Unit 8 Bonus - Scripting

NOTE: This is an optional bonus section. You do not need to read it, but if you're interested in digging deeper, this is for you.

Bash: The Essential Skill for Any Linux Administrator

If you're planning to work with Linux, you’ll use Bash every day -- whether troubleshooting servers, automating tasks, or managing system configurations.

Why is bash important?

Bash is everywhere:
- Bash is the default shell on all major Linux distributions (RedHat, Debian, etc), and most other distributions
- It automates common sysadmin tasks (backups, log analysis, deployments)
- Bash is essential for DevOps and administrative workflows (writing scripts, configuring CI/CD pipelines).

Why learn Bash?

You can automate repetitive or complex tasks.
You can manage anything on your system using Bash (files, processes, services, etc.).
Bash works across almost all major Linux distributions.

Bash scripting turns manual commands into powerful, reusable automation.

Writing Your First Script

Let's create a simple script that prints a message.

Create a script file:
```
touch first-script.sh
```

Make it executable:

chmod +x first-script.sh
# Or, use octal
chmod 755 first-script.sh

Open it in a text editor (e.g., vi):
```
vi first-script.sh
```
Add the following code:
```
#!/bin/bash
echo "Hello, admin!"
```
Run the script:
```
./first-script.sh
```
Expected output:
```
Hello, admin!
```
Key Takeaways:
- The #!/bin/bash shebang line tells the system which interpreter to use to execute the script.
- chmod +x or chmod 755 makes the script executable.
- ./ is required because the script is not in the system’s PATH.

It's worth noting that including a .sh file extension is completely optional.
If you decide to replace the Bash script with an executable binary down the line, having a .sh file extension can hurt you (e.g., if any other programs try to execute it using the .sh filename).

10 Common Control Operators

These operators allow you to chain and control command execution in Bash.

Operator	Purpose	Example
`;`	Run multiple commands sequentially	`mkdir test; cd test`
`&&`	Run second command only if first succeeds	`mkdir test && cd test`
`\|\|`	Run second command only if first fails	`mkdir test \|\| echo "Failed"`
`&`	Run a command in the background	`sleep 60 &`
`\|`	Pipe output from one command to another	`ls \| grep ".txt"`
`()`	Run commands in a subshell	`(cd /tmp && ls)`
`{}`	Run commands in the current shell	`{ cd /tmp; ls; }`
`>`	Redirect output to a file (overwrite)	`echo "log" > file.txt`
`>>`	Redirect output (append)	`echo "log" >> file.txt`
`$(...)`	Capture command output (from a subshell)	`DATE=$(date)`

Why does this matter?
- These operators control execution flow and are fundamental to Bash scripting.

10 Common Conditionals

Bash conditionals allow scripts to make decisions.

Test	Meaning	Example
`[[ -f FILE ]]`	File exists	`[[ -f /etc/passwd ]]`
`[[ -d DIR ]]`	Directory exists	`[[ -d /home/user ]]`
`[[ -n STR ]]`	String is non-empty	`[[ -n "$USER" ]]`
`[[ -z STR ]]`	String is empty	`[[ -z "$VAR" ]]`
`[[ "$A" = "$B" ]]`	Strings are equal	`[[ "$USER" = "root" ]]`
`[[ "$A" != "$B" ]]`	Strings are not equal	`[[ "$USER" != "admin" ]]`
`[[ NUM1 -eq NUM2 ]]`	Numbers are equal	`[[ 5 -eq 5 ]]`
`[[ NUM1 -gt NUM2 ]]`	NUM1 is greater than NUM2	`[[ 10 -gt 5 ]]`
`[[ "$?" -eq 0 ]]`	Last command was successful	`command && echo "Success"`
`[[ -x FILE ]]`	File is executable	`[[ -x script.sh ]]`

Why does this matter?
- These tests are used in if-statements and loops.
Single brackets vs. double brackets?
- Single brackets [ ... ] are used in POSIX shell scripts. Using single brackets is equivalent to using the test command (see help test and help [)
  - These are generally more subject to shell injection attacks, since they allow anything to expand within them.
  - You must quote all variables and subshell command (i.e., "$(...)") within single brackets.
- Double brackets [[ ... ]] are keywords (also builtins) in bash (see help [[).
  - These do not require you to quote variables, and provide some extended functionality.
  - If you're writing a Bash script, there's no reason to use single brackets for conditionals.

10 Bash Scripting Scenarios

Below are 10 real-world examples of using bash from the command line.

Scenario	Solution	Cont'd.
Check if a file exists before deleting	`if [ -f "data.txt" ]; then rm data.txt; fi`
Backup a file before modifying	`cp config.conf config.bak`
Create a log entry every hour	`echo "$(date): Check OK" >> log.txt`
Monitor disk space	`df -h`	`awk '$5 > 90 {print "Low disk: "$1}'`
Check if a service is running	`systemctl is-active nginx`	`systemctl restart nginx`
List large files in a directory	`find /var/log -size +100M -exec ls -lh {} \;`
Change all `.txt` files to `.bak`	`for file in *.txt; do mv "$file" "${file%.txt}.bak"; done`
Check if a user is logged in	`who`	`grep "admin"`
Kill a process by name	`pkill -f "python server.py"`
Find and replace text in files	`sed -i 's/old/new/g' file.txt`

Why does this matter?
- These scenarios show how Bash automates real-world tasks.

Debugging Bash Scripts

Debugging tools help troubleshoot Bash scripts.

Command	Purpose
`set -x`	Print each command before execution
`set -e`	Exit script if any command fails
`trap '...' ERR`	Run a custom error handler when a command fails
`echo "$VAR"`	Print variable values for debugging
`printf "%s\n" "$VAR"`	Print variable values for debugging
`bash -x script.sh`	Run script with debugging enabled

Using set -x and echo (or printf) are some of the most common methods of troubleshooting.

Example Debugging Script

#!/bin/bash
set -xe  # Enable debugging and exit on failure
mkdir /tmp/mydir
cd /tmp/mydir
rm -rf /tmp/mydir

Next Steps

Now that you understand the fundamentals, here’s what to do next:

Practice writing scripts:
- Automate a daily task (e.g., installing a program, creating backups, user management)
Master error handling:
- Learn signals and trap, and learn about logging techniques.
Explore advanced topics:
- Look into writing functions, using arrays, and job control.
Read man bash:
- The ultimate built-in reference.
- This resource has everything you need to know about Bash and then some!
Join ProLUG community:
- Learn from others, contribute, and improve your Linux skillset.

🚀 Happy scripting!

Linux Administration Course