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82 lines
7.0 KiB
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82 lines
7.0 KiB
Plaintext
Episode: 2302
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Title: HPR2302: Bash snippet - nullglob
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Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr2302/hpr2302.mp3
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Transcribed: 2025-10-19 01:02:08
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---
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This in HPR episode 2,300 and 2 entitled Bash snippet, held on, and in part on the series
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Bash Cripting. It is hosted by Dave Morris and in about 7 minutes long, and carries an
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exquisite flag. The summary is, after learning about the held on option, I have started to use it.
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This episode of HPR is brought to you by AnanasThost.com. Get 15% discount on all shared hosting
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with the offer code HPR15. That's HPR15. Better web hosting that's honest and fair at AnanasThost.com.
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Hello, welcome to Hacker Public Radio. My name is Dave Morris, and I've got a, well, I hope it's a short show about a Bash thing.
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Now, I've done a lot of long, long shows, but Bash is more to come probably when I get to it. But
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I recently used a feature that I talked about in one of those longer shows, and I thought that I
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would talk about how I used it. The feature is NullBlob, and it's part of the shopped command that I
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handled that I described and introduced to you in show 2278. I've called this particular show a
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Bash snippet, and it's about NullBlob. I was thinking I might do more of these snippets if I come up
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with some things that I think might be useful. So this option NullBlob, when it controls what is
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returned from an expansion, when no files match the expansion. So when NullBlob is switched on,
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and a pattern doesn't match, nothing is returned. When it's disabled, which is the default,
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then the pattern itself is returned, which is a nuisance, or can be anyway. And when I was talking
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about these back in that show I mentioned, I didn't think I was going to use many of them. I
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think I even said as much, but today I wrote a script where I used NullBlob, and I thought I'd
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just share a bit of the code, just a few lines to demonstrate what I did and why.
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The script I was writing is for handling mail messages that are sent in containing tag and summary
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updates, had one recently, and that prompted me to do this. I used Thunderbird to
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receive and manage my mail, and I've configured it to drop these messages into a directory so I can
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process them. I use Thunderbird's message filters to do this. I think all that I'm using is standard
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Thunderbird stuff. I don't think there's any add-ons there. And because we get spammed this address
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as well, I'd be surprised if we didn't really. Then sometimes it's good just to go through this
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stuff and throw it away or whatever. And sometimes the messages, the valid messages we get,
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need a bit of work before they're ready to be processed as a typo in them or something.
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So the directory where these messages are saved, I call the spool area, and in my script I set a
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variable mail drop to hold this directory. So diving into this bit of code, which is in the
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in the notes I've put line numbers on them. These are not line numbers in the script, they're just
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in the snippet. And I left the comments in case I helped at all. So I use a variable called ng
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to hold the state of null glob before the script actually modifies it. And I do that by using
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shocked minus p null glob, which returns a list of one command. In this case, which can be used
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to revert the option that you're dealing with to its state at that time. Then that's followed
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in line 6 by the shocked command, which actually switches on null glob. Then I have a variable
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messages, which is an array. And I set that to whatever comes back from expanding a file
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expansion pattern, which consists of the the mail drop variable, because that's a directory,
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slash asterisk.eml. So I'm looking for all files of any name, which end in dot eml. And that's
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what Thunderbird calls them when it does this business of saving them as files. So where are the
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get back a list of files or a get back nothing? If I didn't use null glob, then when there was nothing
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in the directory, I would get back the pattern, which is not useful. You probably see in a moment.
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Then on line 8, I use the eval command, which simply takes the string that it's presented with,
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and executes it as a command in the in the current script. And I valve the variable ng,
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expanded variable ng. And that contains the shocked command to turn null glob back on again,
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assuming it was it was off. I should say turn it back off again, assuming it was off when
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when this whole thing started. So then in my script, I have a test for the contents of the messages
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array. And it uses an if command, where in it, it's comparing the number of elements in the array
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with the number zero. So if it's greater than zero, then what happens is a message is printed,
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which files in the spool area, and then the contents of the array are printed out. So a list of
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all the files is listed. If there's nothing in the array, that is the number of elements is zero,
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then the message the spool area is empty, is output, and the script exits. So after this snippet,
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then the various files are manipulated. So for example, there's a list of files,
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and one of them is a spam message. I can simply go through them and delete them. It just saves me
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explicitly typing the RM command to delete them. Sometimes I stash them away in a holding
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area. But the script also lets me edit the files and various other things. But I've not gone
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into that because I just wanted this to be quick. So that's really it. That's it. So the whole point
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of the null globe was a way of getting a list of files, storing in an array for later manipulation.
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The script uses the array. I've got this list of files and goes through them in order. So this isn't
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the only way of doing what I've done. But I like to write scripts that reduce the number of
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sub-processes as much as I can. So I'm not calling other commands like find and WC, and that sort of
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thing to count stuff. And storing stuff in an array is useful for scripts where you want to
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process a list of things I find anyway. It's just the way I do it. I mean, it's not the definitive
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way of doing it. It's just the way that I do it. But I thought I would share the fact that I'd
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use null globe and how I'd used it in case you were puzzled as to why you would ever want to use
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these things as I was when I was talking about them. So hope you find that interesting and maybe you
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have a snippet of Shell script of your own that you like to share sometime. Okay then, bye now.
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You've been listening to Hacker Public Radio at Hacker Public Radio dot org.
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