How to use the command "cat" (with examples)

The “cat” command in Unix is used to print and concatenate files. It is a standard Unix utility that can be used to read and display the contents of a file, as well as to combine multiple files together.

Use case 1: Print the contents of a file to stdout

Code:

cat path/to/file

Motivation: This use case is useful when you want to quickly view the contents of a file without opening it in an editor. It allows you to quickly see the content of the file in the terminal.

Explanation: The “cat” command is followed by the path to the file you want to print. The contents of the file will be displayed in the terminal.

Example output:

This is the content of the file.

Use case 2: Concatenate several files into an output file

Code:

cat path/to/file1 path/to/file2 ... > path/to/output_file

Motivation: This use case is helpful when you want to combine the contents of multiple files and save them into a new file. It allows you to merge the contents of different files into a single file.

Explanation: The “cat” command is followed by the paths to the files you want to concatenate. The “>” symbol is used to redirect the output to a file. The contents of all the files will be combined and saved into the specified output file.

Example output: The contents of file1 and file2 are merged and saved into output_file.

Use case 3: Append several files to an output file

Code:

cat path/to/file1 path/to/file2 ... >> path/to/output_file

Motivation: This use case is similar to the previous one, but instead of creating a new file, it appends the contents of multiple files to an existing file. It allows you to add more content to an existing file.

Explanation: The “cat” command is followed by the paths to the files you want to append. The “»” symbol is used to append the output to a file. The contents of all the files will be concatenated and appended to the specified output file.

Example output: The contents of file1 and file2 are appended to the end of output_file.

Use case 4: Copy the contents of a file into an output file without buffering

Code:

cat -u /dev/tty12 > /dev/tty13

Motivation: This use case is useful when you want to copy the contents of one file directly to another without any buffering. It is especially helpful in systems where real-time data transfer is required.

Explanation: The “-u” option is used to disable buffering and make the copy operation immediate. The “/dev/tty12” and “/dev/tty13” are special device files for terminal input and output. By using these special files, the contents of tty12 will be copied to tty13 immediately.

Example output: The contents of the file are copied from one terminal to another.

Use case 5: Write stdin to a file

Code:

cat - > path/to/file

Motivation: This use case is helpful when you want to save the input from the terminal directly into a file. It allows you to quickly create or append to a file with the content entered in the terminal.

Explanation: The “-” symbol is used to represent stdin as the input source. By using this symbol, the content entered in the terminal will be written to the specified file.

Example output: Whatever input is entered in the terminal will be saved into the file.

Use case 6: Number all output lines

Code:

cat -n path/to/file

Motivation: This use case is useful when you want to display the lines of a file along with line numbers. It makes it easier to refer to specific lines in the file when discussing or editing it.

Explanation: The “-n” option is used to number all the output lines. When you use this option, each line of the file will be displayed with a line number.

Example output:

1 This is line 1.
2 This is line 2.
3 This is line 3.

Use case 7: Display non-printable and whitespace characters (with M- prefix if non-ASCII)

Code:

cat -v -t -e path/to/file

Motivation: This use case is helpful when you want to visualize non-printable and whitespace characters in a file. It allows you to see hidden characters or special formatting in the file.

Explanation: The “-v” option is used to display non-printable characters. The “-t” option is used to display TAB characters as “^I”. The “-e” option is used to display the end of each line as “$”. When you use all these options, the file contents are displayed with all the non-printable characters and special formatting represented appropriately.

Example output:

This line has a TAB character    This line is followed by a newline character$