Advanced Bash Scripting

Chapter 48: Advanced Bash Scripting

Overview

This chapter covers advanced bash scripting techniques for automation.

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48.1 Process Management

Background Processes

# Run in background
command &
./script.sh &

# Wait for background process
wait $PID

# Jobs control
jobs
fg %1
bg %1
Ctrl+Z  # Suspend

Signals

# Trap signals
trap 'echo "Caught SIGINT"' INT
trap 'cleanup' EXIT

# Common signals
# SIGINT (2)  - Ctrl+C
# SIGTERM (15) - Graceful termination
# SIGKILL (9)  - Force kill
# SIGHUP (1)   - Hangup

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48.2 Process Substitution

Examples

# Compare files
diff <(sort file1) <(sort file2)

# While with process substitution
while read line; do
    process $line
done < <(command)

# Read from multiple files
paste <(cut -f1 file1) <(cut -f2 file2)

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48.3 Advanced Arrays

Associative Arrays

# Requires bash 4+
declare -A users
users[admin]=root
users[nginx]=www-data

# List keys/values
echo ${!users[@]}
echo ${users[@]}

Array Operations

# Slicing
${array[@]:start:length}

# Search and replace in array
${array[@]/old/new}

# Conditional filter
${array[@]/*pattern*/}

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48.4 Advanced Functions

Returning Values

# Return status
check_service() {
    systemctl is-active $1
    return $?
}

# Return data via stdout
get_users() {
    awk -F: '$3 >= 1000 {print $1}' /etc/passwd
}
users=$(get_users)

# Using global variable
RESULT=""
store_result() {
    RESULT="computed value"
}

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48.5 Error Handling

Exit Codes

# Exit with status
exit 0    # Success
exit 1    # Error
exit 2    # Misuse
exit 126  # Not executable
exit 127  # Command not found

# Check exit status
command
if [ $? -eq 0 ]; then
    echo "Success"
fi

# Set -e (exit on error)
set -e
command || { echo "Failed"; exit 1; }

Debugging

# Verbose mode
set -v

# Debug mode
set -x

# Track errors
set -o errtrace
set -o pipefail

# Combined
set -euo pipefail

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48.6 Parallel Processing

xargs

# Parallel execution
ls *.log | xargs -P 4 -I {} gzip {}

# With GNU parallel
cat hosts | parallel -j 4 ssh {} 'uptime'

Background Jobs

# Run in parallel
for server in server1 server2 server3; do
    ssh $server 'apt update' &
done
wait

# Limit parallel jobs
sem -j 4 ./process.sh

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48.7 Text Processing

awk

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

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

# Conditional
awk '$3 > 1000 {print $1}' /etc/passwd

# Operations
awk '{sum+=$1} END {print sum}' file

sed

# Replace
sed 's/old/new/g' file
sed -i 's/old/new/g' file

# Delete lines
sed '/pattern/d' file

# Print lines
sed -n '1,10p' file
sed -n '/start/,/end/p' file

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48.8 Secure Scripts

Security Best Practices

# Avoid eval
# Sanitize input
# Use quotes for variables
# Don't use backticks (use $())
# Check file permissions
# Validate arguments

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Why This Matters in DevOps/SRE

Advanced bash enables complex automation:

           Advanced Bash in DevOps/SRE
+------------------------------------------------------------------+
|                                                                   |
|    Parallel Processing:                                            |
|    +----------------------------------------------------------+  |
|    | xargs -P -> Parallel execution                     |  |
|    | GNU Parallel -> Advanced parallelization           |  |
|    | Background jobs -> Async operations                |  |
|    +----------------------------------------------------------+  |
|                                                                   |
|    Text Processing:                                                 |
|    +----------------------------------------------------------+  |
|    | awk -> Data extraction and reporting             |  |
|    | sed -> Stream text editing                       |  |
|    | grep -> Pattern matching                        |  |
|    +----------------------------------------------------------+  |
|                                                                   |
|    Debugging:                                                      |
|    +----------------------------------------------------------+  |
|    | set -x -> Trace execution                       |  |
|    | bashdb -> Debugger                              |  |
|    | shellcheck -> Static analysis                   |  |
|    +----------------------------------------------------------+  |
|                                                                   |
+------------------------------------------------------------------+

Practical Impact:

• Process large datasets efficiently
• Build complex automation pipelines
• Debug shell script issues

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Common Mistakes & Anti-Patterns

1. Not Using Process Substitution

# WRONG: Using temp files
grep pattern file > temp
sort temp
rm temp

# CORRECT: Process substitution
grep pattern file | sort

2. Not Handling Signals Properly

# WRONG: No cleanup on interrupt
while true; do
    work
done

# CORRECT: Handle signals
trap 'cleanup; exit' INT TERM

3. Not Using ShellCheck

# WRONG: Writing scripts without linting
# Hidden bugs

# CORRECT: Use shellcheck
shellcheck myscript.sh

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Summary

In this chapter, you learned:

• ✅ Process management and signals
• ✅ Process substitution
• ✅ Advanced arrays and functions
• ✅ Error handling and debugging
• ✅ Parallel processing
• ✅ Text processing with awk/sed

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Next Chapter

Chapter 48: Ansible for Linux Administration

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Last Updated: February 2026