Why reverse syntax of output redirection doesn't act as input redirection?

Question

command 1: cat file1 > file2 (success)
command 2: file2 < cat file1 (cat : No such file or directory)

I just wanted to know the general syntax of output and input redirections.
If redirection can happen to file or stream, why command 2 fails?

command 2 is not valid: file2 is the "executable" and your try to send it cat as input. What do you actual want to do? — cylgalad, Feb 23 '18 at 08:27
Also basic shell stuff: > sends output to a file, < uses a file as input. — cylgalad, Feb 23 '18 at 08:33
@cylgalad Got it now. Please put the second comment as answer. — Prem, Feb 23 '18 at 08:37
@cylagad The command is not invalid (if file2 is a script), but it's not the same as the first one. — Kusalananda, Feb 23 '18 at 09:12

Kusalananda · Accepted Answer · 2018-03-30T19:21:34.037

The redirection in

cat file1 >file2

is an output redirection, while < specifies an input redirection.

The line

file2 < cat file1

is the same as

file2 file1 <cat

(it does not matter much where the redirection actually occurs as it's handled by the shell in its own parsing step and then removed from the actual command) which means "run file2 with file1 as argument, and redirect the standard input from the file cat".

The error comes from the shell trying to open cat as a file in the current directory. The error occurs before the shell tries to run the command file2.

Related: How is this command legal ? "> file1 < file2 cat"

Why reverse syntax of output redirection doesn't act as input redirection?

1 Answers1