Linux: Use Cat Command to Write to File

Objective: Use cat command command to write to a text file.

To write to a file using the cat command, we can use Here Documents. A here document is a special-purpose code block. It uses a form of I/O redirection to feed a command list to an interactive program or a command, such as ftp or cat.

COMMAND << EOF
...
...
...
EOF

COMMAND << EOF

...

EOF

To write the string Hello World to a file called hello.txt using here document and cat, use the following syntax:

cat << EOF > hello.txt
Hello World
EOF

cat << EOF > hello.txt

Hello World

EOF

If instead, you are looking at concatenating or appending file contents to a new file, then here document is not required. To concatenate files in1.txt and in2.txt to output.txt, use the following syntax.

cat in1.txt in2.txt > output.txt

1	cat in1.txt in2.txt > output.txt

Here’s an interview question that I came across. Are you able to guess the output of the following commands?

cat<<cat>cat
<cat>
cat
cat cat

cat<<cat>cat

<cat>

cat

cat cat

You will get the following output:

<cat>

<cat>

The here document is used with the cat command to write the text <cat> to a file called cat and the cat command is used to print the file cat to the screen.

Mohamed Ibrahim

ibrahim = { interested_in(unix, linux, android, open_source, reverse_engineering); coding(c, shell, php, python, java, javascript, nodejs, react); plays_on(xbox, ps4); linux_desktop_user(true); }