Basic regex in grep in Bash Scripting - Step-by-Step Execution

Concept Flow - Basic regex in grep

Start: Input text file

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Run grep with regex

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grep reads each line

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Check if line matches regex

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Print line

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End: Output matched lines

grep reads each line of a file, checks if it matches the regex, and prints only matching lines.

Execution Sample

Bash Scripting

grep '^a.*e$' sample.txt

This command prints lines from sample.txt that start with 'a' and end with 'e'.

Execution Table

Step	Line Content	Regex Check '^a.*e$'	Match Result	Action
1	apple	Starts with 'a' and ends with 'e'	Yes	Print line
2	banana	Does not start with 'a'	No	Skip line
3	ace	Starts with 'a' and ends with 'e'	Yes	Print line
4	angle	Starts with 'a' and ends with 'e'	Yes	Print line
5	grape	Does not start with 'a'	No	Skip line
6	ale	Starts with 'a' and ends with 'e'	Yes	Print line
7	bee	Does not start with 'a'	No	Skip line

💡 All lines processed, grep ends after last line.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	After 7	Final
Current Line	none	apple	banana	ace	angle	grape	ale	bee	none
Match Result	none	Yes	No	Yes	Yes	No	Yes	No	none
Output Lines	[]	[apple]	[apple]	[apple, ace]	[apple, ace, angle]	[apple, ace, angle]	[apple, ace, angle, ale]	[apple, ace, angle, ale]	[apple, ace, angle, ale]

Key Moments - 3 Insights

Why does grep skip lines that do not start with 'a'?

Because the regex '^a.*e$' requires the line to start with 'a'. Lines like 'banana' fail this check as shown in execution_table row 2.

What does the '.*' in the regex mean?

'.*' means any characters (zero or more) between the start 'a' and the end 'e'. This allows lines like 'apple' and 'angle' to match, as seen in rows 1 and 4.

Why does grep print lines that end with 'e' only?

The regex ends with 'e$', meaning the line must end with 'e'. Lines like 'ale' match this, but 'bee' does not start with 'a' so it is skipped (row 7).

Visual Quiz - 3 Questions

Look at the execution table, what is the match result for line 3 ('ace')?

A Skipped

B No

C Yes

D Error

💡 Hint

Check row 3 under 'Match Result' column in execution_table.

At which step does grep skip a line because it does not start with 'a'?

A Step 2

B Step 1

C Step 4

D Step 6

💡 Hint

Look at 'Match Result' and 'Action' columns for lines not starting with 'a' in execution_table.

If the regex changed to '^b.*e$', which line would grep print at step 7?

A angle

B bee

C apple

D ale

💡 Hint

Check which lines start with 'b' and end with 'e' in the original lines from variable_tracker.

Concept Snapshot

grep with regex:
- Reads file line by line
- Checks each line against regex
- Prints lines matching regex
- '^' means start of line
- '$' means end of line
- '.*' means any characters in between

Full Transcript

This visual trace shows how grep uses a basic regex to filter lines in a file. The regex '^a.*e$' means lines must start with 'a' and end with 'e'. Each line is checked in order. Lines matching the pattern are printed; others are skipped. Variables track the current line, match result, and output lines. Key moments clarify why some lines match or not. The quiz tests understanding of matching and skipping behavior.