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If we want to copy-paste what we just wrote in stdin to stdout, we can use a redirect > or append >>.

How does tee also writing from stdin to stdout different?

kature
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3 Answers3

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tee has one input and two outputs (hence its name, after the tee component used in plumbing). Redirection using > or >> has one input and one output.

Using tee, you redirect stdin both to stdout and (a second copy) to the file specified as argument to tee. Redirection with > or >> can't do that.

For example if you type:

ls -l | tee file-list

you get a directory listing on the terminal and it is simultaneously copied to the file file-list. However if you type

ls -l > file-list

the directory listing is stored only in the file file-list, there is no output on the terminal.

raj
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  • Oh I get it now. Both to stdout and to a file, but why even do that, mere comfortability? – kature Jul 12 '22 at 13:07
  • @kature no. I think you are looking in a specific circumstance where tee has another side-effect. – Philip Couling Jul 12 '22 at 13:08
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    @kature With some long-running operations that produce a lot of output (which is not uncommon in sysadmin tasks) you may need to watch terminal output in real time (to react if something goes wrong) but also to have it logged in a file to analyze later (or just to have documentation what you did). These are cases when tee is useful. – raj Jul 12 '22 at 13:10
  • @raj thanks, that makes sense, maybe you'd like to include that in the end of the answer. – kature Jul 12 '22 at 13:12
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The context of why you are asking this is important.

Example:

cat one_file > another_file

When you do this, cat does NOT open another_file. Your shell opens the other file and sets the stdout of cat to that file descriptor.

This can be problematic if you are trying to write to a file that requires root privileges. So this will most likely not work.

sudo echo 1 > /some/root/owned/file

Above the shell tries to open /some/root/owned/file before executing sudo and does not have permission.

Where as this does work:

echo 1 | sudo tee /some/root/owned/file

That's because tee has been run as root and its tee that opens the file in this context. Not the shell

Philip Couling
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  • Thank you Philip, that's an important distinction in your answer. I asked it without context besides understanding what tee itself does. Now, via Raj's answer I get the concept and via your answer I have made an important distinction. – kature Jul 12 '22 at 13:15
  • This will work, however: sudo sh -c "echo 1 > /some/root/owned/file" – raj Jul 12 '22 at 13:19
  • @raj yes it will. The tee version is commonly seen in tutorials. But both are valid. – Philip Couling Jul 12 '22 at 15:33
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The difference in redirecting output to a file with either >, >> or command | tee -a filename is that with tee you are able to set modes that can handle errors in different ways and that tee can write/append to files and the terminal. Sometimes people use tee because they want to see the output of the command in their terminal and also have it saved to a file, >/>> cannot do that.

InjectedScript
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