How to check disk space and send email alert in Perl?

Checking disk space and sending email alerts in Perl is a common system administration task. Perl's flexibility lets you do this easily using built-in functions and core modules, without needing extra dependencies.

Approach

• Check disk space: Use df command output or Perl's statvfs (if available) to retrieve disk usage statistics.
• Parse usage: Extract total, used, and available space, and calculate percentage used.
• Send email alert: Use Perl's core open command to pipe to a system mailer (e.g. sendmail or mail). Alternatively, construct an email and print it to STDOUT for testing.
• Threshold-based alert: Only send the email if disk usage exceeds a defined threshold.

Key Perl concepts involved

• qx//
• Array and string splitting to parse CLI output.
• Conditional tests to check threshold.
• Opening a pipe with open to send mail.
• Use of sigils - $ for scalars, @ for arrays.
• Context awareness: scalar vs list context (e.g., split in list context returns array).

Example: Check root filesystem disk space and email alert

#!/usr/bin/perl
use strict;
use warnings;

# Configuration
my $threshold_percent = 80;              # Alert if usage exceeds 80%
my $filesystem = '/';                    # Filesystem to check, usually root '/'
my $recipient = 'admin@example.com';    # Email recipient

# Get disk space via 'df -P' for portability
my @df_output = qx(df -P $filesystem);
# Example df -P output line:
# Filesystem 1024-blocks Used Available Capacity Mounted on
# /dev/sda1  20511356 12345678  7135678  64% /

# Skip header, parse next line
my $line = $df_output[1] or die "No df output for $filesystem";
my @fields = split /\s+/, $line;

# df -P format: Filesystem, blocks, Used, Available, Use%, Mounted on
# Indexes:          0          1     2      3       4      5
my $used_percent = $fields[4];
$used_percent =~ s/%//;  # remove trailing % sign

print "Disk Usage for $filesystem: $used_percent%\n";

if ($used_percent >= $threshold_percent) {
    # Send email alert
    my $subject = "Disk space alert: $filesystem usage is $used_percent%";
    my $message = <<"END_MSG";
Warning: Disk usage on filesystem $filesystem has reached $used_percent% which exceeds the threshold of $threshold_percent%.

Please check the system and free up space as needed.

-- System Monitor
END_MSG

    # Try using sendmail directly
    open(my $mail, '|-', '/usr/sbin/sendmail -t') or die "Could not open sendmail: $!";

    print $mail "To: $recipient\n";
    print $mail "Subject: $subject\n";
    print $mail "\n";  # header/body separator
    print $mail $message;

    close($mail);
    print "Alert email sent to $recipient.\n";
} else {
    print "Disk usage is within limits.\n";
}

Explanation

This script:

• Runs df -P / to get disk usage for root filesystem.
• Parses the output line for used percentage.
• Checks if usage exceeds 80% threshold.
• If exceeded, formats a simple plain-text email.
• Opens a pipe to /usr/sbin/sendmail -t (common on Linux/Unix systems) and writes the email headers and body.
• If usage is below threshold, just prints status to STDOUT.

Common Pitfalls and Gotchas

• Different df output formats: Use -P (POSIX) to avoid issues with locale or long device names splitting into multiple fields.
• Sendmail path: On some systems, sendmail may be at a different path (e.g., /usr/bin/sendmail). Adjust accordingly.
• Email sending: This example assumes a local mail transfer agent (MTA) like Sendmail or Postfix is installed and configured.
• Permissions: Running this might require permissions to read disk info or send email.
• Threshold tuning: Set your alert threshold appropriately for your system needs.

Enhancements

• Parse multiple filesystems by looping over df output lines.
• Use the core module File::Statvfs (from Perl 5.12+) for a pure Perl solution without parsing df.
• Use Email::Sender or MIME::Lite for richer HTML emails (requires CPAN modules).
• Integrate logging or retry sends for more robust monitoring.

This approach demonstrates Perl's “There's more than one way to do it” (TMTOWTDI) philosophy — using simple OS commands versus dedicated modules to suit your environment.