Linux Cat Command: Useful Examples

Understanding the Cat Command Syntax

The cat command in Linux follows this syntax structure:

cat [OPTIONS] [FILE(S)]

Command Components

Common Options

Usage Examples

Single File Example

cat -n file.txt

This displays the contents of file.txt with line numbers.

Multiple Files Example

cat file1.txt file2.txt file3.txt

This shows the contents of all three files in the order listed.

Standard Input

If no file is specified, cat waits for keyboard input. Press Ctrl+D to end input.

Combining Options

You can use multiple options together. For example:

cat -ns file.txt

This numbers non-blank lines and suppresses repeated empty lines in file.txt.

cat > newfile.txt
Enter your text here
Press Ctrl+D when done

Viewing Single File Contents

Example:

cat example.txt

tac example.txt

Displaying Multiple File Contents

Creating New Files with Cat

The cat command can create new files and add content to them in one step. Here's how to do it:

echo "This is a quick file" | cat > quickfile.txt

Appending Content to Existing Files

To add content to the end of existing files, use the >> operator:

Key Points

• Use > to create or overwrite a file
• Use >> to append to an existing file without erasing its current content

Combining Multiple Files

The cat command can merge the contents of multiple files into a single file. This process is called concatenation. Here's how to do it:

cat file1.txt file2.txt | sort > sorted_combined.txt

cat log1.txt log2.txt log3.txt | sort -n > combined_logs.txt

Copying File Contents

The cat command can also copy the contents of one file to another. Here's how:

Key Points

• Use > to create a new file or overwrite an existing one
• Use >> to append to an existing file without erasing its current content
• When combining files, the order of files in the command determines the order of content in the output file
• Be careful when using > as it will overwrite the destination file without warning

Displaying Line Numbers

The cat command provides options to add line numbers to file contents, helping with line referencing and text structure analysis.

Examples:

1. Number all lines:

   cat -n example.txt

2. Number only non-blank lines:

   cat -b example.txt

Combining Numbering with Other Options

You can use line numbering with other options:

cat -ns example.txt

This numbers non-blank lines and suppresses repeated empty lines.

Showing Non-Printing Characters

Cat can reveal non-printing characters, useful for debugging or analyzing file formatting.

Examples:

1. Display TAB characters:

   cat -T script.sh

2. Show end-of-line markers:

   cat -E config.txt

3. Reveal all non-printing characters:

   
   cat -A document.txt
   ``

Here's the expanded information organized in tables and using Mermaid.js diagrams where possible, formatted in markdown:

Suppressing Empty Lines

The cat command can remove repeated empty lines from the output, making the content more compact and easier to read.

To suppress repeated empty lines, use the -s option:

cat -s filename.txt

This command will display the file content with only one blank line between paragraphs or sections, regardless of how many empty lines are in the original file.

cat -s /var/log/syslog | less

Reversing File Contents

To display file contents in reverse order (last line first), use the tac command, which is cat spelled backward:

tac filename.txt

The tac command reads the file from bottom to top and outputs each line in reverse order.

tac /var/log/auth.log | less

(head -n 1 data.csv && tail -n +2 data.csv | tac) > reversed_data.csv

Using Cat with More and Less Commands

Cat with More

cat largefile.txt | more

Cat with Less

cat largefile.txt | less

cat /var/log/syslog | less

Viewing Multiple Large Files

To view multiple large files sequentially:

cat file1.txt file2.txt file3.txt | less

Displaying with Line Numbers

To display large files with line numbers:

cat -n largefile.txt | less

cat largefile.txt | grep -n "search_pattern" | less

Binary and Hexadecimal Conversion

The cat command, combined with other Linux utilities, can convert file contents to binary and hexadecimal formats. This is useful for analyzing file structures, debugging, or working with low-level data.

Converting to Binary Format

To convert file contents to binary format, use cat with the xxd command:

cat file.txt | xxd -b

This command displays the file content in binary format, showing both the binary representation and the ASCII characters.

Example output:

00000000: 01001000 01100101 01101100 01101100 01101111  Hello
00000005: 00100000 01010111 01101111 01110010 01101100   Worl
0000000a: 01100100 00001010                             d.

Converting to Hexadecimal Format

To convert file contents to hexadecimal format, use cat with xxd without any options:

cat file.txt | xxd

This command displays the file content in hexadecimal format, showing both the hex values and the ASCII characters.

Example output:

00000000: 4865 6c6c 6f20 576f 726c 640a              Hello World.

For a more compact hexadecimal view, you can use the hexdump command:

cat file.txt | hexdump -C

Example output:

00000000  48 65 6c 6c 6f 20 57 6f  72 6c 64 0a           |Hello World.|
0000000c

cat file1.bin | hexdump -C > file1.hex
cat file2.bin | hexdump -C > file2.hex
diff file1.hex file2.hex

cat image.png | xxd -l 32

These conversion methods are valuable for users who need to analyze file structures, debug binary data, or work with low-level file formats. They provide different ways to view file contents beyond the standard text representation, offering insights into the underlying data structure of files.

Displaying Specific Lines

The cat command, combined with other utilities, can display specific lines or portions of file contents. This is useful for focusing on particular sections of a file without viewing the entire content.

To display specific lines, you can use cat with the sed or awk commands:

cat file.txt | grep "important" | sed -n '1,5p'

Sorting File Contents

Cat can be used with the sort command to alphabetically or numerically sort file contents. This helps organize data or find specific entries in large files.

These techniques allow users to extract and organize specific information from files, making data analysis and file management tasks more manageable.

Viewing CPU Information

The cat command displays system CPU details by reading files in the /proc filesystem. This virtual filesystem provides an interface to kernel data structures, allowing access to real-time system information.

To view CPU information:

cat /proc/cpuinfo

This command shows detailed information about the system's CPU(s):

cat /proc/cpuinfo | grep "model name"

For a summary of CPU cores:

cat /proc/cpuinfo | grep "cpu cores" | uniq

To view the total number of processors:

cat /proc/cpuinfo | grep "physical id" | sort | uniq | wc -l

cat /sys/class/thermal/thermal_zone*/temp

Using cat to view system information provides a quick way to access important hardware details without additional software. This can be useful for system administrators, developers, and users who need to check system specifications or troubleshoot hardware-related issues.