linuxexpert-practice-labs/labs/lpic-1/103-7-regular-expressions

LPIC-1 103.7: Searching text with regular expressions

Filtering out comments, pulling addresses out of a log, editing a copy of a file with sed, and basic against extended regular expressions.

Before you start

The lab creates /root/regex/access.log, a log with IPv4 addresses in it. Work as root (sudo -i), and write answers to the files in /root/lab-answers/.

Tasks

1. Show /etc/ssh/sshd_config without comments or blank lines, first with two grep -v commands, then with one grep -Ev '^(#|$)'. Save the result in /root/regex/sshd-active.txt.
2. Pull every IPv4 address out of /root/regex/access.log with grep -Eo and count each one. Write the counts, as uniq -c prints them, to /root/lab-answers/2.
3. Copy /etc/ssh/sshd_config to /root/sshd_config, then use sed -i.bak on the copy to set the port to 2222, and compare the two files with diff.
4. Write the same pattern as a basic and an extended regex, and check both give the same matches in /root/regex/access.log. Write the basic one on line 1 of /root/lab-answers/4 and the extended one on line 2.

Hints

• grep -v '^#' FILE | grep -v '^$'.
• grep -Eo '([0-9]{1,3}\.){3}[0-9]{1,3}' FILE | sort | uniq -c.
• The port line may be commented out (#Port 22): sed -i.bak 's/^#\?Port .*/Port 2222/'. diff FILE.bak FILE.
• In a basic regex \{, \(, \| and \+ do what {, (, | and + do in an extended one.