I received error "bash: line 1: [: -ne: unary operator expected" after run the below script

Question

This script provides the expected output, but throw the error "bash: line 1: [: -ne: unary operator expected". Techies help to fix this.

#!/bin/bash
USR="root"

# Email
SUBJECT="NTP Service Status Report"
EMAIL="[email protected]"
EMAILMESSAGE="/tmp/ntp_status.txt"

# create new file
>$EMAILMESSAGE

# connect each host and pull up user listing
for host in `cat SAP_Prod_servers_51`
do
echo "--------------------------------" >>$EMAILMESSAGE
echo "* HOST: $host " >>$EMAILMESSAGE
echo "--------------------------------" >>$EMAILMESSAGE
ssh $USR@$host "UP=$(ps -ef | grep -v grep | grep ntpd | wc -l);
if [ "$UP" -ne 1 ];
then
        echo "NTP is down.";
        sudo service ntpd start

else
        echo "NTP Service is running.";
fi" >> $EMAILMESSAGE
done

# send an email using /bin/mail
/bin/mail -s "$SUBJECT" "$EMAIL" < $EMAILMESSAGE

Answer 1

The base problem is that the command being sent over ssh is in double-quotes, so the $( ... ) and $UP are being expanded by the local shell before the command is passed to the ssh command and thence to the remote shell.

Also, quotes don't nest, so the double-quotes in the command string don't do what you want -- the first of each pair is treated as a close-quote (matching whichever was before it), the section that's supposed to be quoted is then not in quotes at all, and the next double-quote starts a new quoted section. Since those are quotes around sections of the argument, the local shell removes them from the argument before passing it to ssh.

Between those two things, and assuming that ntpd is not running on the local computer, here's what actually gets passed to the remote computer as a command:

UP=       0;
if [  -ne 1 ];
then
        echo NTP is down.;
        sudo service ntpd start

else
        echo NTP Service is running.;
fi

The [ -ne ] expression is missing its left operand (remember, the quotes got removed), so [ complains about that. I'm not sure why it thinks that's on line 1, as it looks like line 2 to me.

Solution 1: Use single-quotes around the whole command. Since it doesn't have any single-quotes in it nesting isn't an issue, and single-quotes suppress expansion of everything, so the various $ expansions happen on the remote system.

Solution 2: Pass the commands as a (quoted) here-document instead of an argument (BTW, I also double-quoted $EMAILMESSAGE on general principles, and removed semicolons from the ends of lines where they're not needed):

ssh $USR@$host >> "$EMAILMESSAGE" <<'EOF'
UP=$(ps -ef | grep -v grep | grep ntpd | wc -l);
if [ "$UP" -ne 1 ]
then
        echo "NTP is down."
        sudo service ntpd start

else
        echo "NTP Service is running."
fi
EOF

Solution 3: Skip the variable, and just use grep -q directly as the if test, and either escape the internal double-quotes or switch to single-quotes:

ssh $USR@$host "if ps -ef | grep -v grep | grep -q ntpd
then
        echo \"NTP is down.\"
        sudo service ntpd start

else
        echo \"NTP Service is running.\"
fi" >> "$EMAILMESSAGE"