CODEWITHSUNDEEP
shell
user3685285
user3685285

Reputation: 6586

What does the `<` operator do in shell?

I was reading this question, in which someone is trying to get the number of lines in a file from the shell. From the comments, I saw and implemented that the following:

$ wc -l myfile.txt
30213 myfile.txt

$ wc -l < <filename>
30213

What I don't understand is, what the < operator is doing here. Can someone explain why the < operator chops off the file name from the output in this case?

Upvotes: 0

Views: 242

Answers (2)

tomix86
tomix86

Reputation: 1446

In the first case, the filename is supplied as an argument when calling wc, this causes wc to include it in the output.

In the second case, stdin is redirected from filename, this makes wc "unaware" of the file and thus not able to print its name. This is equivalent to calling cat filename | wc -l.

So to answer your question, operator < redirects input file descriptor to read from a given file. If descriptor number is not explicitly specified, then it defaults to standard input (fd 0). See here for a formal reference and here for a convenient description.

Upvotes: 2

Odd822
Odd822

Reputation: 53

wc outputs the file name as part of the output when it reads from a file.

The < redirects input and output between files/programs, following the direction of the "arrow". For more information on I/O redirection, this is a pretty handy link. The reason that the file name is "chopped off" is because wc is technically no longer reading from a file.

Upvotes: 2

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