The CAT command in Linux is helpful in creating text files, displaying their contents, concatenating text from two or more text files into a new file. We can also append text from terminal to a file or append text to an existing file.

Create a new file

cat > newfile.txt

Concatenating multiple files to a new file

cat file1.txt file2.txt > newfile.txt

displays :

# Output:
# [Contents of file1.txt]
# [Contents of file2.txt]

Concatenate text from four files and save it in alphabetical order

We have four files we want to be concatenated in alphabetical order

cat file1.txt file2.txt file3.txt file4.txt

displays :

B from file1.txt
A from file2.txt
C from file4.txt
D from file3.txt

cat file1.txt file2.txt file3.txt file4.txt | sort > file5.txt

cat file5.txt 

will display :

A from file2.txt
B from file1.txt
C from file4.txt
D from file3.txt

Appending text from the Terminal to a file

cat >> term.txt

Appending text from one file to another

 cat file2.txt >> file1.txt

Displaying Line Numbers

to number all output lines we can use -n option :

cat -n file.txt

Suppress Repeated Empty Lines

If a file have unnecessary spacing, using the -s flag permits reducing multiple adjacent blank lines to a single blank line.

cat -s file.txt

The ‘tac’: Reverse ‘cat’

The tac command is the reverse of cat. It concatenates and displays files in reverse, starting from the last line and ending at the first line.

tac file.txt