Splitting Streams with tee
When you redirect the output to a file, it no longer displays on the screen. To see the output on the screen and send it to a file, you could use the cat or less commands to view the file, but you can also use the tee command. The tee command gets its name from plumbing—a T-shaped pipe fitting that splits the water off in two directions. The tee command takes input and splits the stream to the screen and to a file.
Execute the history command and pipe its data to the tee command:
| | $ history | tee commands.txt |
Instead of redirecting the output to a file, you’re piping it to tee, which receives the data as standard input. This time, you see the results on the screen. The commands.txt file will also have the latest version of your history inside.
The tee command overwrites the target file each time. But if you use the -a flag, you can append to the file instead:
| | $ date | tee -a commands.txt |
The date displays on the screen and is appended to the end of the file:
| | $ tail -n 2 commands.txt |
| | 191 history | tee commands.txt |
| | Sat Mar 2 03:53:24 UTC 2019 |
The tee command has another benefit—you can use it to write to places where normal redirection won’t let you. Remember in Creating Files on the Rest of the Filesystem, you tried to create a file with content in the /var folder but it didn’t work? Time to review that.
Try to save the history to a file in the /var directory. The command fails because your user doesn’t have access to write a file there:
| | $ history > /var/commands.txt |
| | -bash: /var/commands.txt: Permission denied |
You might think that you can use sudo to get this to work, but sudo runs the history command as a privileged user rather than your user, and the redirection isn’t part of the sudo process:
| | $ sudo history > /var/commands.txt |
| | -bash: /var/commands.txt: Permission denied |
But you can get around this using sudo tee:
| | $ history | sudo tee /var/commands.txt |
This time, the file gets created. This works with echo as well:
| | $ sudo echo "Line one" >> /var/mylog.txt |
| | bash: /var/mylog.txt: Permission denied |
| | $ echo "Line one" | sudo tee -a /var/mylog.txt |
| | Line one |
One drawback to this approach is that you still see the output on the screen. But you can suppress the output by sending it to /dev/null instead of a file:
| | $ echo "Line two" | sudo tee -a /var/mylog.txt > /dev/null |
You can use tee to create multiline files with sudo, similar to how you did it with cat:
| | $ sudo tee -a /var/mylog.txt << EOF > /dev/null |
You’ll be presented with a blank cursor, as tee is waiting for data from standard input. Type the following lines, pressing Enter after each line:
| | Line three |
| | Line four |
| | EOF |
The two new lines get written to the file.
If all of this seems too much like a hack, it’s because it is. It’s often just easier to create files in your home directory and use sudo to move them where you want them:
| | $ echo "some text" > ~/myfile.txt |
| | $ sudo mv ~/myfile.txt /var/myfile |
You’ve worked with two of the three standard streams in detail. You know how to redirect streams to files or other programs, so let’s look at how to handle error messages.