Reading files line by line (while read) in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the while read loop do in bash scripting?

It reads a file line by line, assigning each line to a variable for processing inside the loop.

intermediate

How do you safely read lines with spaces or special characters using while read?

Use IFS= before read to preserve leading/trailing spaces and special characters.

intermediate

Why is it better to use while read instead of cat file | while read?

Using while read with input redirection avoids creating a subshell, so variables set inside the loop remain accessible after it ends.

beginner

What happens if you don't use IFS= when reading lines with read?

The read command trims leading/trailing spaces and splits the line on spaces or tabs, which can change the original line content.

beginner

Show a simple bash script snippet to read a file named data.txt line by line and print each line.

while read -r line; do echo "$line" done < data.txt

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

A Prevents backslash escapes from being interpreted

B Reads raw binary data

C Reads only the first word of the line

D Reads the file in reverse order

The -r option tells read not to treat backslashes as escape characters, preserving them literally.

Which command correctly reads a file line by line without losing variable values after the loop?

A while read line; do ... done <<< "$(cat file.txt)"

B cat file.txt | while read line; do ... done

C while read line; do ... done < file.txt

D for line in $(cat file.txt); do ... done

Redirecting the file into the while loop (done < file.txt) avoids subshells, so variables set inside remain after the loop.

What is the purpose of setting IFS= before read?

A To split the line into words

B To ignore empty lines

C To convert tabs to spaces

D To preserve leading and trailing spaces in the line

Setting IFS= disables word splitting, so the entire line including spaces is read as-is.

Which of these is a common mistake when reading lines with read?

A Not redirecting the file into the loop and using a pipe instead

B Using <code>read -r</code> to prevent backslash escapes

C Using <code>IFS=</code> to preserve spaces

D Using a variable to store each line

Using a pipe creates a subshell, so variables set inside the loop are lost after it ends.

What does this script print?

while read -r line; do echo "$line"; done < file.txt

A Nothing, it has a syntax error

B Each line of file.txt printed exactly as is

C The file content reversed

D Only the first word of each line

The script reads each line safely with -r and prints it exactly as it appears in the file.

Explain how to read a file line by line in bash using while read and why it is useful.

Think about how you would read a list of names one at a time.

Describe the role of IFS= and -r options when reading lines with read.

Consider how spaces and backslashes might change the line if not handled.