> ## Documentation Index > Fetch the complete documentation index at: https://notes.kodekloud.com/llms.txt > Use this file to discover all available pages before exploring further. # Use scripting to automate system maintenance tasks > Learn to automate system maintenance tasks on CentOS using Bash scripting for backups, archiving, and managing exit statuses. Automating routine system maintenance on CentOS (or similar Linux distributions) saves time and reduces human error. In this guide, you’ll learn how to: * Write and run simple Bash scripts * Archive directories reliably * Manage multiple backup generations * Use exit statuses in conditions * Examine a real-world example (Anacron) *** ## Table of Contents 1. [Understanding the Bash Shell](#understanding-the-bash-shell) 2. [Creating Your First Script](#creating-your-first-script) 3. [Automating Backups](#automating-backups) * [Archiving a Directory](#archiving-a-directory) * [Keeping Two Generations of Backups](#keeping-two-generations-of-backups) 4. [Using Exit Status in Conditions](#using-exit-status-in-conditions) 5. [Real-World Example: Anacron](#real-world-example-anacron) 6. [Quick Reference: Shell Constructs](#quick-reference-shell-constructs) 7. [Further Resources](#further-resources) *** ## Understanding the Bash Shell When you log in, you land at a shell prompt managed by **bash**, the Bourne Again SHell. It interprets commands you type or reads and executes commands from a file (a *script*) in sequence. ```bash theme={null} $ date Mon Dec 6 16:28:09 CST 2021 ``` Bash supports redirection, pipelines, variables, loops, functions, and more—the same features you use interactively are available in scripts. *** ## Creating Your First Script Follow these steps to build and run a basic maintenance script. ### 1. Create the Script File ```bash theme={null} $ touch script.sh $ vim script.sh ``` ### 2. Add Script Contents ```bash theme={null} #!/bin/bash # Log the date and time of execution date >> /tmp/script.log # Capture the current kernel version cat /proc/version >> /tmp/script.log ``` The first line (`#!/bin/bash`) is the **shebang**, telling the system which interpreter to use. * Lines beginning with `#` are comments. * `>>` appends output rather than overwriting. ### 3. Make the Script Executable ```bash theme={null} $ chmod u+x script.sh # or allow everyone: $ chmod +x script.sh ``` ### 4. Run and Verify ```bash theme={null} $ ./script.sh $ cat /tmp/script.log Mon Dec 6 17:06:16 CST 2021 Linux version 4.18.0-348.2.1.el8_5.x86_64 ... ``` *** ## Automating Backups Backup scripts are ideal for automating directory archiving. Below are two approaches: a simple archive and one that retains the previous generation. ### Archiving a Directory Create `archive-dnf.sh`: ```bash theme={null} #!/bin/bash tar acf /tmp/archive.tar.gz /etc/dnf ``` ```bash theme={null} $ chmod +x archive-dnf.sh $ ./archive-dnf.sh $ ls /tmp archive.tar.gz ``` You can inspect the archive with: ```bash theme={null} tar tf /tmp/archive.tar.gz ``` *** ### Keeping Two Generations of Backups To avoid overwriting a good backup, rename the old archive before creating a new one. 1. Save this as `archive-dnf-2.sh`: ```bash theme={null} #!/bin/bash if test -f /tmp/archive.tar.gz; then mv /tmp/archive.tar.gz /tmp/archive.tar.gz.OLD tar acf /tmp/archive.tar.gz /etc/dnf else tar acf /tmp/archive.tar.gz /etc/dnf fi ``` 2. Make it executable and run: ```bash theme={null} $ chmod +x archive-dnf-2.sh $ ./archive-dnf-2.sh $ ls /tmp archive.tar.gz archive.tar.gz.OLD ``` Moving or deleting files in `/tmp` can remove critical data if misused. Always verify paths and filenames before executing backup scripts. *** ## Using Exit Status in Conditions Every command returns an **exit status**: `0` (success) or nonzero (failure). You can leverage this in `if` statements: ```bash theme={null} #!/bin/bash if grep -q '5' /etc/default/grub; then echo 'Grub has timeout of 5 seconds.' else echo 'Grub DOES NOT have a timeout of 5 seconds.' fi ``` * `grep -q` runs quietly (`-q`) and sets exit status accordingly. * Save as `check-grub-timeout.sh`, make it executable, then: ```bash theme={null} $ chmod +x check-grub-timeout.sh $ ./check-grub-timeout.sh Grub has timeout of 5 seconds. ``` *** ## Real-World Example: Anacron Inspect `/etc/cron.hourly/0anacron` to see conditionals, loops, and file checks in action: ```sh theme={null} #!/bin/sh # Check whether @anacron ran today if test -r /var/spool/anacron/cron.daily; then day=$(cat /var/spool/anacron/cron.daily) fi if [ "$(date +%Y%m%d)" = "$day" ]; then exit 0 fi # Skip jobs when on battery power online=1 for psupply in AC AD0*; do sysfile="/sys/class/power_supply/$psupply/online" if [ -f "$sysfile" ]; then if [ "$(cat "$sysfile" 2>/dev/null)" = 1 ]; then online=1 break else online=0 fi fi done if [ "$online" = 0 ]; then exit 0 fi # …rest of the script… ``` *** ## Quick Reference: Shell Constructs | Construct | Use Case | Example | | | | ----------- | -------------------------- | ------------------------------------------ | - | -------------- | | shebang | Select interpreter | `#!/bin/bash` | | | | if … then | Conditional execution | `if grep -q 'foo' file; then echo yes; fi` | | | | for … do | Iterate over lists | `for file in *.log; do gzip "$file"; done` | | | | `>>` | Append redirection | `date >> /tmp/script.log` | | | | `test -f` | Check file existence | `if test -f /path/to/file; then …` | | | | exit status | Check command success/fail | \`command && echo success | | echo failure\` | *** ## Further Resources * [Bash Reference Manual](https://www.gnu.org/software/bash/manual/bash.html) * [Linux `tar` Documentation](https://www.gnu.org/software/tar/manual/) * [Anacron on CentOS Wiki](https://wiki.centos.org/HowTos/Anacron) Make sure to explore bash built-ins (`help`) and system scripts under `/etc/cron.*` for more real-world patterns and advanced techniques.