The most portable solution is using mailx (or its variants like bsd-mailx or heirloom-mailx), which comes pre-installed on most Unix-like systems:

echo "Body text" | mailx -s "Subject" -a file.txt recipient@example.com

For more advanced features, mutt is a great choice:

mutt -s "Subject" -a file.txt -- recipient@example.com < body.txt

For systems with sendmail installed:

(
  echo "From: you@example.com"
  echo "To: recipient@example.com"
  echo "Subject: File attachment"
  echo "MIME-Version: 1.0"
  echo "Content-Type: multipart/mixed; boundary=\"MAIL_BOUNDARY\""
  echo ""
  echo "--MAIL_BOUNDARY"
  echo "Content-Type: text/plain"
  echo ""
  echo "Email body text"
  echo "--MAIL_BOUNDARY"
  echo "Content-Type: application/octet-stream"
  echo "Content-Disposition: attachment; filename=\"file.txt\""
  echo ""
  cat file.txt
  echo "--MAIL_BOUNDARY--"
) | sendmail -t

For systems with Python installed:

python -c 'import smtplib; from email.mime.text import MIMEText; from email.mime.multipart import MIMEMultipart; msg = MIMEMultipart(); msg["From"] = "you@example.com"; msg["To"] = "recipient@example.com"; msg["Subject"] = "Subject"; msg.attach(MIMEText("Body text")); msg.attach(MIMEText(open("file.txt").read())); smtplib.SMTP("localhost").sendmail("you@example.com", "recipient@example.com", msg.as_string())'

If you have ssmtp configured:

ssmtp recipient@example.com <
Common issues to watch for:

Attachment size limits (usually 10-25MB for most MTAs)
SMTP server authentication requirements
File permissions when reading attachments
MIME type detection (specify explicitly if needed)


The traditional way to send email attachments via command line is using mailx with mutt. Most Unix-like systems have these tools pre-installed:
echo "File attached" | mutt -a /path/to/file.txt -s "Subject" recipient@example.com

For systems with sendmail configured, this method works well:
(
echo "Subject: File Attachment"
echo "MIME-Version: 1.0"
echo "Content-Type: multipart/mixed; boundary=\"MAIL_BOUNDARY\""
echo ""
echo "--MAIL_BOUNDARY"
echo "Content-Type: text/plain"
echo ""
echo "Please find attached file."
echo ""
echo "--MAIL_BOUNDARY"
echo "Content-Type: application/octet-stream"
echo "Content-Disposition: attachment; filename=\"file.txt\""
echo "Content-Transfer-Encoding: base64"
echo ""
base64 /path/to/file.txt
echo "--MAIL_BOUNDARY--"
) | sendmail recipient@example.com

For systems with Python installed (which is nearly all modern distributions), this script offers more control:
#!/usr/bin/env python3
import smtplib
from email.mime.multipart import MIMEMultipart
from email.mime.text import MIMEText
from email.mime.base import MIMEBase
from email import encoders

msg = MIMEMultipart()
msg['From'] = 'sender@example.com'
msg['To'] = 'recipient@example.com'
msg['Subject'] = 'File Attachment'

body = "Please find attached file."
msg.attach(MIMEText(body, 'plain'))

attachment = open("/path/to/file.txt", "rb")
part = MIMEBase('application', 'octet-stream')
part.set_payload(attachment.read())
encoders.encode_base64(part)
part.add_header('Content-Disposition', "attachment; filename=file.txt")
msg.attach(part)

server = smtplib.SMTP('localhost')
server.send_message(msg)
server.quit()

If you have access to an SMTP server with API (like Mailgun or SendGrid), curl works great:
curl -s --user 'api:YOUR_API_KEY' \
    https://api.mailgun.net/v3/YOUR_DOMAIN/messages \
    -F from='Sender ' \
    -F to=recipient@example.com \
    -F subject='File Attachment' \
    -F text='Please find attached file.' \
    -F attachment=@/path/to/file.txt

Some Linux distributions come with specialized tools:
Ubuntu/Debian:
sudo apt install sharutils
uuencode /path/to/file.txt file.txt | mail -s "File Attachment" recipient@example.com

RHEL/CentOS:
yum install mailx
echo "File attached" | mail -s "Subject" -a /path/to/file.txt recipient@example.com

Consider these factors when selecting an approach:

Portability: Python and curl solutions work across most systems
Dependencies: mailx/mutt may need installation on minimal systems
Security: API-based methods (curl) avoid exposing credentials
Complexity: Simple attachments vs. multiple files or HTML content