Daily Linux Command: cat

cat is one of the most commonly used and fundamental commands in Linux systems, primarily used to view, concatenate, and output file contents. It can output file contents to the terminal (screen) or redirect them to other files or commands.

📌 Command Format:

cat [options] [filenames...]

🧾 Common Options:

Option	Description
`-n`	Display line numbers (including empty lines)
`-b`	Display line numbers, but ignore empty lines
`-E`	Display a `$` symbol at the end of each line
`-T`	Display Tabs as `^I`
`-v`	Show non-printing characters (except for newline and Tab)
`-s`	Suppress repeated empty lines to a single empty line
`--help`	Display help information
`--version`	Show the version information of `cat`

💡 Example Usage:

Example 1: View File Contents

cat filename.txt

Outputs the entire content of filename.txt to the terminal.

Example 2: Display Line Numbers (Including Empty Lines)

cat -n filename.txt

Example 3: Display Line Numbers (Excluding Empty Lines)

cat -b filename.txt

Example 4: Show End-of-Line Symbol `$`

cat -E filename.txt

Example 5: Concatenate Multiple Files and Output to a New File

cat file1.txt file2.txt > combined.txt

Concatenates the contents of file1.txt and file2.txt and writes to a new file combined.txt.

Example 6: Use Pipe to Pass `cat` Output to Other Commands

cat filename.txt | grep "keyword"

Finds lines in filename.txt that contain “keyword”.

Example 7: Create a File (Interactive)

cat > newfile.txt

After entering content, press Ctrl+D to save and exit.

Example 8: Append Content to a File

cat >> newfile.txt

Appends input content to the end of newfile.txt.

⚠️ Notes:

cat is not suitable for viewing large files, as it outputs the entire content to the terminal at once. Consider using less or more:
```
cat largefile.txt | less
```
If the file does not exist, cat will return an error:
```
cat: filename.txt: No such file or directory
```

✅ Summary of Use Cases:

Purpose	Command
View file contents	`cat filename`
View and display line numbers	`cat -n filename`
Concatenate files	`cat file1 file2 > merged`
Create a file	`cat > filename`
Append content	`cat >> filename`
View non-printing characters	`cat -A filename`