从Linux命令行阅读邮件附件

Question

Is it possible to read the emails based on the Subject line and then get the base64 attachment or directly get the attachment ? Server : Linux System

Answer 1

Your question seems to presuppose that there is a single attachment and that it can be reliably extracted. In the general case, an email message can have a basically infinite amount of attachments, and the encoding could be one out of several.

But if we assume that you are dealing with a single sender which consistently uses a static message template where the first base64 attachment is always going to be the one you want, something like

case $(formail -zcxSubject: <"$message") in
    "Hello, here is your report for "*)
        awk 'BEGIN { h=1 }
            h { if ($0 ~ /^$/) h=0 ; next }  # skip headers
            /^Content-Disposition: attachment/ { a=1 }  # find att
            a && /^$/ { p=1; next }
            p && /^$/ { exit }
            p' "$message" |
        base64 -d ;;
esac

This will extract the Subject: header and compare it to a glob pattern. I expect this is what you mean by "based on subject" -- if we find a matching subject header, examine this message, otherwise discard.

The crude Awk script attempts to isolate the base64 data and pass it to base64 -d for extraction. This contains a number of pesky and somewhat crude assumptions about the message format, and probably requires significant additional tweaking. Briefly, we skip the headers, then look for MIME headers identifying an attachment, and print that, skipping everything else in the message. If this header is missing, or identifies the wrong MIME part, you will get no results, or (worse) incorrect results. Also, the /^Content-Disposition:/ regex could theoretically match on a line which is not a MIME header, though this seems highly unlikely (but might actually happen if you are looking eg at a bounce message).

A more robust approach would involve a MIME extraction tool or perhaps a custom script to actually parse the MIME structure and extract the part you want. Without details about what exactly you need, I'm not able to provide that. (This would also allow you to use the sender's specified filename; the above script simply prints the decoded payload to standard output.)

Note also that formail has no idea about RFC2047 encoding, so if the subject is not plain ASCII, you have to specify the encoded form in the script.