difference between grep Vs cat and grep

Question

i would like to know difference between below 2 commands, I understand that 2) should be use but i want to know the exact sequence that happens in 1) and 2) suppose filename

Answer 1

Functionally (in terms of output), those two are the same. The first one actually creates a separate process cat which simply send the contents of the file to standard output, which shows up on the standard input of the grep, because the shell has connected the two with a pipe.

In that sense grep regex is also equivalent but with one less process.

Where you'll start seeing the difference is in variants when the extra information (the file names) is used by grep, such as with:

grep -n regex filename1 filename2

The difference between that and:

cat filename1 filename2 | grep -n regex

is that the former knows about the individual files whereas the latter sees it as one file (with no name).

While the former may give you:

filename1:7:line with regex in 10-line file
filename2:2:another regex line

the latter will be more like:

7:line with regex in 10-line file
12:another regex line

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Another executable that acts differently if it knows the file names is wc, the word counter programs:

$ cat qq.in
1
2
3

$ wc -l qq.in           # knows file so prints it
3 qq.in

$ cat qq.in | wc -l     # does not know file
3

$ wc -l