tee

Overview

tee reads stdin and writes it to both standard output and one or more files. It is the standard way to keep a pipeline’s live output while saving a copy for logs, audits, or later processing. Named after a T-splitter in plumbing.

Syntax

command | tee [OPTIONS] [FILE...]
command | tee [OPTIONS] FILE | other-command

With no files, tee only copies stdin to stdout (still useful with options like -a rarely alone).

Common Options

Option	Description
-a / --append	Append to files instead of overwriting
-i / --ignore-interrupts	Ignore SIGINT (less common in scripts)
-p	Diagnose errors writing to non-pipes (GNU; behavior with SIGPIPE)

Safety

• tee file truncates file by default when the pipeline starts — same class of footgun as > file. Use -a to append.
• Writing into system paths needs appropriate privileges (sudo tee pattern below).
• Do not tee secrets into world-readable locations; check umask and directory permissions.

Examples with Explanations

Save output and still see it

ping -c 4 1.1.1.1 | tee ping.log

Append to a running log

./deploy.sh 2>&1 | tee -a deploy.log

2>&1 captures stderr too so the log is complete.

Split to multiple files

./build.sh 2>&1 | tee build.log build-copy.log

Continue the pipeline after saving

dmesg | tee dmesg.full | grep -i error

Full capture in dmesg.full; only errors scroll on the terminal.

Write files that need root (classic sudo tee)

echo 'net.ipv4.ip_forward=1' | sudo tee /etc/sysctl.d/99-forward.conf

Why not sudo echo … > file? The redirect is applied by the local shell before sudo, so the file open happens as you, not root. sudo tee opens the file as root.

Same pattern: append as root

echo 'export PATH="/opt/bin:$PATH"' | sudo tee -a /etc/environment

(Validate distro conventions before editing /etc/environment.)

Suppress terminal output but keep the file

make 2>&1 | tee build.log >/dev/null
# or simply:
make 2>&1 > build.log

tee is only needed when you still want a branch (multiple files, or later pipe stages).

Capture and process with jq later

curl -sS https://api.github.com/repos/jqlang/jq \
  | tee repo.json \
  | jq -r '.stargazers_count'

Timestamped session log with a second process

./long-job.sh 2>&1 | tee -a "job-$(date +%F).log"

Combine with process substitution (bash) for dual processing

./app 2>&1 | tee >(logger -t myapp) | grep -i error

Sends a copy to syslog via logger while grepping errors on the terminal (bash/zsh).

Preserve exit status of the left side (bash pipefail)

set -o pipefail
make 2>&1 | tee build.log

Without pipefail, the pipeline’s exit status is tee’s (usually 0) even if make failed.

Understanding Output

Whatever arrives on stdin is duplicated to each listed file and to stdout, byte-for-byte (text or binary). Order is preserved. If a file cannot be opened, GNU tee reports an error and still attempts other outputs; check exit status in scripts.

Notes & Pitfalls

• Truncation default: tee log overwrites; tee -a log appends.
• Exit codes: with default bash pipes, failures on the left can be masked — enable set -o pipefail in scripts.
• Buffering: when stdout is a pipe, some programs fully buffer; you may see delayed terminal output. Tools like stdbuf -oL can force line buffering when needed.
• Binary data works; avoid viewing huge binary tees in the terminal.
• tee is not a remote multipath tool — for network fan-out use socat, message buses, or log shippers.
• Multiple writers appending to one log without coordination can interleave lines; for concurrent jobs prefer separate logs or a proper logger.

Related Commands

• shell redirects > / >> — single destination, no T-split
• script — record an entire interactive terminal session
• logger — send lines to syslog
• sponge (moreutils) — soak stdin then write (safe in-place pipeline edits)
• pv — progress meter in pipelines (if installed)

Additional Resources

• man tee
• GNU coreutils info: info tee