tee - Linux

Overview

The tee command in Linux reads from the standard input and writes to both standard output and one or more files simultaneously. It is commonly used in shell pipelines to capture the output of a command while still displaying it on the screen. This is particularly useful for logging and debugging.

Syntax

tee [OPTION]... [FILE]...

[OPTION]: Optional flags that alter the behavior of the command.
[FILE]: One or more filenames to write to.

The command can take input from a pipeline or can be redirected to read from a file.

Options/Flags

-a, --append: Append the output to the given files rather than overwriting them.
-i, --ignore-interrupts: Ignore interrupt signals (CTRL+C).
--help: Display an informative help message and exit.
--version: Output version information and exit.

Each option modifies the command’s behavior in specific ways, such as appending to files instead of truncating them, which can be crucial for maintaining logs over time.

Examples

Write output to a file and display it:
```
ls -l | tee output.txt
```
This command lists directory contents in detailed format and writes the output to output.txt, as well as displaying it on the terminal.
Append to an existing file:
```
date | tee -a output.txt
```
This example appends the current date and time to output.txt without overwriting existing content.
Using multiple files:
```
uptime | tee output1.txt output2.txt
```
This sends the output of the uptime command to both output1.txt and output2.txt.
Ignoring interrupt signals:
```
cat | tee -i backup.txt
```
Here, tee will ignore interrupt signals allowing the pipe to handle user interruptions without stopping the logging to backup.txt.

Common Issues

File Overwrites: Without the -a flag, tee will overwrite the file. Always double-check the flags when dealing with important log files.
Permission Errors: If tee is unable to write to a file, ensure you have the necessary permissions or use sudo for elevated permissions.

Integration

tee is often used in scripting and pipelines:

Backup logs while monitoring a command:
```
some_command | tee log.txt | grep "Error"
```
This setup allows live monitoring of errors while also logging all command output.
Combining with sudo for system file editing:
```
echo "127.0.0.1 local.dev" | sudo tee -a /etc/hosts
```
This example adds a line to /etc/hosts with root permissions, useful in scripting environment setups.

cat: Displays the contents of a file.
grep: Searches for patterns in input.
awk: A powerful text processing tool.

Additional Resources:

Linux man pages: tee(1)
GNU coreutils manual: tee invocation

The tee command provides a simple yet powerful way to handle command outputs in Unix-like systems, enhancing both script functionality and user productivity.