Basic regex in grep in Bash Scripting - Mini Project: Build & Apply

Basic regex in grep

📖 Scenario: You are working with a text file that contains a list of fruits and vegetables. You want to find lines that match certain patterns using grep with basic regular expressions.

🎯 Goal: Learn how to use grep with basic regex patterns to search for specific words or patterns in a text file.

📋 What You'll Learn

Create a text file named items.txt with exact lines provided.

Use a variable to store the filename items.txt.

Use grep with a basic regex pattern to find lines starting with a specific letter.

Print the matching lines to the terminal.

💡 Why This Matters

🌍 Real World

Searching text files for specific patterns is common in system administration, log analysis, and data processing.

💼 Career

Knowing how to use <code>grep</code> with regex helps automate searching tasks and quickly find needed information in files.

1

Create the text file items.txt

Create a text file called items.txt with these exact lines, each on its own line:
apple
banana
carrot
apricot
blueberry

Bash Scripting

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

Hint

Use cat with a here-document or any text editor to create items.txt with the exact lines.

2

Set a variable for the filename

Create a variable called filename and set it to the string items.txt.

Bash Scripting

cat  items.txt
apple
banana
carrot
apricot
blueberry
EOF

# Create a variable filename with value 'items.txt'
# Your code here

Hint

Use filename="items.txt" to create the variable.

3

Use grep with basic regex to find lines starting with 'a'

Use grep with the pattern ^a on the file stored in the variable filename to find lines that start with the letter a. Write the command using the variable $filename.

Bash Scripting

cat  items.txt
apple
banana
carrot
apricot
blueberry
EOF

filename="items.txt"

# Use grep with pattern '^a' on $filename
# Your code here

Hint

The pattern ^a means lines starting with 'a'. Use grep '^a' "$filename".

4

Print the matching lines

Run the command to print the lines from items.txt that start with a. The output should be exactly:
apple
apricot

Bash Scripting

cat  items.txt
apple
banana
carrot
apricot
blueberry
EOF

filename="items.txt"

grep '^a' "$filename"

# Run the command to print matching lines
# Your code here

Hint

Just run the grep command from the previous step to see the matching lines.

Practice

1. What does the caret symbol ^ mean in a basic regex pattern used with grep?

easy

A. It matches the end of a line

B. It matches the start of a line

C. It matches any single character

D. It matches zero or more repetitions

Solution

1. Step 1: Understand caret usage in regex

   The caret ^ is used to anchor the pattern to the start of a line in basic regex.

2. Step 2: Compare with other anchors

   The dollar sign $ matches the end of a line, so caret is specifically for the start.

3. Final Answer:

   It matches the start of a line -> Option B

4. Quick Check:

   ^ means start of line [OK]

Hint: Caret (^) always means line start in grep regex [OK]

Common Mistakes:

• Confusing ^ with $ (end of line)
• Thinking ^ matches any character
• Using ^ to mean OR condition

2. Which of the following is the correct basic regex syntax to search for lines containing either "cat" or "dog" using grep?

easy

A. grep 'cat\|dog' file.txt

B. grep 'cat|dog' file.txt

C. grep 'cat\dog' file.txt

D. grep 'cat/dog' file.txt

Solution

1. Step 1: Understand OR in basic regex

   In basic regex with grep, the OR operator is written as \|, not just |.

2. Step 2: Check each option

   grep 'cat|dog' file.txt uses | without escape, which is invalid in basic regex. grep 'cat\|dog' file.txt correctly escapes it as \|. Options C and D use wrong characters.

3. Final Answer:

   grep 'cat\|dog' file.txt -> Option A

4. Quick Check:

   OR in basic grep regex = \| [OK]

Hint: Use \| for OR in basic grep regex [OK]

Common Mistakes:

• Using | without escaping in basic grep
• Confusing \| with \dog or /
• Forgetting to escape special characters

3. Given a file animals.txt with these lines:

cat
caterpillar
dog
catalog
dogma

What is the output of grep '^cat$' animals.txt?

medium

A. cat
caterpillar
catalog

B. caterpillar
catalog

C. cat

D. No output

Solution

1. Step 1: Understand the regex pattern

   The pattern ^cat$ matches lines that start and end exactly with "cat" (no extra characters).

2. Step 2: Check each line against pattern

   Only the line "cat" matches exactly. "caterpillar" and "catalog" have extra characters, so they don't match.

3. Final Answer:

   cat -> Option C

4. Quick Check:

   ^cat$ matches exact line 'cat' [OK]

Hint: Use ^ and $ to match exact whole line [OK]

Common Mistakes:

• Assuming partial matches with ^ and $
• Ignoring line boundaries in regex
• Confusing grep default substring match with anchored match

4. You want to find lines containing either "apple" or "banana" in fruits.txt using basic regex with grep. Which command uses incorrect syntax for the OR operation?

medium

A. grep 'apple|banana' fruits.txt

B. grep 'apple\|banana$' fruits.txt

C. grep '^apple\|banana$' fruits.txt

D. grep 'apple\|banana' fruits.txt

Solution

1. Step 1: Identify correct OR syntax in basic grep

   Basic regex requires escaping the OR operator as \|. Using | alone treats it as a literal | character, not as an OR operator.

2. Step 2: Check each option

   grep 'apple|banana' fruits.txt uses | without escape, treating | as literal, not OR. Options A, B, and C correctly escape OR operator.

3. Final Answer:

   grep 'apple|banana' fruits.txt -> Option A

4. Quick Check:

   Unescaped | in basic grep regex = literal |, not OR [OK]

Hint: Escape | as \| in basic grep regex [OK]

Common Mistakes:

• Not escaping | in basic regex
• Mixing extended regex syntax without -E flag
• Misplacing anchors with OR operator

5. You want to find lines in data.txt that start with "error" or end with "fail" using basic regex with grep. Which command correctly does this?

hard

A. grep '^error\|fail' data.txt

B. grep '^error|fail$' data.txt

C. grep '^error$\|fail$' data.txt

D. grep '^error\|fail$' data.txt

Solution

1. Step 1: Understand the pattern requirements

   We want lines starting with "error" OR lines ending with "fail". So pattern is ^error OR fail$.

2. Step 2: Use correct OR syntax in basic regex

   Basic regex OR is \|. So pattern is ^error\|fail$. grep '^error\|fail$' data.txt matches this exactly.

3. Step 3: Check other options

   grep '^error|fail$' data.txt misses escape on OR, causing error. grep '^error$\|fail$' data.txt incorrectly anchors "error" with $. grep '^error\|fail' data.txt misses anchor on "fail".

4. Final Answer:

   grep '^error\|fail$' data.txt -> Option D

5. Quick Check:

   Start with ^error OR end with fail$ using \| [OK]

Hint: Combine ^ and $ with \| for OR in basic grep regex [OK]

Common Mistakes:

• Not escaping | in basic regex
• Misplacing anchors with OR operator
• Using $ with start anchor or vice versa