cat

cat

The cat command in Unix and Linux is used to concatenate and display the content of files. It's one of the most frequently used commands for viewing file contents, combining files, and redirecting output.

Basic Usage

The basic syntax for the cat command is:

cat [options] [file...]

• options: Command-line options to control the behavior of cat.

• file: The file(s) to be read. If no file is specified, cat reads from standard input.

Examples

Displaying File Contents

To display the contents of a file:

cat file.txt

This command outputs the entire content of file.txt to the standard output (usually the terminal).

Concatenating Files

To concatenate multiple files and display their contents:

cat file1.txt file2.txt

This command displays the contents of file1.txt followed by file2.txt.

Redirecting Output to a File

To concatenate files and redirect the output to a new file:

cat file1.txt file2.txt > combined.txt

This command combines file1.txt and file2.txt into a new file named combined.txt.

Appending to a File

To append the contents of one file to another:

cat file1.txt >> file2.txt

This command appends the contents of file1.txt to the end of file2.txt.

Options

-n Option: Numbering Lines

To number all output lines:

cat -n file.txt

This command displays the content of file.txt with line numbers.

Example output:

     1  First line
     2  Second line
     3  Third line

-b Option: Numbering Non-Empty Lines

To number only non-empty lines:

cat -b file.txt

This command displays the content of file.txt with line numbers for non-empty lines only.

Example output:

     1  First line

     2  Second line
     3  Third line

-s Option: Squeeze Blank Lines

To suppress repeated empty lines:

cat -s file.txt

This command reduces multiple consecutive blank lines to a single blank line.

-E Option: Displaying End-of-Line Characters

To display a $ at the end of each line:

cat -E file.txt

This command helps visualize the end of each line.

Example output:

First line$
Second line$
Third line$

Practical Use Cases

Viewing File Contents

The most common use of cat is to quickly view the contents of a file.

cat /etc/passwd

Creating Files

You can create a new file by using cat and redirecting input from the terminal:

cat > newfile.txt

Type the content you want to add to newfile.txt and press Ctrl+D to save and exit.

Combining Files

cat is useful for combining multiple files into one:

cat part1.txt part2.txt part3.txt > complete.txt

This combines part1.txt, part2.txt, and part3.txt into complete.txt.

Displaying Line Numbers

To help debug scripts or code, you can display line numbers:

cat -n script.sh

Summary

The cat command is a simple yet powerful tool for viewing, combining, and manipulating file content in Unix and Linux environments. Its versatility makes it an essential command for file management and text processing tasks.

help

Usage: cat [OPTION]... [FILE]...
Concatenate FILE(s) to standard output.

With no FILE, or when FILE is -, read standard input.

  -A, --show-all           equivalent to -vET
  -b, --number-nonblank    number nonempty output lines, 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
  -u                       (ignored)
  -v, --show-nonprinting   use ^ and M- notation, except for LFD and TAB
      --help     display this help and exit
      --version  output version information and exit

Examples:
  cat f - g  Output f's contents, then standard input, then g's contents.
  cat        Copy standard input to standard output.

man

NAME
       cat - concatenate files and print on the standard output

SYNOPSIS
       cat [OPTION]... [FILE]...

DESCRIPTION
       Concatenate FILE(s) to standard output.

       With no FILE, or when FILE is -, read standard input.

       -A, --show-all
              equivalent to -vET

       -b, --number-nonblank
              number nonempty output lines, 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

       -u     (ignored)

       -v, --show-nonprinting
              use ^ and M- notation, except for LFD and TAB

       --help display this help and exit

       --version
              output version information and exit

EXAMPLES
       cat f - g
              Output f's contents, then standard input, then g's contents.

       cat    Copy standard input to standard output.