Ny difference between tee and >> when using echo

io-redirectionstdouttee

Is there any difference beteween doing:

echo "hahaha" >> file1

and

echo "hahaha" |tee -a file1

Yes, I noticed that I cannot write to write protected files even aith sudo echo, but I can if I sudo tee. Thanks.

Best Answer

There's no difference in the sense that the data in the file will be the same if echo and tee are executed successfully and if the file is writable by the current user.

The tee command would additionally produce output on its standard output, showing the text that would also be appended to the file. This would not happen in the first command.

Another difference is that if the file can not be written to, then the first command, with the redirection, would not even run the echo, whereas the echo would run in the second command, but tee would fail in writing to the file (tee would still produce text on the terminal though).

This could be significant in the case where you run some long running process that produces output:

long_running_thing >>file

This would not even start long_running_thing if file was not writable.

long_running_thing | tee -a file

This would execute long_running_thing and it would run to the end, but no output would be saved into file if it wasn't writable (and the output would additionally be written to the terminal from tee).

The next thing to be aware of, which you hinted at in the end of the question, is that

sudo echo hello >>file

won't work if file isn't writable by the current user. This is because the redirection is processed before the command is executed (see above).

To append to a root-owned file, use

echo hello | sudo tee -a file

Here, we run tee as root. The echo does not need to be executed by root, but the utility that actually writes to the file needs to be executed as root (or as whatever user owns the file) if it's not owned by the current user.

Another possibility would be to use

sudo sh -c 'echo hello >>file'

echo hello | sudo sh -c 'cat >>file'

This would use a redirection to append data to the file, but in this case, the shell that performs the redirection is running as root, so it would not fail in appending/creating the file due to restrictive permissions/ownership (it may still fail if e.g. file is the name of a directory).

Related Solutions

Append Text to Files – Using Cat or Echo

Use tee to read from standard input and write to standard output and files.

echo "hello" | tee -a file1 file2

-a is the short (and portable/standard) for GNU tee's --append

Bash – Using ‘ping’, ‘cut’ and ‘tee’ together

Your pipe commands, as a non-terminal destination, are buffering your output. It will show up eventually, but only when quite a lot of output builds up or the ping command exits.

You can use ping -c 5 google.com to set a specific number of packets to be sent and then ping will exit. Your output comes back and the pipes should work as expected.

Edit: another workaround uses stdbuf to avoid pipe buffering and awk to avoid some internal cut buffering and lets the ping run continuously:

ping www.google.com | stdbuf --output=0 awk '{print $1}' | tee /tmp/file

Best Answer

Related Solutions

Append Text to Files – Using Cat or Echo

Bash – Using ‘ping’, ‘cut’ and ‘tee’ together

Related Question