tee

command | tee [options] [file...]

Option	Description
tee file	Writes standard input to a file and also passes it to standard output.
tee -a file	Appends to a file instead of overwriting it.
tee file1 file2	Writes to multiple files at the same time.
tee /dev/null	Passes data through to standard output only, effectively the same as cat.

The following examples demonstrate how tee works. Think of tee as a T-shaped pipe fitting that splits the data stream in two directions.

# Data flow:
# command output → tee → saved to file
#                    └→ standard output (to next command)

Writes standard input to a file while also passing it to standard output.

echo "Hello" | tee output.txt
Hello
cat output.txt
Hello

Displays build output in the terminal while saving it to a log file. Using 2>&1 includes standard error as well.

make 2>&1 | tee build.log

The -a flag appends to the file instead of overwriting it. This example appends to a log file while filtering for only errors and warnings.

make 2>&1 | tee -a build.log | grep -E "error|warning"

Writes to multiple files at the same time.

echo "test data" | tee file1.txt file2.txt file3.txt
test data

Use sudo tee to write to a file that requires root privileges. sudo echo > /etc/hosts fails because the shell runs the redirect outside of sudo's permissions, but piping through sudo tee writes the file correctly.

echo "127.0.0.1 myapp.local" | sudo tee -a /etc/hosts
127.0.0.1 myapp.local

Saves an intermediate result to a file mid-pipeline so you can inspect it later.

cat access.log | tee /tmp/debug.log | grep "ERROR" | wc -l
3

The sudo tee pattern is a common idiom for writing to files that require root privileges. sudo echo "..." > /etc/hosts fails because the shell executes the redirect without sudo's elevated permissions, but using sudo tee via a pipe writes the file correctly.

For more on pipes, see Pipe (|). For more on redirection, see Output Redirection (>).