eval variable fails to execute sed command

I wrote a bash script to remove an entry from crontab. I want to execute this command on remote servers and check if the command executed successfully. I'm trying this with the following script:

DELETEENTRY='sed -i '"'"'/# Restarts HP OMO Agent Weekly/d'"'"' /var/spool/cron/root && echo "ok" || echo "error"'
DELETERESULT=$(ssh -q $HOST-- eval $DELETEENTRY)
 if [ "$DELETERESULT" = "ok" ]
 then
   ...........
 fi

If I run the sed command as is, the command runs fine. If I use it with eval, I get the following error:

sed: -e expression #1, char 2: unterminated address regex

I tried multiple quotes of the /# Restarts HP OMO Agent Weekly/d String, but always ending up getting the same error.

Question

How can I use eval to successfully run the DELETENTRY variable?

This ...
> DELETEENTRY='sed -i '"'"'/# Restarts HP OMO Agent Weekly/d'"'"' /var/spool/cron/root && echo "ok" || echo "error"'
... sets the contents of shell variable DELETEENTRY to:

sed -i '/# Restarts HP OMO Agent Weekly/d' /var/spool/cron/root && echo "ok" || echo "error"

When you then perform an

> ssh -q $HOST-- eval $DELETEENTRY

• the shell applies parameter expansion to get

  ssh -q a.host.name-- eval sed -i '/# Restarts HP OMO Agent Weekly/d' /var/spool/cron/root && echo "ok" || echo "error"
  

  You appear to be missing a space before the --, but I'm going to assume that that's a typo in the question, because you would otherwise have a different issue.

• That command will be subject to word splitting (the embedded ' are treated as ordinary characters for this purpose), but that's not actually a problem here, because on the remote side, ssh will join the command and all its arguments back together with spaces to use as input to the shell.

• However, when that shell processes the resulting command, it will perform its own round of quote removal on it, and here your inner single quotes get consumed.

• The command that is ultimately executed by eval is therefore

  sed -i /# Restarts HP OMO Agent Weekly/d /var/spool/cron/root
  

  , which gets chopped into tiny little pieces by word splitting. Hence the error (the first sed expression is just /#).

It's not clear to me why you're using eval here. It's tricky to use correctly at the best of times, and in this case, ssh already does substantially the same job that eval does -- read one or more arguments, form them into a string, and process them as shell input. Note also that this means that using ssh in this way exposes you to all the problems that using eval does.

Additionally, It's usually bad news to store commands in variables, but if you must do so then you should use an array instead of a string so as to take control over word splitting. Maybe something like this, then:

DELETEENTRY=(
  sed -i "'/# Restarts HP OMO Agent Weekly/d'" /var/spool/cron/root
  '&&' echo ok
  '||' echo error
)
DELETERESULT=$(ssh -q "$HOST" -- "${DELETEENTRY[@]}")
# ...

That form of quoted expansion of an array ("${DELETEENTRY[@]}") expands each element to a separate word, which is what one would ordinarily do when expanding a command from a variable. In this particular case, though, the remote ssh is just going to join it all together into a string anyway, so it wouldn't make a difference if you used the other flavor of array expansion ("${DELETEENTRY[*]}").

Note that the eval has been removed. As already discussed, it is not needed here.

The single quotes around the sed expression are stored in the array (and look how much easier that is to read) and conveyed to the remote shell. There, they serve their usual purpose in the shell command that is constructed and run.

There are other alternatives and possible improvements, too, some of which are discussed in another answer.

I recommend serializing the function with declare -f and then load it on the other side and run. That way you can properly check your scripts with shellcheck.

No need to echo anything, exit statuses are transferred over ssh.

func() {
    sed -i '/# Restarts HP OMO Agent Weekly/d' /var/spool/cron/root
}
if ssh -q "$HOST" "$(declare -f func); func"; then
   .........
fi

Because this is one command, you can quote the string for the other side using printf %q. As you have found out, quoting it yourself is hard.

ssh "$HOST" "$(printf "%q " sed -i '/# Restarts HP OMO Agent Weekly/d' /var/spool/cron/root)"

Also, you may consider ansible.