Basic regex in grep in Bash Scripting - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to search for the word 'error' in the file 'log.txt'.

Bash Scripting

grep [1] log.txt

Drag options to blanks, or click blank then click option'

A-i 'error'

B'error'

C'Error'

D-v 'error'

Attempts:

3 left

💡 Hint

Common Mistakes

Using -v option which inverts the match.

Using -i which ignores case, but the task wants exact match.

The command grep 'error' log.txt searches for the exact word 'error' in the file.

2fill in blank

medium

Complete the code to search for lines starting with 'Error' in 'log.txt'.

Bash Scripting

grep [1] log.txt

Drag options to blanks, or click blank then click option'

A'Error$'

B'^error'

C'Error'

D'^Error'

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Error$' which matches lines ending with 'Error'.

Not using ^ so matches anywhere in the line.

The caret ^ means the line starts with the pattern 'Error'.

3fill in blank

hard

Fix the error in the code to search for lines containing digits in 'data.txt'.

Bash Scripting

grep [1] data.txt

Drag options to blanks, or click blank then click option'

A'[0-9]'

B'[a-z]'

C'\\d'

D'[A-Z]'

Attempts:

3 left

💡 Hint

Common Mistakes

Using '\d' which is not recognized by basic grep.

Using character classes that do not include digits.

The pattern '[0-9]' matches any digit. '\d' is not supported by basic grep regex.

4fill in blank

hard

Fill both blanks to create a grep command that finds lines with exactly three digits in a row in 'file.txt'.

Bash Scripting

grep [1][2] file.txt

Drag options to blanks, or click blank then click option'

A'[0-9]'

B'\\{3\\}'

C'[a-z]'

D'\\{2\\}'

Attempts:

3 left

💡 Hint

Common Mistakes

Using '\{2\}' which matches two digits instead of three.

Using '[a-z]' which matches letters, not digits.

The pattern '[0-9]\{3\}' matches exactly three digits in a row in basic grep regex.

5fill in blank

hard

Fill all three blanks to create a grep command that finds lines starting with 'ID' followed by one or more digits in 'records.txt'.

Bash Scripting

grep [1][2][3] records.txt

Drag options to blanks, or click blank then click option'

A'^ID'

B'[0-9]'

C'\\+'

D'\\{1,\\}'

Attempts:

3 left

💡 Hint

Common Mistakes

Using '\+' which is not supported in basic grep regex.

Not anchoring the pattern with '^' to match start of line.

The pattern '^ID[0-9]\{1,\}' matches lines starting with 'ID' followed by one or more digits in basic grep regex.

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