grep (search text patterns) in Linux CLI - Mini Project: Build & Apply

Using grep to Search Text Patterns

📖 Scenario: You are working with a simple text file that contains a list of fruits and their colors. You want to find specific fruits by searching for text patterns inside this file.

🎯 Goal: Learn how to use the grep command to search for text patterns in a file and display matching lines.

📋 What You'll Learn

Create a text file named fruits.txt with exact content

Use a variable to store the search pattern

Use grep with the variable to find matching lines

Display the matching lines as output

💡 Why This Matters

🌍 Real World

Searching text files for specific words or patterns is common when working with logs, configuration files, or data files.

💼 Career

Knowing how to use grep helps in troubleshooting, data analysis, and automating tasks in many IT and software roles.

1

Create the text file fruits.txt

Create a file named fruits.txt with these exact lines:
apple red
banana yellow
cherry red
grape purple
lemon yellow

Linux CLI

# Create the file fruits.txt with the exact lines
# Your code here

Hint

Use echo with -e and redirect output to fruits.txt.

2

Set a variable for the search pattern

Create a variable called pattern and set it to the exact string red.

Linux CLI

echo -e "apple red\nbanana yellow\ncherry red\ngrape purple\nlemon yellow" > fruits.txt
# Set the variable pattern to 'red'
# Your code here

Hint

Use pattern=red to assign the string.

3

Use grep with the variable to find matching lines

Use the grep command with the variable $pattern to search inside fruits.txt and show matching lines.

Linux CLI

echo -e "apple red\nbanana yellow\ncherry red\ngrape purple\nlemon yellow" > fruits.txt
pattern=red
# Use grep with $pattern to search fruits.txt
# Your code here

Hint

Use grep "$pattern" fruits.txt to search using the variable.

4

Display the matching lines

Run the script to display all lines from fruits.txt that contain the word red.

Linux CLI

echo -e "apple red\nbanana yellow\ncherry red\ngrape purple\nlemon yellow" > fruits.txt
pattern=red
grep "$pattern" fruits.txt
# Run the command to show output

Hint

The output should show only lines containing 'red'.

Practice

1. What does the grep command do in Linux?

easy

A. Searches text for lines matching a pattern

B. Deletes files matching a pattern

C. Copies files to another directory

D. Lists all files in a directory

Solution

1. Step 1: Understand the purpose of grep

   The grep command is used to search text for lines that match a given pattern.

2. Step 2: Compare options

   Options B, C, and D describe other commands like rm, cp, and ls, not grep.

3. Final Answer:

   Searches text for lines matching a pattern -> Option A

4. Quick Check:

   grep = search text lines [OK]

Hint: grep finds lines with your pattern in text [OK]

Common Mistakes:

• Confusing grep with file commands like rm or cp
• Thinking grep modifies files instead of searching
• Assuming grep lists files instead of searching content

2. Which of the following is the correct syntax to search for the word error in a file named log.txt using grep?

easy

A. grep log.txt error

B. grep -f error log.txt

C. grep error log.txt

D. grep -e log.txt error

Solution

1. Step 1: Recall grep syntax

   The basic syntax is grep [pattern] [file]. So to search for 'error' in 'log.txt', use grep error log.txt.

2. Step 2: Analyze other options

   grep -f error log.txt uses -f which expects a file of patterns, not a pattern string. Options C and D have arguments reversed or incorrect flags.

3. Final Answer:

   grep error log.txt -> Option C

4. Quick Check:

   grep pattern file = grep error log.txt [OK]

Hint: Use grep pattern filename order to avoid errors [OK]

Common Mistakes:

• Swapping pattern and filename positions
• Using -f flag incorrectly for pattern string
• Confusing -e flag usage

3. Given a file data.txt with the following content:

apple
banana
Apple
BANANA
apple pie
banana split

What is the output of the command grep -i 'apple' data.txt?

medium

A. apple
apple pie

B. Apple
BANANA
banana split

C. apple pie
banana split

D. apple
Apple
apple pie

Solution

1. Step 1: Understand -i option

   The -i option makes grep case-insensitive, so it matches 'apple' and 'Apple'.

2. Step 2: Identify matching lines

   Lines containing 'apple' ignoring case are: 'apple', 'Apple', and 'apple pie'.

3. Final Answer:

   apple Apple apple pie -> Option D

4. Quick Check:

   grep -i matches apple and Apple [OK]

Hint: Use -i to ignore case when searching text [OK]

Common Mistakes:

• Ignoring case sensitivity without -i
• Missing lines with partial matches like 'apple pie'
• Confusing output order

4. You run the command grep -c 'fail' report.txt but it returns 0 even though you know the file contains the word 'fail'. What is the most likely reason?

medium

A. The file report.txt is empty

B. The word 'fail' is in uppercase like 'FAIL' and grep is case-sensitive by default

C. The grep command syntax is incorrect

D. The file report.txt does not exist

Solution

1. Step 1: Understand grep -c behavior

   The -c option counts matching lines. If it returns 0, no exact matches were found.

2. Step 2: Consider case sensitivity

   By default, grep is case-sensitive. If the file has 'FAIL' but you search 'fail', it won't match.

3. Final Answer:

   The word 'fail' is in uppercase like 'FAIL' and grep is case-sensitive by default -> Option B

4. Quick Check:

   grep is case-sensitive unless -i used [OK]

Hint: Use -i to ignore case if matches return zero [OK]

Common Mistakes:

• Assuming grep ignores case by default
• Not checking file content carefully
• Mistaking syntax errors for no matches

5. You want to find how many lines in server.log contain the word timeout ignoring case, and display only the count. Which command should you use?

hard

A. grep -i -c timeout server.log

B. grep -i timeout server.log

C. grep timeout -i -c server.log

D. grep -c timeout server.log

Solution

1. Step 1: Identify options needed

   You need to ignore case (-i) and count matching lines (-c).

2. Step 2: Check correct option order

   Options can be combined as -i -c or -c -i. grep -i -c timeout server.log uses both correctly before pattern and file.

3. Step 3: Analyze other options

   grep -c timeout server.log lacks case insensitivity. grep -i timeout server.log shows matches but not count. grep timeout -i -c server.log has incorrect argument order (pattern before options).

4. Final Answer:

   grep -i -c timeout server.log -> Option A

5. Quick Check:

   Use -i and -c together for case-insensitive count [OK]

Hint: Combine -i and -c to count ignoring case [OK]

Common Mistakes:

• Omitting -i for case-insensitive search
• Using pattern before options causing errors
• Expecting output lines instead of count