Using cat

Syntax#

• cat [OPTIONS]… [FILE]…

Parameters#

cat file.txt

will print the contents of a file.

If the file contains non-ASCII characters, you can display those characters symbolically with cat -v. This can be quite useful for situations where control characters would otherwise be invisible.

cat -v unicode.txt

Very often, for interactive use, you are better off using an interactive pager like less or more, though. (less is far more powerful than more and it is advised to use less more often than more.)

less file.txt

To pass the contents of a file as input to a command. An approach usually seen as better (UUOC) is to use redirection.

tr A-Z a-z <file.txt   # as an alternative to cat file.txt | tr A-Z a-z

In case the content needs to be listed backwards from its end the command tac can be used:

tac file.txt

If you want to print the contents with line numbers, then use -n with cat:

cat -n file.txt

To display the contents of a file in a completely unambiguous byte-by-byte form, a hex dump is the standard solution. This is good for very brief snippets of a file, such as when you don’t know the precise encoding. The standard hex dump utility is od -cH, though the representation is slightly cumbersome; common replacements include xxd and hexdump.

$ printf 'Hëllö wörld' | xxd
0000000: 48c3 ab6c 6cc3 b620 77c3 b672 6c64       H..ll.. w..rld

Display line numbers with output

Use the --number flag to print line numbers before each line. Alternatively, -n does the same thing.

$ cat --number file

 1  line 1
 2  line 2
 3
 4  line 4
 5  line 5

To skip empty lines when counting lines, use the --number-nonblank, or simply -b.

$ cat -b file

 1  line 1
 2  line 2

 3  line 4
 4  line 5

Read from standard input

cat < file.txt

Output is same as cat file.txt, but it reads the contents of the file from standard input instead of directly from the file.

printf "first line\nSecond line\n" | cat -n 

The echo command before | outputs two lines. The cat command acts on the output to add line numbers.

Concatenate files

This is the primary purpose of cat.

cat file1 file2 file3 > file_all

cat can also be used similarly to concatenate files as part of a pipeline, e.g.

cat file1 file2 file3 | grep foo

Write to a file

cat >file

It will let you write the text on terminal which will be saved in a file named file.

cat >>file

will do the same, except it will append the text to the end of the file.

N.B: Ctrl+D to end writing text on terminal (Linux)

A here document can be used to inline the contents of a file into a command line or a script:

cat <<END >file
Hello, World.
END

The token after the << redirection symbol is an arbitrary string which needs to occur alone on a line (with no leading or trailing whitespace) to indicate the end of the here document. You can add quoting to prevent the shell from performing command substitution and variable interpolation:

cat <<'fnord'
Nothing in `here` will be $changed
fnord

(Without the quotes, here would be executed as a command, and $changed would be substituted with the value of the variable changed — or nothing, if it was undefined.)

Show non printable characters

This is useful to see if there are any non-printable characters, or non-ASCII characters.

e.g. If you have copy-pasted the code from web, you may have quotes like ” instead of standard ".

$ cat -v file.txt
$ cat -vE file.txt # Useful in detecting trailing spaces.

e.g.

$ echo '”     ' | cat -vE # echo | will be replaced by actual file.
M-bM-^@M-^]     $

You may also want to use cat -A (A for All) that is equivalent to cat -vET. It will display TAB characters (displayed as ^I), non printable characters and end of each line:

$ echo '” `' | cat -A
M-bM-^@M-^]^I`$

Concatenate gzipped files

Files compressed by gzip can be directly concatenated into larger gzipped files.

cat file1.gz file2.gz file3.gz > combined.gz

This is a property of gzip that is less efficient than concatenating the input files and gzipping the result:

cat file1 file2 file3 | gzip > combined.gz

A complete demonstration:

echo 'Hello world!' > hello.txt
echo 'Howdy world!' > howdy.txt
gzip hello.txt 
gzip howdy.txt

cat hello.txt.gz howdy.txt.gz > greetings.txt.gz

gunzip greetings.txt.gz

cat greetings.txt

Which results in

Hello world!
Howdy world!

Notice that greetings.txt.gz is a single file and is decompressed as the single file greeting.txt. Contrast this with tar -czf hello.txt howdy.txt > greetings.tar.gz, which keeps the files separate inside the tarball.