Getting "unary operator expected" in my sh script

Question

I’m trying to write a startup script for my service using sh but having trouble with the following line

#!/bin/sh
…
if [ `ps --pid $ppid 2> /dev/null | grep -c $ppid 2> /dev/null` -eq '1' ]; then

I get the error

Starting jboss-as: /etc/init.d/jboss: line 106: [: -eq: unary operator expected

I can’t tell what is wrong with the eq conditional. Any advice? - Dave

Answer 1

The only way I could reproduce your error is when the ppid variable is empty.

#! /bin/sh

ppid=''


if [ `ps --pid $ppid 2> /dev/null | grep -c $ppid 2> /dev/null` -eq '1' ]; then
    echo running;
else
    echo not running;
fi

You can check for variable emptyness using

if [ -z "$ppid" ]; then 
  echo "ppid is empty;"
fi

EDIT: Combining both:

#! /bin/sh

ppid=


if [[ ! -z "$ppid" && $(ps --pid $ppid 2> /dev/null | grep -c $ppid 2> /dev/null) -eq '1' ]]; then
    echo running;
else
    echo not running or pid null;
fi

LAST EDIT: Actually your problem comes from not quoting variable expansion:

#! /bin/sh

ppid=


if [ $(ps --pid "$ppid" 2> /dev/null | grep -c "$ppid" 2> /dev/null) -eq '1' ]; then
    echo running;
else
    echo not running or pid null;
fi

See #5: http://fahdshariff.blogspot.co.uk/2013/10/shell-scripting-best-practices.html#BP5