grep (search text patterns) in Linux CLI - Step-by-Step Execution

Concept Flow - grep (search text patterns)

Start grep command

↓

Read input text line

↓

Check if line matches pattern?

No→Skip line

Yes↓

Print matching line

↓

More lines?

Yes→Read next line

No↓

End

grep reads each line of input, checks if it matches the pattern, prints it if yes, and repeats until all lines are processed.

Execution Sample

Linux CLI

echo -e "apple\nbanana\napple pie" | grep apple

Searches lines containing 'apple' from the given input and prints them.

Execution Table

Step	Input Line	Pattern Match?	Action	Output
1	apple	Yes	Print line	apple
2	banana	No	Skip line	apple
3	apple pie	Yes	Print line	apple apple pie
4	No more lines	-	End	-

💡 No more lines to read, grep finishes execution.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
Current Line	-	apple	banana	apple pie	No more lines
Output		apple	apple	apple apple pie	apple apple pie

Key Moments - 2 Insights

Why does grep print only some lines and skip others?

grep prints lines only if they match the pattern, as shown in execution_table rows 1 and 3 where 'apple' is found, but skips row 2 where 'banana' does not match.

What happens when grep reaches the end of input?

As in execution_table row 4, when no more lines are available, grep stops reading and ends execution.

Visual Quiz - 3 Questions

Look at the execution table, what is the output after step 2?

A "apple\napple pie"

B "apple"

C ""

D "banana"

💡 Hint

Check the Output column after step 2 in the execution_table.

At which step does grep skip printing a line?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look for 'Skip line' in the Action column of execution_table.

If the pattern was 'banana', which step would print output?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Check which input line matches 'banana' in the Input Line column.

Concept Snapshot

grep command searches text line by line.
It prints lines matching the given pattern.
Non-matching lines are skipped.
Stops when no more lines remain.
Syntax: grep pattern [file or input]
Useful for quick text search in files or streams.

Full Transcript

The grep command reads input text line by line. For each line, it checks if the line contains the search pattern. If yes, grep prints that line. If not, it skips it. This repeats until all lines are processed. For example, given three lines: 'apple', 'banana', and 'apple pie', searching for 'apple' prints the first and third lines only. When no more lines remain, grep ends. This simple flow helps quickly find text patterns in files or command outputs.