How to Pipe Command Output to Sendmail with Custom Subject Line in Linux/Unix

When migrating between servers with different mail utilities, many sysadmins encounter the frustrating difference between mail and sendmail command syntax. The particular challenge comes when trying to:

Pipe command output to email
Set a custom subject line
Maintain error output (2>&1)
Do it without modifying original scripts

The mail command commonly accepts the subject flag -s directly:

command | mail -s "Subject" recipient@domain.com

But sendmail expects email headers in the piped content itself. Here's why your current approach fails:

/usr/local/bin/php -f file.php 2>&1 | /usr/sbin/sendmail -s "Subject" test@email.com
# The -s flag doesn't work as expected with sendmail

To properly format email headers when piping to sendmail:

/usr/local/bin/php -f /path/to/file.php 2>&1 | (
  echo "Subject: My Test Email Subject"
  echo "To: test@email.com"
  echo "Content-Type: text/plain; charset=UTF-8"
  echo
  cat
) | /usr/sbin/sendmail -t

1. Command Subshell: The parentheses create a subshell to process headers and content

2. Email Headers: We explicitly define:
- Subject line
- Recipient (To: field)
- Content type

3. Empty Line: Critical separator between headers and body

4. cat: Passes through the piped content as email body

5. -t flag: Tells sendmail to extract recipients from headers

1. Adding CC and From fields:

command | (
  echo "From: cron@server.com"
  echo "To: admin@domain.com"
  echo "Cc: team@domain.com"
  echo "Subject: Daily Backup Report"
  echo "Content-Type: text/plain; charset=UTF-8"
  echo
  cat
) | sendmail -t

2. HTML formatted email:

command | (
  echo "To: user@domain.com"
  echo "Subject: HTML Report"
  echo "Content-Type: text/html; charset=UTF-8"
  echo "MIME-Version: 1.0"
  echo
  echo "<html><body>"
  echo "<h1>Report Output</h1>"
  echo "<pre>"
  cat
  echo "</pre>"
  echo "</body></html>"
) | sendmail -t

Problem: Email arrives with empty body

Fix: Ensure there's exactly one empty line between headers and body

Problem: Special characters appear broken

Fix: Set proper charset in Content-Type header

Problem: Email goes to spam

Fix: Add proper From header and consider SPF/DKIM configuration

When moving scripts from systems using the basic mail command to those using sendmail, one common pain point is properly formatting email subjects when piping command output. Many legacy cron jobs and automated scripts rely on this functionality for notifications.

The sendmail command has different parameter requirements than mail. For piping output with a subject, the correct syntax is:

command | sendmail -f "from@domain.com" -t <<EOF
To: recipient@email.com
Subject: Your Custom Subject Here

$(cat)
EOF

For your specific case of piping PHP script output, here's the working version:

/usr/local/bin/php -f /path/to/file.php 2>&1 | sendmail -f "cron@yourserver.com" -t <<EOF
To: test@email.com
Subject: My Test Email Subject

$(cat)
EOF

-f flag: Sets the envelope sender address (important for SPF/DKIM)
-t flag: Tells sendmail to read recipients from message headers
Here document (EOF): Allows embedding the email headers and body

If you encounter issues, try these troubleshooting steps:

# Test sendmail availability
which sendmail

# Verify permissions
ls -la /usr/sbin/sendmail

# Check mail queue
mailq

If available on your system, mailx provides a simpler syntax:

/usr/local/bin/php -f /path/to/file.php 2>&1 | mailx -s "My Subject" test@email.com