Overview
The tee command is useful for saving intermediate results in a pipeline to a file while simultaneously passing them to the next command. The '-a' option, in particular, is essential for adding new data while preserving existing file content.
Key Features
- Writes standard input data to standard output and files simultaneously.
- Appends to file content instead of overwriting using the '-a' option.
- Useful for saving intermediate results in pipelines.
- Can write to multiple files at once.
Key Options
This section explains the main options for the 'tee' command, with a focus on the '-a' option.
File Handling Options
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Description:
`tee-a-a-a-a-a` Executes the command.
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Usage Examples
The following examples demonstrate how to append content to files using the '-a' option of the 'tee' command. All examples use executable 'tee -a' commands.
Appending Content to an Existing File
echo "This is a new line." | tee -a test.txtAppends 'This is a new line.' to test.txt and simultaneously outputs it to the screen.
Appending Content to Multiple Files Simultaneously
echo "Log message" | tee -a log1.txt log2.txtAppends 'Log message' to both log1.txt and log2.txt simultaneously.
Appending Command Output to a File
ls -l | tee -a output.logAppends the output of the 'ls -l' command to output.log and also displays it on the screen.
Creating a File and Appending Content (Creates if it doesn't exist)
echo "First line" | tee -a new_file.txt
echo "Second line" | tee -a new_file.txtCreates a new file if it doesn't exist and appends content. (tee -a creates the file if it doesn't exist)
Tips & Notes
Useful tips and points to consider when using the 'tee' command and the '-a' option.
Note on Command Name
- Note: 'tee-a-a-a-a-a' is not an actual Linux command. This guide is written to explain the usage of the '-a' (append) option of the 'tee' command. When actually using it, you should enter the command as 'tee -a'.
Difference from Redirection (>>)
The '>>' operator also appends content to a file, but the 'tee' command has the advantage of outputting the content to standard output as well, allowing it to be passed to subsequent pipeline commands.
- `echo "hello" >> file.txt` (Appends only to the file, no screen output)
- `echo "hello" | tee -a file.txt` (Appends to the file and also outputs to the screen)
Permission Issues
Attempting to use 'tee -a' on a file without write permissions will result in a 'Permission denied' error. In such cases, you may need to use 'sudo' to execute the command with administrator privileges.
- `echo "Important log" | sudo tee -a /var/log/system.log`