Bash Script to Count Lines in a File

Use the command wc -l filename in a Bash script to count the number of lines in a file, or assign it to a variable with lines=$(wc -l < filename).

Examples

Inputfile.txt content: Hello World

Output2 file.txt

Inputempty.txt content:

Output0 empty.txt

Inputsingle_line.txt content: Just one line

Output1 single_line.txt

How to Think About It

To count lines in a file, think of reading the file and counting each line break. The wc -l command does this efficiently by counting newline characters. The script just needs to run this command on the file and show the result.

Algorithm

1

Get the filename as input.

2

Use the <code>wc -l</code> command to count lines in the file.

3

Print or store the line count result.

Code

bash

#!/bin/bash

filename="$1"

if [[ ! -f "$filename" ]]; then
  echo "File not found: $filename"
  exit 1
fi

lines=$(wc -l < "$filename")
echo "Number of lines in $filename: $lines"

Output

Number of lines in file.txt: 2

Dry Run

Let's trace counting lines in a file named 'file.txt' with 2 lines.

1

Check if file exists

file.txt exists, so continue

2

Count lines

Run wc -l < file.txt, output is '2'

3

Print result

Print 'Number of lines in file.txt: 2'

Step	Action	Value
1	Check file existence	file.txt found
2	Count lines with wc -l	2
3	Print output	Number of lines in file.txt: 2

Why This Works

Step 1: File existence check

The script first checks if the file exists using [[ -f filename ]] to avoid errors.

Step 2: Counting lines

The command wc -l < filename counts the number of newline characters, which equals the number of lines.

Step 3: Output formatting

The script stores the count in a variable and prints a friendly message with the filename and line count.

Alternative Approaches

Using awk

bash

#!/bin/bash
filename="$1"
if [[ ! -f "$filename" ]]; then
  echo "File not found: $filename"
  exit 1
fi
lines=$(awk 'END {print NR}' "$filename")
echo "Number of lines in $filename: $lines"

Awk reads the file and prints the total record count, which equals lines; useful if you want more processing later.

Using sed

bash

#!/bin/bash
filename="$1"
if [[ ! -f "$filename" ]]; then
  echo "File not found: $filename"
  exit 1
fi
lines=$(sed -n '$=' "$filename")
echo "Number of lines in $filename: $lines"

Sed can print the last line number with '$=' which equals the total lines; slightly less common but effective.

Complexity: O(n) time, O(1) space

Time Complexity

The command reads through the entire file once to count newline characters, so time grows linearly with file size.

Space Complexity

The command uses constant extra space since it counts lines on the fly without storing the whole file.

Which Approach is Fastest?

All methods (wc, awk, sed) read the file once, so they have similar performance; wc -l is simplest and fastest for just counting lines.

Approach	Time	Space	Best For
wc -l	O(n)	O(1)	Simple line count
awk 'END {print NR}'	O(n)	O(1)	Line count with text processing
sed -n '$='	O(n)	O(1)	Line count using stream editor