
name="${filename%.*}"

String Operations

String manipulation

# Uppercase/lowercase (Bash 4+)
upper="${string^^}"
lower="${string,,}"

# Remove prefix/suffix
no_prefix="${string#prefix}"   # Remove shortest match
no_prefix="${string##prefix}"  # Remove longest match
no_suffix="${string%suffix}"   # Remove shortest match
no_suffix="${string%%suffix}"  # Remove longest match

# Replace substring
new_string="${string/old/new}"   # Replace first occurrence
new_string="${string//old/new}"  # Replace all occurrences

# String length
length="${#string}"

# Substring
substr="${string:start:length}"

Check if string contains substring

if [[ "$string" == *"substring"* ]]; then
    echo "Contains substring"
fi

# Using grep
if echo "$string" | grep -q "pattern"; then
    echo "Contains pattern"
fi

String comparison

# Equality
if [[ "$str1" == "$str2" ]]; then
    echo "Strings are equal"
fi

# Pattern matching
if [[ "$string" == pattern* ]]; then
    echo "Matches pattern"
fi

# Regular expression
if [[ "$string" =~ ^[0-9]+$ ]]; then
    echo "String is numeric"
fi

Array Operations

Array basics

# Create array
arr=("item1" "item2" "item3")

# Add to array
arr+=("item4")

# Access elements
echo "${arr[0]}"      # First element
echo "${arr[@]}"      # All elements
echo "${#arr[@]}"     # Array length

# Iterate over array
for item in "${arr[@]}"; do
    echo "$item"
done

# Iterate with index
for i in "${!arr[@]}"; do
    echo "Index $i: ${arr[$i]}"
done

Check if array contains value

contains() {
    local value=$1
    shift
    local arr=("$@")
    
    for item in "${arr[@]}"; do
        if [[ "$item" == "$value" ]]; then
            return 0
        fi
    done
    return 1
}

if contains "target" "${arr[@]}"; then
    echo "Array contains target"
fi

Process Management

Background processes

# Run in background
command &
pid=$!

# Wait for background process
wait $pid

# Check if process is running
if kill -0 $pid 2>/dev/null; then
    echo "Process is running"
fi

Process timeout

# Using timeout command (GNU coreutils)
timeout 30s command

# Manual implementation
command &
pid=$!
sleep 30
if kill -0 $pid 2>/dev/null; then
    kill $pid
    echo "Command timed out"
fi

Parallel execution

# Using xargs
find . -name "*.txt" -print0 | xargs -0 -P 4 -I {} process_file {}

# Using GNU parallel (if available)
parallel process_file ::: file1 file2 file3

# Manual parallel execution
for file in *.txt; do
    process_file "$file" &
done
wait  # Wait for all background jobs

Input/Output Redirection

Standard redirections

# Redirect stdout to file
command > file

# Redirect stderr to file
command 2> file

# Redirect both stdout and stderr
command &> file
command > file 2>&1

# Append instead of overwrite
command >> file

# Redirect stderr to stdout
command 2>&1

# Discard output
command > /dev/null 2>&1

Here documents

# Basic here document
cat <<EOF
This is a multi-line
string with variable expansion: $var
EOF

# Without variable expansion
cat <<'EOF'
This is a multi-line
string with literal $var
EOF

# Indented here document
cat <<-EOF
	This ignores leading tabs
	Useful for indenting
EOF

Error Handling

Check command exit status

if command; then
    echo "Command succeeded"
else
    echo "Command failed"
fi

# Or store exit code
command
exit_code=$?
if [[ $exit_code -eq 0 ]]; then
    echo "Success"
fi

Robust error handling

set -euo pipefail

# Custom error handler
error_exit() {
    echo "Error: $1" >&2
    exit "${2:-1}"
}

# Usage
[[ -f "$file" ]] || error_exit "File not found: $file" 2

Trap signals

# Cleanup on exit
cleanup() {
    rm -f "$TEMP_FILE"
    echo "Cleanup complete"
}
trap cleanup EXIT

# Handle specific signals
trap 'echo "Interrupted"; exit 130' INT
trap 'echo "Terminated"; exit 143' TERM

# Error line number
trap 'echo "Error on line $LINENO"' ERR

Text Processing

Using grep

# Basic search
grep "pattern" file

# Case-insensitive
grep -i "pattern" file

# Recursive search
grep -r "pattern" directory/

# Show line numbers
grep -n "pattern" file

# Invert match (show non-matching lines)
grep -v "pattern" file

# Extended regex
grep -E "pattern1|pattern2" file

Using sed

# Replace text
sed 's/old/new/' file           # Replace first occurrence per line
sed 's/old/new/g' file          # Replace all occurrences
sed 's/old/new/gi' file         # Case-insensitive replace

