CODEWITHSUNDEEP
shellunixless-unix
Shnd
Shnd

Reputation: 2050

Why is it that we can redirect the input of 'less' command, but we can't run less without any arguments?

as far as i learned in programming, when i want to add the input redirection to my program, i write my program as when it executed, it waits for the user's input (with std::cin or something similar). but i got confused by less command.

We all know that we can do something like this:

ls -la | less

but when we try to execute

less

without any arguments we get an error. how come ?!

Upvotes: 2

Views: 685

Answers (2)

Dietrich Epp
Dietrich Epp

Reputation: 213568

The less accepts input from stdin or from a file.

Since it doesn't make sense to accept input from a terminal (just to display the same input back to a terminal), the less program probably checks if stdin is a terminal (with isatty) and refuses to run.

See man 3 isatty

Upvotes: 5

John Kugelman
John Kugelman

Reputation: 361869

Less is a pager, a UNIX term for a program that shows output one screenful at a time. If you pipe a program's output to less, it shows the output one page a time. If you pass file names on the command line, it shows those files page by page.

If you do neither, there's nothing to page through. It throws an error because there's nothing sensible it can do. What do you want it to show?

Upvotes: 2

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