Bash script with netcat expression in if statement

Question

I have a script I'm using to help with docker-compose orchestration called wait_to_start.sh that I obtained from here:

#!/bin/bash

echo $WAIT_COMMAND
echo $WAIT_START_CMD

is_ready() {
    eval "$WAIT_COMMAND"
}

# wait until is ready
i=0
while ! is_ready; do
    i=`expr $i + 1`
    if [ $i -ge $WAIT_LOOPS ]; then
        echo "$(date) - still not ready, giving up"
        exit 1
    fi
    echo "$(date) - waiting to be ready"
    sleep $WAIT_SLEEP
done

#start the script
exec $WAIT_START_CMD

However, I'm having trouble getting it working. I'd like to use netcat to test the service is running.

couchbase:
  container_name: couchbase
  image: couchbase/server:community-3.0.1
  volumes:
    - /opt/couchbase/var:/opt/couchbase/var
  ports:
    - "8091:8091"

myapp:
  container_name: myapp
  image: myapp
  command: wait_to_start.sh
  volumes_from:
    - couchbase
  links:
    - couchbase:couchbase
  environment:
    - WAIT_COMMAND=[ `nc -z -w3 localhost 8091` -eq 0 ]
    - WAIT_START_CMD=mycmd
    - WAIT_SLEEP=2
    - WAIT_LOOPS=10

However it doesn't work.

When I run netcat directly on the command line it works. It returns 0 on success. The above doesn't work.

I was getting:

bash: [: =: unary operator expected

So I played around and found that echoing WAIT_COMMAND produces

[ -eq 0 ]

What am I doing wrong? I'm a bit of a bash noob. It must be something simple.

Command substitution (the backticks) get you a command's output not its return code. — Etan Reisner
– Etan Reisner, Commented Oct 8, 2015 at 13:34
Ah right, because I was following the original example which used curl. — hookenz
– hookenz, Commented Oct 8, 2015 at 22:24
This could be simplified with just WAIT_COMMAND="nc -z -w3 couchbase 11211". — gniourf_gniourf
– gniourf_gniourf, Commented Oct 8, 2015 at 23:18
Also since that script uses eval (which is a horrible thing) you can put an arbitrarily complicated command in there. And just sticking an ! at the start of the nc command should work. — Etan Reisner
– Etan Reisner, Commented Oct 8, 2015 at 23:32

matz · Accepted Answer · 2015-10-08 13:11:16Z

1

Instead of

WAIT_COMMAND=[ nc -z -w3 localhost 8091 -eq 0 ]

simply use

WAIT_COMMAND="nc -z -w3 localhost 8091"

The [ command is not necessary to evaluate the return code of a program.

answered Oct 8, 2015 at 13:11

matz

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1 Comment

hookenz Over a year ago

That doesn't work. Because nc returns zero. And the script is looking for eq to zero is an error. I still want it to work the same way.

hookenz · Accepted Answer · 2015-10-09 00:39:50Z

1

Thanks to the tips and comments and lots of experimentation. What worked in the end was this:

[ $(nc -z -w3 couchbase 11211; echo $?) -eq 0 ]

I also found that if I try to set WAIT_COMMAND directly in the script to test it, it gets evaluated early. So I had to use single quotes.

i.e.

WAIT_COMMAND='[ $(nc -z -w3 couchbase 11211; echo $?) -eq 0 ]'

In docker-compose.yml it looks like this:

environment:
    WAIT_COMMAND=[ $(nc -z -w3 couchbase 11211; echo $?) -eq 0 ]

Or simply: (thanks Etan!)

WAIT_COMMAND="! nc -z -w3 couchbase 11211"

edited Oct 9, 2015 at 0:39

answered Oct 8, 2015 at 23:32

hookenz

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