Unit
- Shell definition: A shell is a command-line interpreter that provides a user interface for Unix-like operating systems. It interprets commands typed by the user and can also execute sequences of commands stored in a file, which is called a shell script.
- Common shells: The most widely used shells include Bash (Bourne Again SHell), Zsh (Z shell), and Sh (original Bourne shell). Bash is the default on most Linux distributions and offers a rich set of features for scripting.
- Script vs. interactive: Interactive shell use involves typing commands one at a time. A script collects those commands into a file, making the process repeatable and auditable. This reduces human error and saves time on routine operations.
- Automation benefits: Shell scripts automate repetitive tasks such as file backups, log monitoring, user account management, and software deployment. Organizations that adopt scripting see reduced manual effort and increased consistency across environments.
- Portability considerations: While Bash scripts are common, portability across different Unix-like systems may require using the POSIX-compliant Sh shell or testing scripts in multiple environments. Knowing which features are Bash-specific is essential for cross-platform scripts.
- Common use cases: Shell scripting is used in DevOps for CI/CD pipeline steps, in system administration for cron jobs, in data processing for log parsing, and in development for build automation. Almost any command-line task can be scripted.
- First script example: A minimal script begins with a shebang line (
#!/bin/bash) followed by commands. The shebang tells the system which interpreter to use. After writing the script, it must be made executable withchmod +x. - Script execution methods: Scripts can be run by specifying the path (
./script.sh), by invoking the interpreter directly (bash script.sh), or by sourcing the script (source script.sh) which runs it in the current shell environment. - Debugging scripts: To trace script execution, use
bash -x script.shor addset -xinside the script. The-xoption prints each command before execution, showing variable expansions. For strict error handling, useset -eto exit on any non-zero return status, andset -uto treat unset variables as an error. - Exit status: Every command in a shell script produces an exit code ranging from 0 to 255. A zero exit status indicates success, while any non-zero value signals an error or abnormal condition. You can capture the exit status of the last command using the special variable
$?. Check exit codes to make scripts more robust. - Variables and quoting: Define variables with
VAR=value(no spaces around equals). To reference a variable, use$VARor${VAR}. Always quote variable expansions, e.g.,"$VAR", to preserve whitespace and prevent word splitting. Without quotes, a variable containing spaces may be treated as multiple arguments. - Command substitution: Command substitution allows storing the output of a command into a variable. Use the modern syntax
$(command)or the older backtick`command`. For example,current_dir=$(pwd)saves the current directory path. The$(...)syntax nests easily and is preferred in modern scripts.
Code Example
Bash
#!/bin/bash
# My first shell script
echo 'Hello, World!'
echo 'Current date:'
dateThis script prints a greeting and the current date. The shebang line #!/bin/bash specifies the interpreter. Save it as hello.sh, run chmod +x hello.sh, then execute it with ./hello.sh.
Security Consideration
MEDIUM RISK
Vulnerability: Scripts executed without proper permissions or from untrusted sources can introduce malicious code.
Mitigation: Always verify the contents of a script before running it. Avoid setting world-writable permissions on script files. Use chmod to restrict write access to the owner only.
Reference: CIS Benchmarks for Linux
Quick Revision
- A shell script is a file containing a sequence of commands executed by a shell interpreter.
- The shebang line (
#!/bin/bash) defines which interpreter to use. - Scripts automate tasks, reduce manual errors, and enable repeatable operations.
- Make scripts executable with
chmod +xand run them with a path or interpreter invocation.