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Very simple foreach line alias to xargs - is it usefule? (beehaw.org)
submitted 6 days ago* (last edited 6 days ago) by thingsiplay@beehaw.org to c/linux@lemmy.ml
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I created a simple alias for xargs, with the intend to pipe it when needed. It will simply run a command for each line of it. My question to you is, is this useful or are there better ways of doing this? This is just a little bit of brainstorming basically. Maybe I have a knot in my head.

# Pipe each line and execute a command. The "{}" will be replaced by the line.
# Example:
#   find . -maxdepth 2 -type f -name 'M*' | foreach grep "USB" {}
alias foreach='xargs -d "\n" -I{}'

For commands that already operate on every line from stdin, this won't be much useful. But in other cases, it might be. A more simplified usage example (and a useless one) would be:

find . -maxdepth 1 | foreach echo "File" {}

It's important to use the {} as a placeholder for the "current line" that is processed. What do you think about the usefulness? Have you any idea how to use it?

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[–] gnuhaut@lemmy.ml 5 points 6 days ago (2 children)

Don't use ls if you want to get filenames, it does a bunch of stuff to them. Use a shell glob or find.

Also, because filenames can have newlines, if you want to loop over them, it's best to use one these:

for x in *; do do_stuff "$x"; done # can be any shell glob, not just *
find . -exec do_stuff {} \;
find . -print0 | xargs -0 do_stuff # not POSIX but widely supported
find . -print0 | xargs -0 -n1 do_stuff # same, but for single arg command

When reading newline-delimited stuff with while read, you want to use:

cat filenames.txt | while IFS= read -r x; do_stuff "$x"; done

The IFS= prevents trimming of whitespace, and -r prevents interpretation of backslashes.

[–] thingsiplay@beehaw.org 2 points 5 days ago* (last edited 5 days ago)

Some additional thoughts to be aware of by looking closer to each line (previously I just glanced over).

This point is not directly affecting your example, but I want to make you aware of something I fall into myself. Its one of those Bash quirks. Other shells might handle it differently, only speaking about Bash here. For a regular for loop over files, its important to note that if no file exists, the variable will be set to the search string. So example for x in *.png; do, if no .png file is found, then x will be set to *.png literally. So depending on what you do in the loop this could be catastrophic. But Bash has an option for this specifically: shopt -s nullglob . Using this option, if no file is found, then x will be set to an empty string. More about Bash options: https://www.gnu.org/software/bash/manual/html_node/The-Shopt-Builtin.html

for x in *.abcdefg; do echo "$x"; done
shopt -s nullglob
for x in *.abcdefg; do echo "$x"; done

BTW one can also do a read line by line without cat, by reading the file directly: (for some reasons Beehaw won't let me type the lower than character, so replace that, here a screenshot too):

while IFS= read -r line; do echo "Line: ${line}" ; done \< filenames.txt
[–] thingsiplay@beehaw.org 2 points 6 days ago (1 children)

Those find and ls commands were just to illustrate how the actual alias work, to get a sense of. I'm aware of those issues with filenames. It's not about ls or find here.

[–] gnuhaut@lemmy.ml 3 points 6 days ago (1 children)

Yeah sorry then. It would be good to not use ls in your example though, someone who doesn't know about that might read this discussion and think that's reasonable.

As for your original question, doing the foreach as a personal alias is fine. I wouldn't use it in any script, since if anyone else reads that, they probably already know about xargs. So using your foreach would be more confusing to any potential reader I think.

[–] thingsiplay@beehaw.org 2 points 6 days ago

I guess you are right. I even point such things out on others, so fair enough. I will update the example, as I don't want someone to see this and take it as a good example. For the alias, I never use aliases in scripts anyway. These are always for interactive usage for me at least. In scripts i follow some other rules, such as use longer names for options (unless it is a really common one).