The Tee command in Linux is one or more files that reads standard input and writes the result to both standard output and standard output. Basically, the command output is displayed on the terminal, and the output is redirected to the file, for example, the user can use the tee command to add a new line to the root user file.
tee command basic syntax:
[command] | tee [options] [filename]
tee is basically used to display the standard output command and save it to a file, for example, we execute the 'hostnamectl command to print the hostname and other details of the system, and then save the standard output as' host_info.txt' :
$ hostnamectl | tee host_info.txt
Static hostname: ywnz
Icon name: computer-laptop
Chassis: laptop
Machine ID: 7a138ee71db94e8785d1a4dbe54dde7e
Boot ID: 34d27941b74941a9a29bc424f66613bc
Operating System: openSUSE Leap 15.2
CPE OS Name: CPE: / o: packages: leap: 15.2
Kernel: Linux 5.3.18-lp152.75-default
Architecture: x86-64
You can view the contents of the file 'host_info.txt' in the cat command:
$ cat host_info.txt
If you want to write output to multiple files, you can add a list of files separated by Spaces as an argument:
$ command | tee file1 file2… fileN
By default, the contents of the file are overwritten every time the tee command is run, and if you want to output as additional content, you need to use a or append:
$ command | tee -a file
To allow the tee command to exit normally when execution is interrupted:
$ command | tee -i file
Hide the output:
$ command | tee file >/dev/null
The tee command allows you to write to files owned by the root user or someone else. For example, if you want to add a new line to the following file, you will receive a "permission denied" error because sudo does not perform the redirection of the output:
$ sudo echo "add a newline" > /etc/sudoers
bash: /etc/sudoers: Permission denied
tee can be used:
$ echo "add a newline" | sudo tee -a /etc/sudoers
Using tee in the shell:
$ vi /tmp/testbash.sh
#! /bin/bash
LOGFILE=/tmp/test-logs-$(date +%d%m%Y)
echo "Append the logs routinely" | tee -a $LOGFILE
