Hello,
I have to learn how to pipe effectively in Linux, I was wondering if anyone know any books / papers / links that explains this from the ground up? What I have found so far has been pretty gash and while I get the fact that it’s stdin and stdout and std2(error) or whatever I don’t understand how linked programs know what info is coming in and out etc?
Hope this makes sense
Regards,
Ultirian
Joking aside, lemme take a crack at your question here:
So programs get a file descriptor for standard input, standard output and standard error. That’d be stdin, stdout and stderr.
unix allows you to use the pipe | to connect stdout of program A to stdin of program B.
You can also use output redirection to, for example, connect stderr and stdout, like so: 2>&1
Pipes and redirects are a feature of all unix shells.
df -h / | grep -v Filesystem | sed 's/ \{1,\}/g'
Why would anyone care about the data structure this creates?
Well - it creats a comma separated string of a fedora/sles/any BTRFS linux file systems root file systems root disk.
Ok who cares?
Well if I want to cut the percent left of the OS I can do this:
df -h / | grep -v Filesystem | sed 's/ \{1,\}/g' | cut -d',' -f5
for me that returns 56% as a string.
Ok cool. Let’s say you use something like ansible, and you understand python. Let’s say you run that as a shell module and then run a python-like “split” on the “,” you have now created an array. You can now access this data structure using traditional references.
For example, in pseudo-code:
diskspace = df -h / | grep -v Filesystem | sed 's/ \{1,\}/g
print diskspace[5]
If that doesn’t make sense, you should want it to. Start by man paging everything between the pipes. I’ve just provided a nugget of gold. However, I’m happy to answers to any questions because I’m willing to pay it forward.
It creates a byte stream. Every time.
Well, the pipes.
Respectfully, not true.
oh?
Yes, if you run something like:
shell: df -h / | grep -v Filesystem | sed 's/ \{1,\}/g
register: cool
- debug:
msg: cool.stdout
...
Do something with cool.stdout
cool.stdout is stringCounter point:
- name
shell: zfs send storage/music
register: zfslol
- debug:
msg: zfslol.stdout
I do not expect ansible to react well with this. please don’t try this.
Huh? Is that even comparable? ZFS vs df -h?
Well, I thought we were talking about the stdout/stdin
Maybe I misunderstood your intent
Wait? How did you even come to that?
I gave specific command!
Anyways -
What does df -h / report on zfs
Same shit
Same shit like a string of shitty spaced data? Like below?
Yep
Dude! You can trim out the table header line with grep -v stuff and then remove the space and replace with commas.
I know you well enough to know you understand how a comma separated string can be used on a “split” method to do all sorts of fun stuff programmatically.
You could tear that string up with a split method and python in 5 seconds, no doubt!
yeah, you could also use python’s .split() method. By default, it cleans out all whitespace and splits.
oh, lol
What do I know? I’m a self-taught firefighter - lol.
I only question because you’re educated, and I’m a damn hacker haha
educated
Yep, got me that 2 semesters of Womens Studies* at the local UC.
*womens studies is chasing skirts
I just have no life outside tech
I don’t want to drift off topic, but you know I’ve been here since 2015 right?
@sgtawesomesauce When some people were learning the right way — I was cutting people out cars, loading people people cut in half by I-Beams into choppers, delivering babies, and saving kittens out of trees while reading books on data structures and other stupid computer stuff right?
Forget it — pipes are useful. Good luck OP…