Bash Script to Merge Two Files Easily

Use the Bash command cat file1 file2 > merged_file to merge two files by concatenating their contents into a new file.

Examples

Inputfile1.txt content: Hello file2.txt content: World

Outputmerged_file content: Hello World

Inputfile1.txt content: Line1 Line2 file2.txt content: Line3 Line4

Outputmerged_file content: Line1 Line2 Line3 Line4

Inputfile1.txt content: (empty) file2.txt content: Only this line

Outputmerged_file content: Only this line

How to Think About It

To merge two files, think of simply putting one file's content after the other. The cat command reads files in order, and redirecting its output to a new file combines them. This is like stacking two pages one after another.

Algorithm

1

Get the names of the two files to merge.

2

Use a command to read the first file's content.

3

Append the second file's content after the first.

4

Redirect the combined content into a new file.

5

Print a message confirming the merge.

Code

bash

#!/bin/bash

file1="$1"
file2="$2"
merged_file="merged_output.txt"

cat "$file1" "$file2" > "$merged_file"
echo "Files '$file1' and '$file2' merged into '$merged_file'."

Output

Files 'file1.txt' and 'file2.txt' merged into 'merged_output.txt'.

Dry Run

Let's trace merging 'file1.txt' with content 'Hello' and 'file2.txt' with content 'World'.

1

Read file1.txt

Content read: 'Hello'

2

Read file2.txt

Content read: 'World'

3

Concatenate and write to merged_output.txt

merged_output.txt content: 'Hello\nWorld'

Step	Action	Result
1	Read file1.txt	Hello
2	Read file2.txt	World
3	Write merged_output.txt	Hello World

Why This Works

Step 1: Using cat to read files

The cat command reads the contents of files in the order given.

Step 2: Redirecting output

Using > sends the combined output into a new file, creating or overwriting it.

Step 3: Merging files

By listing two files with cat, their contents are joined one after the other.

Alternative Approaches

Using tee command

bash

cat file1.txt file2.txt | tee merged_output.txt > /dev/null
echo "Merged using tee."

This uses a pipe and tee to write output; slightly more complex but useful for simultaneous display.

Appending second file to first

bash

cp file1.txt merged_output.txt
cat file2.txt >> merged_output.txt
echo "Merged by appending."

Copies first file then appends second; useful if you want to keep original files unchanged.

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

Time Complexity

The time depends on reading both files fully, so it is proportional to the sum of their sizes (n and m).

Space Complexity

The script writes output directly to a file without extra memory, so space is constant.

Which Approach is Fastest?

Using cat file1 file2 > merged_file is fastest and simplest; alternatives add overhead with pipes or copying.

Approach	Time	Space	Best For
cat with redirection	O(n + m)	O(1)	Simple and fast merging
tee with pipe	O(n + m)	O(1)	Merging with output display
copy then append	O(n + m)	O(1)	Preserving original files