On Linux I'm having text files and each contains 1 or 2 base64 blocks of code. Following is part of the file and 1 base64 block.

Content-Transfer-Encoding: base64

RGVhciBMdWlzLA0KICANCkdvb2QgbW9uaW5nIQ0KT3VyIEJhbmsgYWNjb3VudCBpcyB1bmRlcmdvaW5nIGF1ZGl0IGZvciB0aGUgZmlzY2FsIHllYXIgYXMgcmVxdWlyZWQgYnkgdGhlIGNoaW5hIGZvcmVpZ24gY3VycmVuY3kgY29udHJvbCBwb2xpY3kgYW5kIHRoZSBhY2NvdW50IHdpbGwgbm90IGJlIGF2YWlsYWJsZSB1bnRpbCB0aGUgQ2hpbmVzZSB0YXggYnVyZWF1cyBhcmUgc 2F0aXNmaWVkIHRoYXQgYWxsIGFwcGxpY2FibGUgdGF4ZXMgaGF2ZSBiZWVuIHBhaWQgdXAuIFBsZWFzZSBob2xkIHBheW1lbnQgc28gaSBjYW4gZnVybmlzaCB5b3Ugd2l0aCBvdXIgcmV2aXNlZCBiYW5raW5nIGluZm9ybWF0aW9uLiAgDQoNCiAgDQpCZXN0IHJlZ2FyZHMNCiANCkRhdmlk


Which command can I use to extract first base64 out of the file?

My aim is to decode output to be readable

Example file decode command: https://superuser.com/a/663397

Example phrase decode command: https://askubuntu.com/a/178546

I don't know how to extract only base64 part of code or if it contains two base64, only the first one. I may use sed or awk, but I don't know how I assume I will lookup for:

  1. base64
  2. ------=_Part_

Trim the blank lines and I'm having base64 But what if there are 2 base64 sections? I want to have the first one.

  • If you mean picking apart a MIME multipart message, that is something your email client will do. Or you can cobble something up in Perl with MIME::Parser from CPAN
    – vonbrand
    Dec 31, 2015 at 16:08
  • grep -xm1 '^[[:alnum:] ]\{100,\}'
    – Costas
    Dec 31, 2015 at 17:53
  • 2
    It seems like all the 64bit-encoded data is one line. If that’s the case, I’d try to: (1) grep out all the lines containing headers, delimiters, and empty ones, then (2) take the first line with head -1 and (3) pass it to base64 -d. Jan 2, 2016 at 12:50

1 Answer 1


I'm not sure I quite understand all of your question, but it seems that you have part of it figured out already and just need a way to trim out the text that you want to input into the base64 command.

I want to answer this part:

i assume i will lookup for a) base64 and b) ------=Part

trim the blank lines and im having base64 but what if there are 2 base64, i want first one

You can print all the lines from the first instance of base64 to the first-after-that instance of =_Part_ with the following sed command:

sed -n '/base64/,${p;/=_Part_/q;}' inputfile


-n suppresses the default action of printing each line.

/base64/,$ applies the following code block from the first instance of base64 to the end of the file. ($ means last line in this context.)

p means to print the line.

/=_Part_/ is a pattern which limits the q command that follows it so that it is only executed if the line contains =_Part_.

The q command quits sed, causing all subsequent lines to not be processed at all.

The upshot of all this is very similar to the simpler sed -n '/base64/,/=_Part_/p' inputfile which would print from base64 to =_Part_—but this simpler version would print multiple such sections instead of stopping after the first.

  • 1
    true, but i think the comments above might also have it, too - its just a single line - base64 doesn't incorporate newlines anyway (or so i think). so sed -n '/base64/{n;n;p;q;}' - maybe... but this is a more general solution overall.
    – mikeserv
    Jan 3, 2016 at 7:22
  • 1
    @mikeserv, you could be right. The example he posted isn't a single line (see cmUgc), but maybe it is a made-up example? Anyway, for OP: explanation of mikeserv's line is just, when base64 is first seen in the file, next line, next line, print that line, then quit.
    – Wildcard
    Jan 3, 2016 at 7:25

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