cat

Displaying File Contents

The most straightforward use of cat is displaying the contents of a file to the terminal. Simply provide the filename as an argument:

cat myfile.txt

This command will output the content of myfile.txt to your standard output (usually your terminal). Let’s say myfile.txt contains:

This is the first line.
This is the second line.

Running the command above will display this text in your terminal.

Concatenating Files

cat shines when it comes to joining multiple files. You can concatenate many files into a single output, either to the terminal or to a new file:

cat file1.txt file2.txt file3.txt > combined.txt

This command concatenates file1.txt, file2.txt, and file3.txt and redirects the output to a new file named combined.txt. If combined.txt already exists, it will be overwritten. To append to an existing file, use >> instead of >:

cat file1.txt file2.txt file3.txt >> combined.txt

This appends the content of file1.txt, file2.txt, and file3.txt to the end of combined.txt.

Using cat with Wildcards

Combining cat with shell wildcards allows you to process multiple files matching a specific pattern. For instance:

cat *.txt > all_text_files.txt

This command concatenates all files ending with .txt in the current directory into all_text_files.txt.

Displaying Numbered Lines

Sometimes, it’s helpful to see line numbers alongside the file content. The -n option provides this functionality:

cat -n myfile.txt

This will display myfile.txt with line numbers added at the beginning of each line.

Handling Multiple Files with -s (Suppressing Messages)

When concatenating files, and one of the files doesn’t exist, cat will typically print an error message. The -s (silent) option suppresses these messages:

cat -s file1.txt file2.txt non_existent_file.txt > output.txt

This command will still concatenate file1.txt and file2.txt into output.txt, without displaying an error message for non_existent_file.txt.

Creating Files with cat

You can use cat to create new files and populate them with content. Use the redirection operator > and echo to achieve this:

cat > newfile.txt << EOF
This is the first line of the new file.
This is the second line.
EOF

This creates newfile.txt containing the text within the EOF markers. The EOF indicates the end of the input. You can replace EOF with any other unique string.

Combining cat with Other Commands

The power of cat truly emerges when used in conjunction with other Linux commands within pipes. For example, to count the lines in a file:

cat myfile.txt | wc -l

This pipes the output of cat myfile.txt to the wc -l command, which counts the number of lines.

This demonstrates just a fraction of the possibilities offered by the seemingly simple cat command. Experimenting with different options and combinations will reveal its full potential.