cat Cheat Sheet

Practical uses of cat for viewing files, numbering lines, revealing control characters and combining files.

A quick reference for cat (concatenate): print files, join them, and pipe their contents elsewhere.

View a file

cat file.txt

Prints the whole file to the terminal. Good for short configuration files such as /etc/hostname or /etc/resolv.conf. For anything long, use a pager (less file.txt) so it does not scroll off screen.

Number every line

cat -n file.txt

Prefixes each line with a line number. Handy when someone reports an error at “line 42” of a config file.

Number non-blank lines

cat -b file.txt

Like -n, but blank lines are not counted. Useful for files with many spacer lines.

Show hidden and control characters

cat -A file.txt

Displays line endings as $, tabs as ^I, and other control characters in caret notation. Use it to spot Windows \r line endings (^M$), stray trailing whitespace, or tabs-vs-spaces problems that are invisible otherwise. -A is equivalent to -vET.

Squeeze repeated blank lines

cat -s file.txt

Collapses runs of blank lines into a single blank line, making padded files easier to read.

Display multiple files

cat file1 file2
cat /etc/cron.d/*

Prints the files back to back, in order. Useful for reviewing a set of small config fragments at once.

Combine files into a new file

cat file1 file2 > combined.txt

Writes the concatenated contents to combined.txt, overwriting it if it exists.

Append to a file

cat file.txt >> combined.txt

Adds the contents of file.txt to the end of combined.txt without erasing what is already there.

Related commands

• tac — same as cat but prints lines in reverse order (last line first).
• head -n 20 file — first 20 lines only.
• tail -n 20 file — last 20 lines; tail -f follows a log file as it grows.

Keep cat for viewing and joining; reach for head, tail or less when a file is too large to dump in full.