# Delete lines
sed '/pattern/d' file           # Delete matching lines
sed '1d' file                   # Delete first line
sed '$d' file                   # Delete last line

# Print specific lines
sed -n '5p' file                # Print line 5
sed -n '5,10p' file             # Print lines 5-10
sed -n '/pattern/p' file        # Print matching lines

# In-place editing
sed -i 's/old/new/g' file       # Linux
sed -i '' 's/old/new/g' file    # macOS

Using awk

# Print specific columns
awk '{print $1, $3}' file

# Filter by column value
awk '$3 > 100' file

# Sum a column
awk '{sum += $1} END {print sum}' file

# Custom field separator
awk -F: '{print $1}' /etc/passwd

# Pattern matching
awk '/pattern/ {print $1}' file

Date and Time

Get current date/time

# Current timestamp
now=$(date +%s)

# Formatted date
date=$(date +"%Y-%m-%d")
datetime=$(date +"%Y-%m-%d %H:%M:%S")

# ISO 8601 format
iso_date=$(date -u +"%Y-%m-%dT%H:%M:%SZ")

Date arithmetic

# Days ago (GNU date)
yesterday=$(date -d "yesterday" +%Y-%m-%d)
week_ago=$(date -d "7 days ago" +%Y-%m-%d)

# Days ago (BSD date - macOS)
yesterday=$(date -v-1d +%Y-%m-%d)
week_ago=$(date -v-7d +%Y-%m-%d)

Network Operations

Check if host is reachable

if ping -c 1 -W 1 example.com &> /dev/null; then
    echo "Host is reachable"
fi

Download files

# Using curl
curl -O https://example.com/file
curl -o output.txt https://example.com/file

# Using wget
wget https://example.com/file
wget -O output.txt https://example.com/file

# Follow redirects and show progress
curl -L --progress-bar -o file https://example.com/file

HTTP requests

# GET request
curl https://api.example.com/endpoint

# POST request with JSON
curl -X POST https://api.example.com/endpoint \
    -H "Content-Type: application/json" \
    -d '{"key": "value"}'

# Check HTTP status
status=$(curl -s -o /dev/null -w "%{http_code}" https://example.com)

Temporary Files and Directories

Create temporary files safely

# Create temporary file
TEMP_FILE=$(mktemp)
trap 'rm -f "$TEMP_FILE"' EXIT

# Create temporary directory
TEMP_DIR=$(mktemp -d)
trap 'rm -rf "$TEMP_DIR"' EXIT

# Create in specific location
TEMP_FILE=$(mktemp /tmp/myapp.XXXXXX)

Miscellaneous

Generate random numbers

# Random number 0-32767
random=$RANDOM

# Random number in range 1-100
random=$((RANDOM % 100 + 1))

# Better random (if available)
random=$(shuf -i 1-100 -n 1)

URL encoding

urlencode() {
    local string="$1"
    local strlen=${#string}
    local encoded=""
    local pos c o
    
    for (( pos=0; pos<strlen; pos++ )); do
        c=${string:$pos:1}
        case "$c" in
            [-_.~a-zA-Z0-9] ) o="$c" ;;
            * ) printf -v o '%%%02x' "'$c" ;;
        esac
        encoded+="$o"
    done
    echo "$encoded"
}

JSON parsing (with jq)

# Extract value
value=$(echo '{"key": "value"}' | jq -r '.key')

# Extract from array
items=$(echo '[{"name": "a"}, {"name": "b"}]' | jq -r '.[].name')

# Create JSON
json=$(jq -n --arg name "value" '{key: $name}')

9.0 KiB Raw Blame History

Common Shell Scripting Patterns

File and Directory Operations

Check if file/directory exists

Create directory if it doesn't exist

Find and process files safely

Read file line by line

Get file information

String Operations

String manipulation

Check if string contains substring

String comparison

Array Operations

Array basics

Check if array contains value

Process Management

Background processes

Process timeout

Parallel execution

Input/Output Redirection

Standard redirections

Here documents

Error Handling

Check command exit status

Robust error handling

Trap signals

Text Processing

Using grep

Using sed

Using awk

Date and Time

Get current date/time

Date arithmetic

Network Operations

Check if host is reachable

Download files

HTTP requests

Temporary Files and Directories

Create temporary files safely

Miscellaneous

Generate random numbers

URL encoding

JSON parsing (with jq)

9.0 KiB

Raw Blame History