Use bash variables in subsequent bash command

Question

I am writing a bash script which shows me all tcp connections which are important for me. For that I use netstat -atp. It looks like this:

num_haproxy_443_established=$(netstat -atp | grep ESTABLISHED | grep :https | grep haproxy | wc -l)
printf "\n show ESTABLISHED connections port 443 \n"
printf " %s" $num_haproxy_443_established

In the bash script I have some of these calls and now I would like to optimize it and call netstat -atp only once and reuse the results. I tried:

netstat_res=$(netstat -atp)

num_haproxy_443_timewait=$("$netstat_res" | grep TIME_WAIT | grep :https | grep haproxy | wc -l)
printf " %s" $num_haproxy_443_timewait

After executing the script I always get 0: command not found as error message. How can I use a variable inside $(...) ?

Thanks!

Fair enough, but you not piping the contents of netstat_res to grep by $(echo "$netstat_res" | grep TIME_WAIT | grep :https | grep haproxy | wc -l) — Inian
– Inian, Commented Jun 20, 2017 at 9:11
If you set your variable to netstat_res="netstat -atp" it would work but then you'd call netstat every time (not sure why you would want to re-use its results anyway) — laurent
– laurent, Commented Jun 20, 2017 at 9:12
Sometimes I want to monitor the load on a loadbalancer. Therefore I would run this script every second for about 30 seconds or so. When there are 1000 or more connections I don't want to run netstat 8 times or more but only once. — Hamburml
– Hamburml, Commented Jun 20, 2017 at 9:16
I think an awk with print is probably going to be a cleaner solution or answer. Similar to what @anubhava proposed. — jww
– jww, Commented Jun 20, 2017 at 9:25

m13r · Accepted Answer · 2017-06-20 09:20:00Z

1

If you have something like A="foo" then $("$A") will be resolved to call the program foo in the subshell.

So you just have to echo the content of the variable and then grep from it:

num_haproxy_443_timewait=$(echo "$netstat_res" | grep TIME_WAIT ...)

edited Jun 20, 2017 at 9:20

answered Jun 20, 2017 at 9:11

m13r

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Comments

anubhava · Accepted Answer · 2017-06-20 09:22:26Z

1

You can use shell array to store your netstat command:

# your netstat command
netstat_res=(netstat -atp)

# then execute it as
num_haproxy_443_timewait=$("${netstat_res[@]}" |
awk '/TIME_WAIT/ && /:TIME_WAIT/ && /haproxy/{++n} END{print n}')

echo $num_haproxy_443_timewait

Also note how you can avoid multiple grep calls by a single awk call.

Related BASH FAQ: I'm trying to put a command in a variable, but the complex cases always fail!

edited Jun 20, 2017 at 9:22

answered Jun 20, 2017 at 9:17

anubhava

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3 Comments

m13r Over a year ago

no arrays needed for this... the newlines will be preserved in the bash variable.. only echo "$netstat_res" is needed

anubhava Over a year ago

ok I see your point but caching netstat results and reusing might give wrong results depending upon life of the cache.

anubhava Over a year ago

To make netstat run faster I suggest using netstat -atnp

Raindy Long · Accepted Answer · 2017-06-20 09:16:08Z

0

At your case, $netstat_res is the result but not the COMMAND, if you want to save the result and use it not only once, save the result to the file is the better way, such as:

    netstat -atp > /tmp/netstat_status.txt
    num_haproxy_443_timewait=$(cat /tmp/netstat_status.txt | grep TIME_WAIT | grep :https | grep haproxy | wc -l)
    printf " %s" $num_haproxy_443_timewait

answered Jun 20, 2017 at 9:16

Raindy Long

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3 Comments

Hamburml Over a year ago

Yes, $netstat_res is the result. I want to use the result as input for the next commands like grep. I don't want to save the result as a file because that's unnecessary disk usage.

Raindy Long Over a year ago

@Hamburml Oh I see, but "echo $netstat_res" will lose the "\n" separtor, how to fix it?

Hamburml Over a year ago

But it works with echo, see stackoverflow.com/a/44648712/3069477

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