Linux cat Command: Concatenate and Display File Contents

The cat (concatenate) command is one of the most frequently used commands in Linux. It reads data from files and outputs their contents. It can also be used to create and combine files.

Basic Syntax

cat [options] [file(s)]

Common Options

• -n: Number all output lines
• -b: Number non-blank output lines
• -s: Suppress repeated empty output lines
• -A: Show all non-printing characters
• -v: Show non-printing characters except tabs and line feeds
• -E: Display $ at end of each line
• -T: Display TAB characters as ^I

Real-World Examples

1. View File Contents

# Display single file
$ cat file.txt

# Display multiple files
$ cat file1.txt file2.txt

# Display with line numbers
$ cat -n file.txt

2. File Creation and Input

# Create new file from keyboard input
$ cat > newfile.txt
Type your text here
Press Ctrl+D when done

# Append to existing file
$ cat >> existing.txt
Add more text
Press Ctrl+D when done

3. File Combination

# Combine files
$ cat file1.txt file2.txt > combined.txt

# Append file to another
$ cat appendfile.txt >> mainfile.txt

Common Use Cases

1. Quick File Viewing

   # View configuration file
   cat /etc/nginx/nginx.conf
   
   # View system information
   cat /proc/cpuinfo
   

2. File Creation

   # Create script file
   cat > script.sh << 'EOF'
   #!/bin/bash
   echo "Hello, World!"
   EOF
   

3. File Concatenation

   # Combine log files
   cat access.log.* > combined_logs.txt
   
   # Merge configuration files
   cat config.d/* > full_config.conf
   

Tips and Tricks

1. Display Special Characters

   # Show all special characters
   cat -A file.txt
   
   # Show line endings
   cat -E file.txt
   

2. Handle Large Files

   # Use with head/tail
   cat large_file.txt | head -n 20
   
   # Use with less for pagination
   cat large_file.txt | less
   

3. Multiple File Operations

   # Sort and combine files
   cat file*.txt | sort > sorted_combined.txt
   
   # Filter and save
   cat logs.txt | grep "error" > errors.txt
   

Best Practices

1. Use Appropriate Tools

   # For large files, prefer less
   less large_file.txt
   
   # For viewing end of file
   tail -f log_file.txt
   

2. Handle Binary Files

   # Check if file is binary
   file myfile
   
   # Use hexdump for binary files
   hexdump -C binary_file
   

3. Error Handling

   # Check if file exists
   cat file.txt 2>/dev/null || echo "File not found"
   

Common Errors and Solutions

1. No Such File

   # Check file existence first
   [ -f file.txt ] && cat file.txt
   

2. Permission Denied

   # Use sudo if needed
   sudo cat /etc/shadow
   

3. Binary File Warning

   # Use appropriate viewer
   strings binary_file
   

Related Commands

• less: View files with pagination
• more: Simple file viewer
• head: View beginning of file
• tail: View end of file
• tac: View file in reverse

Advanced Usage

1. Here Documents

cat << EOF > config.ini
[Settings]
User = ${USER}
Home = ${HOME}
Date = $(date)
EOF

2. Process Substitution

# Compare command outputs
cat <(ls -l) <(ls -la)

# Process multiple inputs
cat <(sort file1.txt) <(sort file2.txt) > sorted_all.txt

3. Text Processing

# Remove blank lines
cat -s input.txt > output.txt

# Number non-blank lines
cat -b code.txt > numbered_code.txt

Scripting Examples

1. File Processing

#!/bin/bash
# Process multiple files
for file in *.txt; do
    echo "=== $file ==="
    cat -n "$file"
    echo
done

2. Log Analysis

#!/bin/bash
# Extract and format log entries
cat access.log | while read line; do
    if [[ $line == *"ERROR"* ]]; then
        echo "[!] $line"
    fi
done

3. File Generation

#!/bin/bash
# Generate configuration file
cat > config.json << EOF
{
    "user": "$(whoami)",
    "date": "$(date)",
    "system": "$(uname -a)"
}
EOF

Remember that while cat is simple, it’s incredibly versatile. It’s not just for viewing files – it’s a fundamental tool in text processing pipelines and shell scripting. However, for large files or specific viewing needs, consider using specialized tools like less, tail, or grep.

FAQ

What does the cat command do?

The cat command is used to read and concatenate files. It can display file contents, create new files, and combine multiple files into one.

How can I display line numbers using cat?

To display line numbers, use the -n option with the cat command:

cat -n file.txt

This will number all output lines.

Can cat be used to create files?

Yes, you can create files using cat by redirecting input to a new file:

cat > newfile.txt

Type your content and press Ctrl+D to save.

How do I combine multiple files using cat?

To combine files, list them in order and redirect the output to a new file:

cat file1.txt file2.txt > combined.txt

This will merge the contents of file1.txt and file2.txt into combined.txt.

What should I do if cat displays a “Permission Denied” error?

If you encounter a “Permission Denied” error, you may need to use sudo to run the command with elevated privileges:

sudo cat /etc/shadow

Be cautious when using sudo to avoid unintended changes to system files.