RHCSA Practical Lab Series – Searching for Strings in a File

🔍 Lab 11: Extracting Specific Lines from a File

📌 Objective

In this lab, you will:

✔ Search for lines containing a specific string (error) in a given file
✔ Redirect the results into a separate output file (/root/errors.log)
✔ Ensure the output preserves the exact formatting of the original file

📌 Step 1: Locate the Target File

🔹 Confirm that the file exists before performing the search:

[root@node1 ~]# ls -l /var/log/messages

📌 Explanation:

• /var/log/messages → Common system log file containing various events.
• Ensure the file exists before proceeding.

✅ If the file exists, proceed to Step 2.

📌 Step 2: Search for a Specific String (error)

🔹 Use grep to find lines containing error in /var/log/messages:

[root@node1 ~]# grep error /var/log/messages

📌 Explanation:

• grep error /var/log/messages → Displays all lines that contain the string "error".

🔹 Redirect the output to a new file (/root/errors.log) for future reference:

[root@node1 ~]# grep error /var/log/messages > /root/errors.log

📌 Explanation:

• > /root/errors.log → Saves the output to /root/errors.log instead of displaying it on the screen.

✅ Now, all matching lines are stored in /root/errors.log.

📌 Step 3: Verify the Output

🔹 View the contents of /root/errors.log to confirm the extracted lines:

[root@node1 ~]# cat /root/errors.log

📌 Expected Output (example):

Mar 28 12:34:56 node1 kernel: [123456.789] error: disk failure detected
Mar 28 12:35:01 node1 sshd[2501]: error: Authentication failed
...

✅ If lines containing error appear, the extraction was successful.

📌 Troubleshooting:

If /root/errors.log is empty, confirm that the original file contains error messages:

[root@node1 ~]# grep -i error /var/log/messages

(Use -i to make the search case-insensitive.)

✅ Final Summary

✔ Used grep to extract lines containing error from /var/log/messages
✔ Stored the extracted lines in a separate output file (/root/errors.log)
✔ Verified the output to ensure correct extraction

📌 Next Lab: Archiving and Compressing Files with tar and bzip2

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