Reading files line by line (while read) in Bash Scripting - Step-by-Step Execution

Concept Flow - Reading files line by line (while read)

Open file

↓

Read line

↓

Is line empty?

Yes→End loop

No↓

Process line

↓

Read next line

↩Back to Is line empty?

The script opens a file and reads it line by line. For each line, it processes the content until no more lines remain.

Execution Sample

Bash Scripting

while IFS= read -r line; do
  echo "$line"
done < input.txt

Reads each line from input.txt and prints it to the screen.

Execution Table

Step	Action	Line Read	Condition (line empty?)	Branch Taken	Output
1	Read first line	Hello, world!	No	Process line	Hello, world!
2	Read second line	This is a test.	No	Process line	This is a test.
3	Read third line	Line three here.	No	Process line	Line three here.
4	Read fourth line		Yes	End loop

💡 Reached end of file; empty line read signals loop termination.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
line		Hello, world!	This is a test.	Line three here.

Key Moments - 3 Insights

Why do we use 'IFS=' before read?

Setting IFS= prevents trimming of leading/trailing whitespace in the line, ensuring the line is read exactly as is (see execution_table rows 1-3 where lines keep spaces).

What does the '-r' option do in 'read -r'?

The '-r' option tells read not to treat backslashes as escape characters, so lines with backslashes are read literally (important for accurate line reading).

Why does the loop end when an empty line is read?

When read reaches the end of the file, it returns an empty line and the condition detects this to stop the loop (see execution_table row 4).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'line' after step 2?

A "This is a test."

B "Hello, world!"

C "Line three here."

D ""

💡 Hint

Check the 'Line Read' column at step 2 in the execution_table.

At which step does the loop stop reading lines?

A Step 3

B Step 4

C Step 2

D Step 1

💡 Hint

Look at the 'Condition (line empty?)' and 'Branch Taken' columns in the execution_table.

If we remove 'IFS=' from the read command, what might happen?

A Leading/trailing spaces in lines might be trimmed

B The loop will never end

C Backslashes will be treated literally

D The file will not open

💡 Hint

Refer to the key_moments about IFS and how lines are read exactly.

Concept Snapshot

Reading files line by line in bash:
Use 'while IFS= read -r line; do ... done < file'
- IFS= preserves spaces
- -r prevents backslash escapes
- Loop ends when read returns empty line
- Process each line inside the loop

Full Transcript

This example shows how to read a file line by line in bash using a while loop with 'read'. The loop reads each line into the variable 'line' and prints it. The 'IFS=' ensures spaces are preserved, and '-r' prevents backslash escapes. The loop stops when no more lines are available, indicated by an empty line. This method is useful for processing files safely and accurately.