Linux and Unix cat command
cat is one of the most commonly-used commands in Linux. It can be used to:
- Display text files
- Copy text files into a new document
- Append the contents of a text file to the end of another text file, combining them
Displaying Text Files
The simplest way to use cat is to simply give it the name of a text file. It will display the contents of the text file on the screen. For instance:
...will read the contents of mytext.txt and send them to standard output (your terminal screen). If mytext.txt is very long, they will zoom past and you will only see the last screen's worth of your document.
(If you want to view the document page-by-page or scroll back and forth through the document, you can use a pager or viewer such as pg, more, or less.)
If you specify more than one file name, cat will display those files one after the other, catenating their contents to standard output. So this command:
cat mytext.txt mytext2.txt
Will print the contents of those two text files as if they were a single file.
Copy A Text File
Normally you would copy a file with the cp command. You can use cat to make copies of text files in much the same way.
cat sends its output to stdout (standard output), which is usually the terminal screen. However, you can redirect this output to a file using the shell redirection symbol ">".
For instance, this command:
cat mytext.txt > newfile.txt
...will read the contents of mytext.txt and send them to standard output; instead of displaying the text, however, the shell will redirect the output to the file newfile.txt. If newfile.txt does not exist, it will be created. (If newfile.txt already exists, it will be overwritten and its previous contents will be lost, so be careful.)
Similarly, you can catenate several files into your destination file. For instance:
cat mytext.txt mytext2.txt > newfile.txt
...will read the contents of mytext.txt and mytext2.txt and write the combined text to the file newfile.txt. Again, if newfile.txt does not already exist, it will be created; if it already exists, it will be overwritten.
Append A Text File's Contents To Another Text File
Instead of overwriting another file, you can also append a source text file to another using the redirection operator ">>".
cat mytext.txt >> another-text-file.txt
...will read the contents of mytext.txt, and write them at the end of another-text-file.txt. If another-text-file.txt does not already exist, it will be created and the contents of mytext.txt will be written to the new file.
This works for multiple text files as well:
cat mytext.txt mytext2.txt >> another-text-file.txt
...will write the combined contents of mytext.txt and mytext2.txt to the end of another-text-file.txt.
Incorporating Standard Input Into cat Output
cat will read from standard input if you specify a hyphen ("-") as a file name. For example, if you have a file, list.txt, which contains the following text:
apples oranges butter bread
...you could use the echo command to output text, pipe that output to cat, and instruct cat to catenate it with the file's contents, like this:
echo "My Shopping List" | cat - list.txt
...which would output the following text:
My Shopping List apples oranges butter bread
In short, cat is a simple but very useful tool for working with the data in text files on your system. This includes many important files like system logs and configuration files, and any other human-readable data stored in a file.
cat [OPTION]... [FILE]...
These options are available on GNU cat, which is standard on most Linux distributions. If you are using a different Unix-like operating system (BSD, for example), some of these options may not be available; check your specific documentation for details.
|-A, --show-all||Equivalent to -vET.|
|-b, --number-nonblank||Number non-empty output lines. This option overrides -n.|
|-e||Equivalent to -vE.|
|-E, --show-ends||Display "$" at end of each line.|
|-n, --number||Number all output lines.|
|-s, --squeeze-blank||Suppress repeated empty output lines.|
|-t||Equivalent to -vT.|
|-T, --show-tabs||Display TAB characters as ^I.|
|-v, --show-nonprinting||Use ^ and M- notation, except for LFD and TAB.|
|--help||Display a help message, and exit.|
|--version||Output version information, and exit.|
Read the contents of file.txt and display them on the screen.
cat file1.txt file2.txt
Reads the contents of file1.txt and file2.txt, and displays them in order on the terminal screen.
cat file.txt > newfile.txt
Read the contents of file.txt and write them to newfile.txt, overwriting anything newfile.txt previously contained. If newfile.txt does not exist, it will be created.
cat file.txt >> another-file.txt
Read the contents of file.txt and append them to the end of another-file.txt. If another-file.txt does not exist, it will be created.
cat -s file.txt
Display the contents of file.txt, omitting any repeated blank lines.
cp — Copy files and directories.
ed — A simple text editor.
less — Scrolling text viewer.
more — Display text one screen at a time.
pico — A simple text editor.
pg — Browse page by page through text files.
tac — Output the contents of files in reverse order.
tee — Route a file's contents to multiple outputs.
touch — Update the timestamp of a file or directory.