Anchors (^, $) in Bash Scripting - Mini Project: Build & Apply

Using Anchors (^, $) in Bash Scripting

📖 Scenario: You work in a small company where you need to filter log entries. The logs contain many lines, but you want to find only those lines that start or end with specific words.

🎯 Goal: You will write a bash script that uses anchors ^ and $ in regular expressions to find lines starting with a word and lines ending with a word from a list of log entries.

📋 What You'll Learn

Create a variable with multiple log lines

Create a variable with a word to match at the start of lines

Use grep with the anchor ^ to find lines starting with the word

Use grep with the anchor $ to find lines ending with the word

Print the filtered results

💡 Why This Matters

🌍 Real World

Filtering logs or text files to find lines that start or end with specific words is common in system administration and data processing.

💼 Career

Understanding anchors in regular expressions helps automate text filtering tasks, which is valuable for roles in DevOps, system administration, and scripting.

1

Create a variable with log lines

Create a variable called logs that contains these exact lines separated by newlines: error: file not found, warning: low disk space, info: update completed, error: access denied, info: user logged in

Bash Scripting

# Create the variable logs with the exact lines
# Your code here

Hint

Use double quotes and \n to separate lines inside the string.

2

Create a variable with the start word to match

Create a variable called start_word and set it to the exact string error

Bash Scripting

logs="error: file not found\nwarning: low disk space\ninfo: update completed\nerror: access denied\ninfo: user logged in"
# Create the variable start_word with value "error"
# Your code here

Hint

Use double quotes to assign the string.

3

Use grep with ^ anchor to find lines starting with the word

Use echo and grep with the pattern ^$start_word to filter lines from logs that start with the word stored in start_word. Save the result in a variable called start_matches

Bash Scripting

logs="error: file not found\nwarning: low disk space\ninfo: update completed\nerror: access denied\ninfo: user logged in"
start_word="error"
# Use grep with ^ anchor to find lines starting with start_word
# Your code here

Hint

Use double quotes around the pattern and variable to expand it correctly.

4

Use grep with $ anchor to find lines ending with a word and print results

Create a variable called end_word and set it to completed. Then use echo and grep with the pattern $end_word$ to find lines in logs that end with end_word. Save the result in end_matches. Finally, print start_matches and end_matches with echo.

Bash Scripting

logs="error: file not found\nwarning: low disk space\ninfo: update completed\nerror: access denied\ninfo: user logged in"
start_word="error"
start_matches=$(echo "$logs" | grep "^$start_word")
# Create end_word variable with value "completed"
# Use grep with $ anchor to find lines ending with end_word and save to end_matches
# Print start_matches and end_matches
# Your code here

Hint

Remember to use double quotes and variable expansion in the grep pattern. Use echo to print the results.

Practice

1. What does the anchor ^ represent in a bash regular expression?

easy

A. It matches the end of a line.

B. It matches any digit.

C. It matches any whitespace character.

D. It matches the start of a line.

Solution

1. Step 1: Understand the role of anchors in regex

   Anchors are special characters that match positions in text, not actual characters.

2. Step 2: Identify the meaning of ^

   The ^ anchor matches the start of a line in bash regex.

3. Final Answer:

   It matches the start of a line. -> Option D

4. Quick Check:

   ^ means start of line [OK]

Hint: Remember ^ means start, $ means end of line [OK]

Common Mistakes:

• Confusing ^ with end of line anchor
• Thinking ^ matches a character instead of position
• Mixing ^ with other regex symbols like * or .

2. Which of the following grep commands correctly finds lines that end with the word done?

easy

A. grep 'done$' file.txt

B. grep '^done' file.txt

C. grep 'done^' file.txt

D. grep '^done$' file.txt

Solution

1. Step 1: Understand the meaning of $ anchor

   The $ anchor matches the end of a line in regex.

2. Step 2: Match lines ending with 'done'

   Using done$ matches lines where 'done' is at the end.

3. Final Answer:

   grep 'done$' file.txt -> Option A

4. Quick Check:

   Use $ to match line end [OK]

Hint: Use $ after text to match line endings [OK]

Common Mistakes:

• Using ^ after text instead of $
• Using ^done to match line end
• Confusing ^ and $ anchors

3. What will be the output of this command if file.txt contains the lines:

apple
banana
apple pie
pineapple
apple

grep '^apple$' file.txt

medium

A. apple
apple pie
apple

B. apple pie
pineapple

C. apple
apple

D. banana

Solution

1. Step 1: Understand the regex ^apple$

   This matches lines that start and end exactly with 'apple' and nothing else.

2. Step 2: Check each line in file.txt

   Lines 'apple' and 'apple' match exactly. 'apple pie' and 'pineapple' do not match because of extra text.

3. Final Answer:

   apple apple -> Option C

4. Quick Check:

   ^ and $ match exact whole line [OK]

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

Common Mistakes:

• Assuming partial matches with anchors
• Ignoring that $ matches end of line
• Confusing lines with extra words as matches

4. You wrote this command to find lines starting with 'Error' in a log file:
grep 'Error^' logfile.txt
But it returns no results. What is the problem?

medium

A. The file does not contain 'Error'.

B. The anchor ^ should be at the start, not after 'Error'.

C. grep does not support anchors like ^.

D. The command needs double quotes instead of single quotes.

Solution

1. Step 1: Analyze the position of the anchor ^

   The ^ anchor must be at the start of the pattern to match line start.

2. Step 2: Correct the pattern placement

   Use ^Error to match lines starting with 'Error'. Placing ^ after 'Error' is invalid.

3. Final Answer:

   The anchor ^ should be at the start, not after 'Error'. -> Option B

4. Quick Check:

   ^ must be at pattern start [OK]

Hint: Place ^ only at pattern start to match line start [OK]

Common Mistakes:

• Putting ^ after text instead of before
• Assuming grep ignores anchors
• Using wrong quotes causing shell issues

5. You want to extract lines from data.txt that start with a digit and end with a period. Which grep pattern correctly uses anchors to do this?

hard

A. grep '^[0-9].*\.$' data.txt

B. grep '^[0-9].*$' data.txt

C. grep '^\d.*\.$' data.txt

D. grep '^[0-9].*.$' data.txt

Solution

1. Step 1: Match line start with a digit

   The pattern ^[0-9] matches lines starting with any digit.

2. Step 2: Match line end with a literal period

   Use \.$ to match a literal '.' at the end. The dot must be escaped to avoid matching any character.

3. Step 3: Combine pattern correctly

   ^[0-9].*\.$ matches lines starting with digit, any characters, ending with a period.

4. Final Answer:

   grep '^[0-9].*\.$' data.txt -> Option A

5. Quick Check:

   ^ start, $ end, escape . for literal dot [OK]

Hint: Escape dot to match literal period at line end [OK]

Common Mistakes:

• Not escaping dot, matching any char instead of period
• Using \d which grep may not support by default
• Missing $ anchor at line end