Consider the following input file:
1
2
3
4
Running
{ grep -q 2; cat; } < infile
doesn't print anything. I'd expect it to print
3
4
I can get the expected output if I change it to
{ sed -n 2q; cat; } < infile
Why doesn't the first command print the expected output ?
It's a seekable input file and per the standard under OPTIONS:
-q
Quiet. Nothing shall be written to the standard output, regardless of
matching lines. Exit with zero status if an input line is selected.
and further down, under APPLICATION USAGE (emphasize mine):
The
-qoption provides a means of easily determining whether or not a pattern (or string) exists in a group of files. When searching several files, it provides a performance improvement (because it can quit as soon as it finds the first match)[...]
Now, per the same standard (in Introduction, under INPUT FILES)
When a standard utility reads a seekable input file and terminates without an error before it reaches end-of-file, the utility shall ensure that the file offset in the open file description is properly positioned just past the last byte processed by the utility[...]
tail -n +2 file
(sed -n 1q; cat) < file
...
The second command is equivalent to the first only when the file is seekable.
Why does grep -q consume the whole file ?
This is gnu grep if it matters (though Kusalananda just confirmed the same happens on OpenBSD)
