How to append one file to another in Linux from the shell?

In Linux, you can use various methods to append the contents of one file to another from the shell. Below, I will introduce several commonly used methods:

1. Using the cat Command

One of the simplest methods is to use the cat command. The cat command (an abbreviation for 'concatenate') is commonly used for reading, creating, and merging files. To append the contents of file A to the end of file B, you can use the following command:

bash
cat fileA >> fileB

Here, >> is the redirection operator that appends the content of file A to the end of file B without overwriting it.

Example:

Suppose we have two files, text1.txt and text2.txt, where text1.txt contains:

shell
Hello,

and text2.txt contains:

shell
World!

After executing the command cat text1.txt >> text2.txt, the content of text2.txt becomes:

shell
World!
Hello,

2. Using echo and tee Commands

Another method is to use echo combined with the tee command. The tee command reads standard input and writes its content to standard output and one or more files. You can do this:

bash
echo "$(cat fileA)" | tee -a fileB

Here, $(cat fileA) is command substitution, which first outputs the content of fileA as a string. The tee -a fileB command appends this string to fileB.

Example:

Continuing with the above files, this time using echo and tee:

bash
echo "$(cat text1.txt)" | tee -a text2.txt

The result is that text2.txt will again be appended with the content Hello,, becoming:

shell
World!
Hello,
Hello,

3. Using sed or awk

If you need more complex file processing, such as adding content after specific lines, you can use sed or awk. For example, using awk:

bash
awk '1;END{system("cat fileA")}' fileB > temp && mv temp fileB

This command processes fileB, making no changes during processing (the 1; statement prints all lines), executes system("cat fileA") at the end to append the content of fileA to the output, and then saves the output to a temporary file before renaming it back to fileB.

Summary

Based on your specific needs, you can choose the method that best suits your requirements for appending the contents of one file to another. For simple file merging, the cat command is often the most straightforward choice. If you need to control the output or perform more complex text processing during merging, you may need to use tools like tee, sed, or awk.