Re: A bit of grep and regex help?

Davorin Vlahovic wrote:

On 2007-02-16, Robert Newson <ReapNewsB@xxxxxxxxxxxxxxxxxxx> wrote:

Davorin Vlahovic wrote:

On 2007-02-15, Benny Nielsen <spam@xxxxxxxxxxx> wrote:

awk -F: '{if ($3 > 499) print $0}' < /etc/passwd

You don't need redirection. This works and is more efficient:

awk -F: '{if ($3 > 499) print $0}' /etc/passwd

Stupid question: why is [g]awk more efficient reading from a file specified on the command line as an argument as opposed to finding none there and using stdin?

If you use redirection the shell opens the file and feeds the data into
another process.

And here I was thinking redirection was implemented via close and open (ie reopen), the shell process having nothing to do as the stdin of the command was connected /for it by the shell/ directly to the file, ie the shell roughly did:

if (child_of fork())
{
close(stdin)
open(file) as stdin
exec(new_command)
}

as opposed to the shell roughly doing:

if (child_of fork())
{
if (redirection)
{
make a pipe()
if (child_of fork())
{
connect write of pipe to stdout
exec(cat file)
}
connect read of pipe to stdin
}
exec(new_command)
}

If you use file as an argument to awk the system has no overhead
concerning routing the data between two processes.

I think you're thinking of:

$ cat file | command

which /requires/ two processes, a pipe for the data, is unefficient, is a useless use of cat and is *NOT* the same as:

$ command < file

which reopens stdin connected to the file [for the command]. (I think you'll find lsof agrees that a file redirected to stdin is connected directly to that file and not a pipe [from another process, eg the shell].)

.

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