what is the point of '"tac" ?

zaper · 05-04-2018, 01:29 PM

i was bored so i decided to browse some man pages, in cat's man page there was a reference to a command called "tac", hmm interesting i decided to investigate turns out it is literally just cat but where line number 10 would be line number 1 line 2 would be 8 etc so i just wanted to ask why does it exist ? what's it's point ?

scasey · 05-04-2018, 01:49 PM

Did you look at man tac?

Code:

tac - concatenate and print files in reverse

zaper · 05-04-2018, 01:51 PM

yes i can also read a man page i meant why would you ever use it ? why is it there ?

jlinkels · 05-04-2018, 02:07 PM

tac is a very old command. Sometimes users wanted to view a file in reverse. Like for example a log file and view it in reverse.

Code:

tac mylogfile | more

Remember that in those days users might have used a teletype. So they had to use tac to view something in reverse.

jlinkels

zaper · 05-04-2018, 02:14 PM

but linux was made in 1991 who still used teletypes ? or is it a unix carryover ?

scasey · 05-04-2018, 02:18 PM

Quote:

Originally Posted by zaper

yes i can also read a man page i meant why would you ever use it ? why is it there ?

Oh. Sorry. Mis-read your OP.
Frankly, I didn't know about it until this thread. I tend to use tail to see the most recent entries in a file, but I can see how tac could be useful, since tail still displays in chronological sequence, where the reverse display might well be useful...it is, after all, how I sort messages in my email clients

jlinkels: TTY? THAT brought back memories...paper tape and all

jlinkels · 05-04-2018, 02:21 PM

Quote:

Originally Posted by zaper

but linux was made in 1991 who still used teletypes ? or is it a unix carryover ?

It is a Unix carryover.

jlinkels

zaper · 05-04-2018, 02:23 PM

ehm interesting why don't you just use a text editor and jump to last line ? not trying to be mean genuinely curios

zaper · 05-04-2018, 02:24 PM

Quote:

Originally Posted by jlinkels

It is a Unix carryover.

jlinkels

oh ok that makes sense thanks

zaper · 05-04-2018, 02:28 PM

I'm just going to ask this here because i don't want to open another thread for something so minor but any idea why calling it with no argv does not write to stdin unlike cat ?

hydrurga · 05-04-2018, 03:12 PM

Quote:

Originally Posted by zaper

ehm interesting why don't you just use a text editor and jump to last line ? not trying to be mean genuinely curios

What if you want to pipe the output to another process rather than read it in an editor?

zaper · 05-04-2018, 03:41 PM

Quote:

Originally Posted by hydrurga

What if you want to pipe the output to another process rather than read it in an editor?

you cat the file and then pipe that into another program ?

hydrurga · 05-04-2018, 03:47 PM

Quote:

Originally Posted by zaper

you cat the file and then pipe that into another program ?

Imagine you want to process the data in a log file, but in reverse chronological order - tac fits the bill for piping the output to your data processor.

Imagine that you want to quickly look at the latest log. You know that something in the last x hours has caused the problem, but don't know when, so using tail is out of the question (how much tail would you want to see?) - tac piped through more fits the bill.

It may be seldom used, but tac still has its uses.

zaper · 05-04-2018, 04:16 PM

fair enough any idea what running it with no argv does ?

hydrurga · 05-04-2018, 04:26 PM

Quote:

Originally Posted by zaper

fair enough any idea what running it with no argv does ?

Your previous post talked about "writing to stdin". What does that mean, if I can ask?

If you run tac with no arguments, it takes its input from stdin.