Day-5 Regular Expressions, Pipelines & 1/O Redirections

Grep: Global Regular Expression Print

Purpose:
grep is used to search for a specific pattern or expression inside files or outputs of other commands and display matching lines.

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Basic Examples

1. Search for a word in a file:

2. Examples of Grep: Let us pickthe information about root from the file /etc/passwd (/etc/passwd contains information about all the users present in the system)

grep root /etc/passwd

This prints lines containing "root" from /etc/passwd, which stores system users info.

Output example:

root:x:0:0:root:/root:/bin/bash
operator:x:11:0:operator:/root:/sbin/nologin

2. Case-insensitive search:
   Use -i to ignore case differences (match Kernel, kernel, KERNEL, etc.).

grep -i kernel ktfile

Output example:

Welcome to Kernel Tech
Welcome to kernel Tech
Welcome to KERNEL TECH

3. Show lines matching the word and 2 lines after it:
   Use -A option (After context).

grep -nA2 wheel /etc/group

-n shows line numbers; -A2 shows 2 lines after match.

4. Show lines matching the word and 2 lines before it:
   Use -B option (Before context).

grep -nB2 wheel /etc/group

5. Exclude lines containing a word:
   Use -v to invert the match (show lines not containing the word).

grep -v Kernel ktfile

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Additional options

• Show matched words in color:

grep --color root /etc/passwd

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Combining grep with other commands using pipes (|)

What is the pipe (|)?

• The pipe operator takes the output of one command and passes it as input to another command

How to use grep with pipe — basic syntax

command1 | grep 'pattern'

• command1 produces output.

• grep filters that output to only show lines containing 'pattern'.

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Example 1: Search running processes with ps and grep

ps aux | grep firefox

• Lists all processes and filters only those with the word firefox.

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Example 2: Show only .txt files in a directory listing

ls -l | grep '.txt'

• Lists directory files, filters only .txt files.

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Example 3: Filter system log for errors

tail -n 100 /var/log/syslog | grep 'error'

• Shows last 100 lines of syslog, filters lines containing error.

• Example -4

cat ktfile | grep -i kernel

More practical examples:

ls -l | grep -i ktfile
ifconfig | grep -i eth0

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Summary of grep options used:

🔁 I/O Redirection in Linux

🟥 1. Using > and >>

• > will create the file if it doesn't exist, or overwrite it if it does.

• >> will create the file if it doesn't exist, or append to it if it does.

🧪 Examples:

echo "Hello" > test.txt       # Creates test.txt with "Hello"
echo "World" >> test.txt      # Appends "World" to test.txt

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🟩 2. Using the tee Command

• The tee command reads from standard input and writes to both standard output and one or more files.

✅ Syntax:

command | tee filename

• Use -a to append instead of overwrite.

🧪 Example:

ls | tee files.txt

• Displays output of ls on the screen and also saves it to files.txt.

ls | tee -a files.txt

• Appends output of ls to files.txt.

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📝 Summary Table: