4.3Redirection

Redirection connects command input and output to files instead of the terminal.

Output Redirection

# Write output to a file (overwrite)
curl -s https://api.example.com/users > users.json

# Append output to a file
echo "Test run completed at $(date)" >> test-log.txt

# Write to a file and display on screen simultaneously
curl -s https://api.example.com/users | tee users.json

# Append with tee
echo "Another result" | tee -a test-log.txt

Error Redirection

Linux has three standard streams:

• stdin (0): Standard input
• stdout (1): Standard output (normal output)
• stderr (2): Standard error (error messages)

# Redirect only errors to a file
npm run test 2> errors.log

# Redirect both stdout and stderr to the same file
npm run test > output.log 2>&1

# Redirect stdout and stderr to separate files
npm run test > output.log 2> errors.log

# Discard all output (useful for scripts where you only care about the exit code)
curl -s https://api.example.com/health > /dev/null 2>&1

# Discard errors only (suppress "Permission denied" noise from find)
find / -name "config.json" 2>/dev/null

Input Redirection

# Read input from a file
sort < unsorted-list.txt

# Here document (inline input)
cat <<EOF > test-config.json
{
  "baseURL": "https://staging.example.com",
  "timeout": 30000
}
EOF

# Here string (single line input)
grep "error" <<< "This has an error in it"