Shell Pipes & Redirects

Also Known As

TL;DR

Explanation

Standard streams: stdin (0), stdout (1), stderr (2). Redirect stdout: cmd > file (overwrite), cmd >> file (append). Redirect stderr: cmd 2> error.log. Redirect both: cmd > out.log 2>&1. Pipe: cmd1 | cmd2 connects cmd1's stdout to cmd2's stdin. Named pipes (mkfifo) persist as filesystem objects. Pipes are synchronous — cmd1 runs concurrently with cmd2, with a kernel buffer between them. Understanding pipes enables composing tools: cat access.log | grep '500' | awk '{print $1}' | sort | uniq -c | sort -rn

Common Misconception

Why It Matters

Common Mistakes

• Losing stderr in deployment scripts — use 2>&1 to capture errors alongside stdout in logs.
• 2>&1 > file vs > file 2>&1 — order matters; always put the file redirect first: > file 2>&1.
• Using cat file | grep instead of grep file — unnecessary pipe; grep reads files directly.
• Not quoting pipes in PHP exec() — shell special characters in arguments can break the pipe.

Code Examples

# Wrong order — stderr not captured in file:
deploy.sh 2>&1 > deploy.log
# stderr goes to terminal (original stdout at time of 2>&1)
# stdout goes to deploy.log
# Errors missing from log!

# PHP: exec without capturing stderr:
exec('composer install', $output); // Errors from composer lost

# Correct order — both streams to file:
deploy.sh > deploy.log 2>&1
# stdout → file, then stderr → same as stdout (now the file)

# PHP: capture stderr too:
exec('composer install 2>&1', $output, $exitCode);
if ($exitCode !== 0) {
    file_put_contents('/var/log/deploy_errors.log', implode("\n", $output));
}

References

• ↗ https://www.gnu.org/software/bash/manual/bash.html#Redirections

Tags

Related Terms