Comments on: Let’s have a look at cat (Linux, bash)

By: AnonymousCoward

AnonymousCoward — Thu, 14 May 2009 10:40:54 +0000

While cat certainly is a very useful tool, you should remember that it’s essentially bound to the drive IO and not the CPU. A useless use of cat (UUOC) causes a file to be read repeatedly and unnecessarily and the performance degradation can’t be overcome with more cores or faster clock speeds.

Consider:
$ cat file | grep foo
versus:
$ grep foo file

This can seem pretty trivial with tiny files, but if the file is large or the storage is slow the effects can be extremely painful.

I ran the following unscientific benchmark using a 7MB text file:

$ time grep foo foo.csv > /dev/null

real 0m0.172s
user 0m0.093s
sys 0m0.046s

$ time cat foo.csv | grep foo > /dev/null

real 0m4.238s
user 0m3.951s
sys 0m0.264s

Imagine if that file’s size was greater than a GB!

By: John Swindle

John Swindle — Thu, 14 May 2009 03:48:17 +0000

Somehow, Sean neglected what seems to me to be another obvious use of cat. With the “double greater-than” operator, (>>) cat will add text to an existing file. I use this about once a week.

I have yet to use cat to write a program of any kind, but the World Bank hack does sound like an interesting challenge.

By: Barry Rountree

Barry Rountree — Thu, 14 May 2009 01:14:32 +0000

No mention of ‘tac’?

tac – concatenate and print files in reverse

Definitely a why-the-hell-would-anyone-need-that-command that turns out to be a lifesaver once or twice in a career.