Speak stdin/stdout

[Miserlou]

I think stdin will be easy but stdout will be hard.

• Stdin
• Stdout

[madeddie]

Could you elaborate what you mean with "speak"?

[Miserlou]

Just read from stdin as you'd expect the tool to.

The complications there are how this actually gets used by the program, as I can imagine two use cases:

The simpler, but less useful, making stdin available in the command context:

$ echo "hello" | loop "echo $STDIN" --num 3
hello
hello
hello

The more complicated-but-useful, making it available in the command context:

$ cat my-list-of-files-to-create.txt
hello.jpg
goodbye.jpg
$ cat my-list-of-files-to-create.txt | loop "touch $ITEM" --for-stdin
$ ls
hello.jpg
goodbye.jpg

Or something like that. Does that make sense?

[madeddie]

Yeah, makes sense, although that's very much already available in the --for flag. Having the option to pipe things does make things easier.

On Unix this could also be accomplished with:
cat my-list-of-files-to-create.txt | paste -sd',' - | xargs loop 'echo $ITEM' --for

• cat my-list-of-files-to-create.txt could be any command
• paste -sd',' pastes all lines together into one string with a comma as separator
• xargs executes the command with stdin added at the end

Could you explain what you have in mind for stdout? Let Loop reuse the output of the command it executes?

[Miserlou]

This could be accomplished like you described, but I find that to be a pretty hideous solution. The goal of the tool is to make it easy and intuitive to write loops for common scenarios, so it should be able to read from stdin directly without the need for any more tools (die, xargs, die!)

stdout is probably more complicated and I probably don't know enough to get it right on the first try. I'm imagining something like..

$ loop "echo my-cool-filename-$COUNT.jpg" --num 3 | upload_file.sh 
uploading my-cool-filename-1.jpg..
uploading my-cool-filename-2.jpg..
uploading my-cool-filename-3.jpg..

I don't know if the pipeing could happen upon every iteration of the loop, or if it would have to be buffered and then all sent out at once over newlines. I imagine the latter, unfortunately.

[Miserlou]

Gah, there are even more problems with that.. ex, does it execute

upload_file.sh uploading my-cool-filename-1.jpg my-cool-filename-2.jpg my-cool-filename-3.jpg

or

upload_file.sh my-cool-filename-1.jpg
upload_file.sh my-cool-filename-2.jpg
upload_file.sh my-cool-filename-3.jpg

Even more annoying, the whole thing is made easier simply by loop "upload_file.sh my-cool-filename-$COUNT.jpg" --num 3, so perhaps that's not a great example!

[madeddie]

Yeah, I agree about stdin and the one-liner, it's also not very platform agnostic.

About the stdout, piping doesn't work like that, you can't spawn multiple processes on a single pipe like that (unless you use something like xargs). There might be a really advanced idea I'm not familiar with out there, but I think that's a dead end.

Something like a specific flag for an output using command could work though, like the find command:
loop "echo my-cool-filename-$COUNT.jpg" --num 3 --exec upload_file.sh

[madeddie]

Ah yes, so you noticed the problem, indeed like you said, the command piped to would get all output in one execution.

For loops where the command generates output that you want to use, instead of being able to use the $ITEM or $COUNT value directly in the loop command, the --exec might still be worthwhile.

Mind you, on a unix platform, I would personally still tell people to use xargs, just because I'm old-skool like that; do one thing and do it well :D

[Miserlou]

For the purposes of this ticket, I think that stdin is a lot more important. "Do a thing for thing in list of things" is going to be one of the most common use cases, so that's really the one I want to design around.

I think stdout doesn't need to be too fancy. It's okay to a) grow a big-ass list of command responses and pipe them out all at once after the loop finishes or b) ignore it entirely for now.

I don't think I really see much benefit to --exec since there's already a command string that gets looped, so loop "echo my-cool-filename-$COUNT.jpg" --num 3 --exec upload_file.sh simply becomes loop "upload_file.sh my-cool-filename-$COUNT.jpg" --num 3, which is the whole point of the the thing to begin with!

[Miserlou]

I'm thinking more about that --exec use case and I think I get what you're saying, but wouldn't it still be more intuitive to loop 'calc $COUNT*2 | cat' than to loop 'calc $COUNT*2' --exec "cat"?

[madeddie]

Yeah, true. Although that might get just as ugly as my paste -sd example :)
I'll see about other use-cases, but for now, I think you're right.

[Miserlou]

I implemented some versions of the stdin stuff. Not sure if I'm happy with it yet.

Examples:

$ cat README.md | loop 'echo $ITEM' -n 5
![Logo, with help from Linn Kristiansen](https://i.imgur.com/TQp8nu3.png)

# loop [![Build Status](https://travis-ci.org/Miserlou/Loop.svg)](https://travis-ci.org/Miserlou/Loop) [![crates.io](https://img.shields.io/crates/v/loop-rs.svg)](https://crates.io/crates/loop-rs)

UNIX's missing `loop` command.

and:

$ cat my-list-of-files-to-create.txt | loop 'touch $ITEM'
$ ls
hello.jpg 
goodbye.jpg

It can now also read directly from the keyboard with -i:

$ loop 'echo $ITEM | tr a-z A-Z' -i
hello
world^D
HELLO
WORLD

[Miserlou]

The problem as it stands is that it can't support the trivial use case:

$ echo "hello" | loop "echo $ITEM" --num 3
hello
hello
hello

since "hello" is only once, it'll only execute one time.

[madeddie]

That depends on if you'd like this to happen:

$ cat my-list-of-files-to-create.txt | loop 'echo $STDIN' --num 3
hello.jpg 
goodbye.jpg
hello.jpg 
goodbye.jpg
hello.jpg 
goodbye.jpg

[Miserlou]

I'm not sure. Does that seem like the right behavior to you?

Perhaps that should be defined with an additional argument, ex:

$ echo "hello" | loop 'echo $STDIN' --num=3 --continue
hello
hello
hello

Not sure about that. Need a better concept for it maybe.