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Help with sed commands (lemmy.dbzer0.com)

submitted 3 months ago* (last edited 3 months ago) by orsetto@lemmy.dbzer0.com to c/linux@lemmy.ml

21 comments fedilink hide all child comments

Hi all! I have always only used sed with s///, becouse I've never been able to figure out how to properly make use of its full capabilities. Right now, I'm trying to filter the output of df -h --output=avail,source to only get the available space from /dev/dm-2 (let's ignore that I just realized df accepts a device as parameter, which clearly solves my problem).

This is the command I'm using, which works:

df -h --output=avail,source \
    | grep /dev/dm-2 \
    | sed -E 's/^[[:blank:]]*([0-9]+(G|M|K)).*$/\1/

However, it makes use of grep, and I'd like to get rid of it. So I've tried with a combiantion of t, T, //d and some other stuff, but onestly the output I get makes no sense to me, and I can't figure out what I should do instead.

In short, my question is: given the following output

$ df -h --output=avail,source 
Avail Filesystem
  87G /dev/dm-2
 1.6G tmpfs
  61K efivarfs
  10M dev
...

How do I only get 87G using only sed as a filter?

EDIT:

Nevermind, I've figured it out...

$ df -h --output=avail,source \
    | sed -E 's/^[[:blank:]]*([0-9]+(G|M|K))[[:blank:]]+(\/dev\/dm-2).*$/\1/; t; /.*/d'
85G

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[–] gnuhaut@lemmy.ml 8 points 3 months ago* (last edited 3 months ago) (9 children)

You can do something like this to emulate grep:

sed -n '/myregex/p'

The -n suppresses auto-printing. That command should interpreted as: find line matching /myregex/ and then print (p) it.

You can then combine this with s (substitute):

sed '/myregex/ s/from/to/ p`

The complete command is then something like:

LANG=C df -h --output=avail,source | sed -n '/\/dev\/nvme0n1p2/ s/^\s*\([0-9.]\+[KMGT]\).*/\1/ p'

Note that the output can be something like 2.3G, but in my locale that would be 2,3G which is why I added LANG=C.

Easier IMHO is awk:

awk '/myregex/ {print $1}'

prints the first field.

[–] bizdelnick@lemmy.ml 2 points 3 months ago (1 children)

You can then combine this with s (substitute):
sed '/myregex/ s/from/to/ p'

This is not combining commands. In your example p is a modifier to the s/// command.

[–] gnuhaut@lemmy.ml 2 points 3 months ago

Yeah you're right, thanks. I did figure this out on my own already, see my other comment:

Oh yeah that, so technically (and I was confused about this), the p in s/from/to/p is not the same as the p command, it’s a flag for the s command that tells it to print the output.

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