iostat returning the same values in a while loop

[rdindir]

Hello All,

I wanted to write a script that will report me the idle percentage of the cpu. For this I was planning on using "iostat".

When I run iostat with its -w parameter, the cpu idle values change ("iostate -t proc -w 1"). This is ok. It continuously prints the proc parameters every second. The values show change in the cpu idle percentage. As can be seen below

tty cpu
tin tout us ni sy in id
3 56 8 0 3 0 88
1 81 4 0 3 0 93
0 27 3 0 2 0 95
0 27 3 0 3 0 94
1 27 3 0 2 0 94

But when I do this in a loop, the values do not change. I have created a loop like so...

while test "1"=="1"; do iostat -t proc; sleep 0.8; done

This does invoke iostat. But the values do now change. Here is the output

tty cpu
tin tout us ni sy in id
2 61 8 0 3 0 88
tty cpu
tin tout us ni sy in id
2 61 8 0 3 0 88
tty cpu
tin tout us ni sy in id
2 61 8 0 3 0 88
tty cpu
tin tout us ni sy in id
2 61 8 0 3 0 88
tty cpu
tin tout us ni sy in id
2 61 8 0 3 0 88
tty cpu
tin tout us ni sy in id
2 61 8 0 3 0 88

Why would the values not change? I am confused on this behavior...

Regards

 

[yuripv79]

test "1"=="1"

Use true.

It looks like the first line printed has that line showing everything as 0/100, looks like a bug.

 

[yuripv79]

Ugh, I can't read, from iostat(8):

The first statistics that are printed are averaged over the system uptime.

 

[rdindir]

Use true.

I am using "sh", and tests did not have anything regarding true in its manual.

It looks like the first line printed has that line showing everything as 0/100, looks like a bug.

A bug in the script above? A bug in iostat?

 

[SirDice]

I am using "sh", and tests did not have anything regarding true in its manual.

infinite loop:

while true; do
...
done

 

[yuripv79]

A bug in the script above? A bug in iostat?

Not a bug, read my next reply

I am using "sh", and tests did not have anything regarding true in its manual.

true is system utility that does nothing successfully (or it could be a shell built-in), in any case, using it works.

 

[rdindir]

Ugh, I can't read, from iostat(8):

The first statistics that are printed are averaged over the system uptime.

Does that mean, we should at least take 2 readings, possibly specifying a suitable wait time. Like so

iostat -w 0.1 -c 2

Here getting 2 readings with 0.1 sec apart, and taking the last reading would maybe give me a fairly close reading. Or maybe average the two readings maybe.

 

[rdindir]

infinite loop:

while true; do
...
done

I did not know. Thanks.

 

[rdindir]

Not a bug, read my next reply


true is system utility that does nothing successfully (or it could be a shell built-in), in any case, using it works.

I did not know. It even has its own manual Thanks.

 

[yuripv79]

Here getting 2 readings with 0.1 sec apart, and taking the last reading would maybe give me a fairly close reading. Or maybe average the two readings maybe.

First one is average over all uptime so adding it up will break current readings. If you really need it in a while loop, may be something like this (going with your idea)?

while true; do iostat -w 0.01 -c 2 | tail -1; sleep 1; done

 

[rdindir]

First one is average over all uptime so adding it up will break current readings. If you really need it in a while loop, may be something like this (going with your idea)?

while true; do iostat -w 0.01 -c 2 | tail -1; sleep 1; done

Or maybe take 3 readings and average the last 2. That might be better.

 

[rdindir]

I ended up using "ps -Ceao %cpu -p $(pgrep -S idle)" to get the usage of the cpu, which is close to the values reported by the "top" tool. From the forum, there was one answer that suggested this (https://forums.freebsd.org/threads/cpu-usage-shell-script.62707/). The returned value is the usage of all the cores you have, I believe. So must be divided by the number of cores you have.

Thank you all.

 

[Alain De Vos]

When you run iostat the first message is invalid.
Just run iostat once with parameters to print regularly.

 

[Kai Burghardt]

If you insist on using iostat(8), you can pipe into the entire while statement, specifically read:

iostat -t proc -w 1 | while read tty_input tty_output cpu_user cpu_nice cpu_system cpu_interrupt cpu_idle
do
    # `iostat(8)` regularly prints a column header.
    expr "${cpu_idle}" : '[[:digit:]]' >&- || continue
    
    printf 'I am %3d%% idle.\n' ${cpu_idle}
done