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Comments on For scripting what are the pros and cons of command line arguments versus capturing input at the start?

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For scripting what are the pros and cons of command line arguments versus capturing input at the start?

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Let's say I have a script that needs the user to set X number of variables at the start. One can either

  • Pass the arguments in on the command line.
  • Start the program and then have the user input the variables with Python's input() function or PHP's fopen("php://stdin", "r") for example.

What would the pros and cons be and when would I decide to use one method versus the other?

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General comments (1 comment)
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I largely agree with existing answers, but wanted to add this:

In many cases, there is no clear answer one way or the other. Take the program wc which counts characters and lines. Should the interface be wc file.txt or cat file.txt | wc? Compelling arguments can be made for the value of either. Yet we have to pick one.

Actually we don't. wc supports both. More complex programs support the well known - parameter, so cat file.txt | some_program --file - as a way of making it a bit more coherent.

Interactive input falls under this as well. ssh-keygen will interactively ask for the key type. But with ssh-keygen -t ed25519 it will not ask.

CLI arguments, standard input (from pipe) and interactive input form the three pillars of Unix-style human-computer interaction. All programs should aspire to provide first-class support for all three eventually, even if they won't, just like "all kids" aspire to be astronauts. Imagine a stool with two legs. You can make it work, and it's certainly better than a stool with one leg, and way better than none, but once you get to three legs it gets really good.

Of course you have to start somewhere. For your first leg, just pick one. Whatever you feel is a likely use case for your script, and let users rely on Unix to "improvise" the missing legs:

  • cat if they want to use args but you support stdin
  • Shell redirection if they want stdin but you want args
  • expect if you support interactive but they want scriptable

As your program matures, you can use the experience gained from maintaining it to decide which next method of input you should add support for.

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Useless use of `cat` (1 comment)
Useless use of `cat`
__blackjack__‭ wrote over 1 year ago

Useless use of cat. It shouldn't be cat file.txt | wc but < file.txt wc (or wc < file.txt). :-)

I don't think just picking any one of the three for a start is a good idea. It is harder to script something with expect than to provide command line options. And it is harder to route some output into a program that wants filenames and doesn't read from stdin. There are shells that make this quite easy, but I would not assume the users shell, or their knowledge about such things. And of course the user might want to use another language than shell scripts to call/automate.