Why does taking stdin from a file differ from receiving it over a pipe?

Question

Using bash I often want to get the headings of a large csv file and search the rest for a particular entry. I do this as follows.

$ (head -1; grep mike) < tmp.csv
name,age,favourite colour
mike,38,blue

But taking the input from cat, or any other command doesn't work - it seems grep never gets passed the remainder of the file.

$ cat tmp.csv | (head -1; grep mike)
name,age,favourite colour

Why is there different behaviour in these two cases?

Answer 1

The difference between reading from a pipe and reading from a file is that you can lseek on a file, but not on a pipe.

The behavior here looks (as seen through strace) like it's coming from head, not bash. head will read a buffer and find the appropriate number of lines, then lseek backwards to the point where the last-output line ended, leaving the file handle open at that place. As above, this works if it's reading a file, but not if it's reading from a pipe.

I can't think of any case other than what you're doing where this behavior in head makes sense, but there it is. Learn something new every day, I tell ya ...