How to use the last command in the while loop?

Question

I am wondering how to modify this code.

ls
while [ 1 > 0 ]
do
fc -s
sleep 1
done

For fc -s, you can refer to this question for details.

My question are as follows:

How will it run? If you name it as test.sh.
If I want fc -s represent the first line ls, how will I modify it? That is to say, I want to Run ls then sleep one second, then ls, then sleep one second,... Of course ls may be placed by some other command.

Welcome to Unix & Linux! Could you try elaborating your explanations a bit of what you're asking? I wasn't able to follow what your questions are? Specifically with #2. — slm, Jul 18 '13 at 11:44
What does it imply: "ls may be placed by some other command." — unxnut, Jul 18 '13 at 12:00
"I want to Run ls then sleep one second, then ls, then sleep one second,..." The answer to this is watch. No need to make your own. — user, Jul 18 '13 at 12:45
watch is such a powerful tool that I cannot control the output. @MichaelKjörling — eccstartup, Jul 18 '13 at 12:54

score 4 · Answer 1 · edited Jul 18 '13 at 13:17

4

Why don't you use the watch command, which does exactly what you want? This is what it says in the manual:

watch - execute a program periodically, showing output fullscreen

To run the command every second:

watch -n 1 command

edited Jul 18 '13 at 13:17

unxnut

answered Jul 18 '13 at 12:36

user2516278

Yes, but how to get the command before the while loop? – eccstartup Jul 18 '13 at 12:39
You need to be more specific here. What is this script good for? – user2516278 Jul 18 '13 at 12:55
I need to a command like watch. But I don't want it presented fullscreen. I want all output shows again and again in the terminal. And knowing how to get the command before the while loop is another problem which is not as important. – eccstartup Jul 18 '13 at 13:02
#/!bin/bash while true; do $1 sleep 1 done
save this as test.sh, make it executable and execute ist e.g. test.sh ls
– user2516278 Jul 18 '13 at 13:12
#/!bin/bash while true do $1 sleep 1 done – user2516278 Jul 18 '13 at 13:20
@user2516278: You should add that to your answer. – Jul 18 '13 at 13:22
@Evan having probs with the new line in code tag :-( – user2516278 Jul 18 '13 at 13:24
@user2516278: Prepend code lines with four spaces. See http://meta.stackexchange.com/questions/22186/how-do-i-format-my-code-blocks. – Jul 18 '13 at 13:25

score 3 · Accepted Answer · answered Jul 18 '13 at 13:16

3

You can try something along the lines of this:

 $ cat script.sh
 while sleep 1
 do
     eval "$@"
 done
 $ ls
 $ sh script.sh !!

The script will run the command passed to it over and over again. !! evaluates to the last command.

If you want the command to be part of the script and don't want to type it in multiple locations, try this:

 $ cat script.sh
 cmd=ls
 while sleep 1
 do
     eval "$cmd"
 done
 $ sh script.sh

answered Jul 18 '13 at 13:16

This can quite easily break -- reparsing with eval is almost always a bad idea. http://sprunge.us/TMij – Chris Down Jul 18 '13 at 13:24
@ChrisDown: It's definitely not ideal. That particular problem can be fixed by removing the eval statement. (s/eval "$@"/"$@"/). – Jul 18 '13 at 13:31

2 Answers2