Table of Contents

The tee command reads from standard input and writes simultaneously to standard output and one or more files. It is useful for logging pipelines while still seeing output in real time.

Overview

Use tee when you need to:

  • Save command output to a file while viewing it in terminal
  • Capture logs from long-running scripts
  • Feed output to additional pipeline stages
  • Append or overwrite output files explicitly

Syntax

tee [options] [file...]

Common forms:

# Write output to terminal and file
some_command | tee output.log

# Append instead of overwrite
some_command | tee -a output.log

# Write to multiple files
some_command | tee out1.log out2.log

Common Options

Option Purpose Example
-a Append to files instead of overwriting cmd | tee -a run.log
-i Ignore interrupt signals cmd | tee -i run.log
-p Diagnose errors writing to non pipes (GNU) cmd | tee -p out.log

Core Behavior

  • Input always comes from standard input.
  • Output is written to standard output and each named file.
  • Without -a, files are truncated before writing.
  • Exit status reflects write behavior and pipeline context.

Examples

# Save script output while keeping it visible
./build.sh | tee build.log

# Append daily run output
./backup.sh | tee -a backup.log

# Save and continue processing
cat app.log | tee snapshot.log | grep -i error

# Split output into two log files
journalctl -u nginx -n 500 | tee nginx-recent.log nginx-audit.log

# Capture both stdout and stderr
./deploy.sh 2>&1 | tee deploy.log

Pipeline Patterns

Capture intermediate output for debugging:

generate_data | tee raw.out | transform_data | tee transformed.out | summarize

Log and inspect in pager:

long_command 2>&1 | tee command.log | less

Safe Usage Guidelines

  1. Use -a for append-only logs; omit it when fresh logs are desired.
  2. Redirect stderr (2>&1) when full diagnostics are needed.
  3. Choose deterministic file paths for script logs.
  4. Be mindful of sensitive data when logging command output.

Troubleshooting

Log file was overwritten unexpectedly

Cause: tee defaults to truncation.

Fix:

cmd | tee -a existing.log

Missing stderr content in log

Cause: Only stdout was piped.

Fix:

cmd 2>&1 | tee run.log

Pipeline succeeded even when command failed

Cause: Shell pipeline exit behavior may return last command status.

Fix:

  • In Bash scripts, use set -o pipefail
  • Check exit codes explicitly

Notes

tee is most useful in observability-heavy workflows and CI logs where real-time output and persistent records are both required.

For command details and implementation-specific behavior, run:

man tee
tee --help
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