If you are like me you might have used the sort(1) CLI utility more than once in your life. Today, I’ve found a trick that I’ve never used before, and hopefully it will help someone else in the future.

Say that we have the following file to sort:

fpdy 01 08 wcfo
juvi 01 02 ejan
urbx 04 03 ckbw
fkzq 01 08 myaz
fjie 04 09 rhvo
almv 04 02 adhs
cuah 07 04 gbyt
chok 09 06 nqwo
emjd 01 04 ledx
npto 02 10 nqsc

Now, supposed that I wanted to sort first by the third column and then for the second one, then one would do sort -k 3 foo.txt. Easy. But what if instead the source file looked like this and I wanted the same results?

fpdy 0108 wcfo
juvi 0102 ejan
urbx 0403 ckbw
fkzq 0108 myaz
fjie 0409 rhvo
almv 0402 adhs
cuah 0704 gbyt
chok 0906 nqwo
emjd 0104 ledx
npto 0210 nqsc

Well, tricky, right? Not really: the -k accepts the format F[.C], where F is the field number (2 in our case) and C is the character position within the field (4 in our case) so if we run the following we will achieve what we are looking for:

$ sort -k 2.4 foo.txt
almv 0402 adhs
juvi 0102 ejan
urbx 0403 ckbw
cuah 0704 gbyt
emjd 0104 ledx
chok 0906 nqwo
fkzq 0108 myaz
fpdy 0108 wcfo
fjie 0409 rhvo
npto 0210 nqsc

Why 4 and not 3, I don’t know, but I think it’s taking the separator into account. I need to investigate more. But so far I haven’t the need to do something as fine-grained as this, so I can still sleep well at night.

Today Thorsten Ball asked a simple question on Twitter:

Does something like a reverse-curl exist? A tool that allows me to listen on a socket and return one HTTP response?

— Thorsten Ball (@thorstenball) October 27, 2017

After a brief exchange of tweets, I said:

Or we could write one in Go 😺

— Lean (inkel) (@inkel) October 27, 2017

Twenty minutes later lruc was born.

Oh wow! :)

— Thorsten Ball (@thorstenball) October 27, 2017

It’s still very fresh and missing many features, but basically it is a web server that allows you to configure it to always respond with a custom response without too much hassle. The usage is very simple:

Usage of lruc:
  -addr string
        Address to listen for requests (default ":8080")
  -body -
        Response body. Use - to read from a stdin (default "Hello, World!")
  -code int
        HTTP response code (default 200)
  -content-type string
        Content-Type (default "text/plain")

Say that you want to create a server that always respond with a 404 Not Found and a body of No se pudo encontrar lo que buscaba (Spanish for Couldn’t find what you were looking for (sort of)) on port 7070, then you could execute the following:

lruc -addr :7070 -code 404 -body "No se pudo encontrar lo que buscaba"

Or say that you want to always return an image, then you could do something like:

< image.png lruc -content-type image/png -body -

# Or in an useless use of cat
cat image.png | lruc -content-type image/png -body -

This seems like an interesting tool to keep working on, so watch github.com/inkel/lruc for updates.

PS: did I said already that I love Go?