Why Shell Scripting?

Shell scripts automate command-line tasks:

Batch operations

System administration

Build processes

Scheduled tasks

As an agent, you'll use shell commands constantly.

Script Basics

Creating a Script

#!/bin/bash
# This is a comment
echo "Hello, World!"

Running a Script

# Make executable
chmod +x script.sh

# Run it
./script.sh

# Or run with bash
bash script.sh

Variables

Setting Variables

name="World"
count=42
path="/home/user"

# No spaces around =
name = "Wrong"   # ERROR
name="Right"     # OK

Using Variables

echo "Hello, $name"
echo "Count is ${count}"
echo "Path: $path/subdir"

Special Variables

$0    # Script name
$1    # First argument
$2    # Second argument
$@    # All arguments
$#    # Number of arguments
$?    # Exit status of last command
$    # Current process ID

Command Substitution

# Store command output in variable
today=$(date +%Y-%m-%d)
files=$(ls -1 | wc -l)

Conditionals

If Statements

if [ condition ]; then
    # commands
elif [ other_condition ]; then
    # commands
else
    # commands
fi

Test Conditions

# String comparisons
[ "$a" = "$b" ]     # Equal
[ "$a" != "$b" ]    # Not equal
[ -z "$a" ]         # Empty
[ -n "$a" ]         # Not empty

# Number comparisons
[ "$a" -eq "$b" ]   # Equal
[ "$a" -ne "$b" ]   # Not equal
[ "$a" -lt "$b" ]   # Less than
[ "$a" -gt "$b" ]   # Greater than

# File tests
[ -f "$file" ]      # Is regular file
[ -d "$dir" ]       # Is directory
[ -e "$path" ]      # Exists
[ -r "$file" ]      # Is readable
[ -w "$file" ]      # Is writable
[ -x "$file" ]      # Is executable

Examples

# Check if file exists
if [ -f "config.yaml" ]; then
    echo "Config found"
else
    echo "Config missing"
fi

# Check argument count
if [ $# -lt 1 ]; then
    echo "Usage: $0 <filename>"
    exit 1
fi

Loops

For Loop

# Loop over list
for item in apple banana cherry; do
    echo "$item"
done

# Loop over files
for file in *.txt; do
    echo "Processing $file"
done

# Loop over range
for i in {1..5}; do
    echo "Number $i"
done

# C-style for loop
for ((i=0; i<5; i++)); do
    echo "Index $i"
done

While Loop

count=0
while [ $count -lt 5 ]; do
    echo "Count: $count"
    ((count++))
done

Reading Lines

# Read file line by line
while read -r line; do
    echo "Line: $line"
done < file.txt

# Read from command
ls -1 | while read -r file; do
    echo "File: $file"
done

Functions

Defining Functions

# Function definition
greet() {
    echo "Hello, $1!"
}

# With local variables
calculate() {
    local a=$1
    local b=$2
    echo $((a + b))
}

Using Functions

greet "World"
# Output: Hello, World!

result=$(calculate 5 3)
echo "Result: $result"
# Output: Result: 8

Return Values

is_even() {
    if [ $(($1 % 2)) -eq 0 ]; then
        return 0  # True/success
    else
        return 1  # False/failure
    fi
}

if is_even 4; then
    echo "4 is even"
fi

Common Patterns

Error Handling

# Exit on error
set -e

# Check command success
if ! command; then
    echo "Command failed"
    exit 1
fi

# Or with ||
command || { echo "Failed"; exit 1; }

Default Values

# Use default if variable is empty
name=${1:-"default"}
port=${PORT:-8080}

Reading Input

echo "Enter your name:"
read -r name
echo "Hello, $name!"

# With prompt
read -p "Enter value: " value

Processing Arguments

while getopts "hf:v" opt; do
    case $opt in
        h) show_help; exit 0 ;;
        f) file="$OPTARG" ;;
        v) verbose=true ;;
        *) echo "Unknown option"; exit 1 ;;
    esac
done

Text Processing

Pipes and Filters

# Chain commands
cat file.txt | grep "pattern" | sort | uniq

# Count lines
wc -l < file.txt

# Extract columns
awk '{print $2}' file.txt

# Replace text
sed 's/old/new/g' file.txt

String Operations

str="Hello, World!"

# Length
echo ${#str}           # 13

# Substring
echo ${str:0:5}        # Hello

# Replace
echo ${str/World/Bash} # Hello, Bash!

# Remove prefix
file="document.txt"
echo ${file%.txt}      # document

# Remove suffix  
path="/home/user/file"
echo ${path##*/}       # file

Practical Scripts

Backup Script

#!/bin/bash
set -e

src="/data"
dest="/backups"
date=$(date +%Y%m%d)

tar -czf "$dest/backup-$date.tar.gz" "$src"
echo "Backup complete: backup-$date.tar.gz"

Log Rotation

#!/bin/bash
log_dir="/var/log/myapp"
keep_days=7

find "$log_dir" -name "*.log" -mtime +$keep_days -delete
echo "Cleaned logs older than $keep_days days"

Health Check

#!/bin/bash
url="http://localhost:8080/health"

if curl -s "$url" | grep -q "ok"; then
    echo "Service is healthy"
    exit 0
else
    echo "Service is unhealthy"
    exit 1
fi

Batch Processing

#!/bin/bash
for file in input/*.csv; do
    name=$(basename "$file" .csv)
    ./process.py "$file" > "output/${name}.json"
    echo "Processed: $name"
done

Best Practices

Use Strict Mode

#!/bin/bash
set -euo pipefail

-e: Exit on error
-u: Error on undefined variables
-o pipefail: Pipe fails if any command fails

Quote Variables

# Bad - breaks with spaces
file=$1
cat $file

# Good
file="$1"
cat "$file"

Use Functions

Break scripts into functions for clarity and reuse.

Add Help

show_help() {
    echo "Usage: $0 [options] <file>"
    echo "Options:"
    echo "  -h  Show help"
    echo "  -v  Verbose output"
}

Log Actions

log() {
    echo "[$(date +%H:%M:%S)] $*"
}

log "Starting process"

Conclusion

Shell scripting is essential for automation:

Variables and substitution

Conditionals and loops

Functions for organization

Pipes for text processing

Start with simple scripts, add complexity as needed.

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