Does the cat command pass back a file or the contents of the file

catcommand linegrep

Compare the following commands:

cat f.txt | grep "someText" 
grep "someText" f.txt

They both seem to work. But the documentation for cat says cat outputs the contents of the file rather than the file name and grep command takes file name but not file contents (correct me if I am wrong), then why does the first command work since it is feeding grep with the file contents rather than the file name.

Another question: they both work but why would one use the first line instead of the second, the first one is just redundant?

Best Answer

In your first example

cat f.txt | grep "someText"

grep doesn't get a filename argument, only a string to search for. In that case grep will read the text to search from standard input. In this case that standard input is piped in from the output of cat f.txt, which outputs the content of the file not the filename.
What you also could have done to make grep read from stdin is to use:

< f.txt grep "someText"

Using cat is quite often redundant on its own (independent of grep) and can be replaced by the input redirection as above. I would always use the second form in your example, unless you have to do some preprocessing on the input.

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