Redirect operators in Linux and Unix (macOS)

Friday, July 12, 2024 at 10:26 AM | 3 min read

Last modified on Wednesday, June 3, 2026 at 8:17 PM

#command line, #linux, #macOS, #redirect operator, #stdin, #stdout

Redirecting

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Table of Contents

stdin and stdout explained

In order to understand what redirect operators do in Linux or Unix (macOS), one first has to understand what stdin and stdout mean.

In the case of a personal Linux (or macOS) device, for example, by default, stdin refers to a device's keyboard, and stdout refers to Command Line (Terminal). And these pathways are called streams.

That being said, we don't always want to output stdout to Command Line, which is not persistent by default. Perhaps we want to save it somewhere. It is possible to change our default input and output locations. This means making the OS to obtain information from somewhere other than the stdin or send results to somewhere other than the stdout. This functionality is known as redirection.

The output redirector operator (>)

The default output redirector operator is the most popular and well known of redirectors. As previously mentioned, by default, when not using redirection, stdout is made in Terminal. However, this output is not persistent because it is not saved to Terminal. If it was, we would have an awful lot of clutter there!

In Linux Command Line, if I were to run the following in Terminal:

$HOME

the following would be returned in Terminal:

bash: /home/maria: Is a directory

I type $HOME with my laptop keyboard, my stdin, and it is output to the Terminal window, my stdout. But it is not saved anywhere, so I can't refer to that particular output later. As also previously mentioned, I can redirect this output somewhere else, and even save the data using the output redirector (>). Let's say I run the following command instead:

printenv 'HOME' > home.txt

Then, if I use the cat command to output the contents of home.txt:

cat home.txt

It results in the following output in Terminal:

/home/maria

However, if I simply ran the following without using the output redirector operator:

printenv 'HOME'

the following would be returned in Terminal:

/home/maria

And /home/maria would not be redirected as output to home.txt.

The printenv command prints the value of an environment variable, and $HOME is one of them. In this case, printenv is printing the value of the global environment variable called HOME. And since I am not appending $ to the beginning of HOME, it is not being treated as a variable containing a value (HOME=/home/maria), but simply as the value of $HOME.

The input redirector operator (<)

The default input redirector (<) redirects stdin somewhere else. For example, if I type the following in Terminal followed by the Return (Enter) key:

printenv 'HOME' < home.txt

this would result in the following in Terminal:

/home/maria

And if I open up home.txt with the vim command:

vim home.txt

The following would appear:

/home/maria

If I were to combine the input and output operator, for example:

printenv 'HOME' < home.txt > home2.txt

This command both redirects stdin and stdout, and also creates a new file called home2.txt if it does not already exist, thanks to the output operator (>).

When I run the following in Terminal:

cat home2.txt

it returns the following:

/home/maria

When I run the following in Terminal:

cat home.txt

it returns the following:

/home/maria

However, because I am using the (single) regular output operator (>), it overwrites the previous content of home2.txt with a new instance of /home/maria. What if I didn't want to overwrite the contents of home2.txt?

The output redirector append operator (>>)

If I just want to append the contents of stdin (i.e., home.txt) to stdout (i.e., home2.txt), I would run the following in Terminal:

printenv 'HOME' < home.txt >> home2.txt

Then, when I run the following command in Terminal:

cat home2.txt

it returns the following:

/home/maria
/home/maria

And if I run the following in Terminal:

cat home.txt

it returns the following:

/home/maria

The contents of home.txt remains the same because it is the stdin, but home2.txt now includes a second line of text, which has been appended to the original first line of text. No data was overwritten or lost.

An important thing to note: Usually one should use the output redirector append operator (>>) instead of the output redirector operator (>) so as not to lose any file data (unless there is a specific reason to use >)!