"unary operator expected" in condition in Bash [duplicate] - linux

Question

In my script I am trying to error check if the first and only argument is equal to -v, but it is an optional argument. I use an if statement, but I keep getting the unary operator expected error.
This is the code:
if [ $1 != -v ]; then
echo "usage: $0 [-v]"
exit
fi
To be more specific:
This part of the script above is checking an optional argument and then after, if the argument is not entered, it should run the rest of the program.
#!/bin/bash
if [ "$#" -gt "1" ]; then
echo "usage: $0 [-v]"
exit
fi
if [ "$1" != -v ]; then
echo "usage: $0 [-v]"
exit
fi
if [ "$1" = -v ]; then
echo "`ps -ef | grep -v '\['`"
else
echo "`ps -ef | grep '\[' | grep root`"
fi

Answer 1

Quotes!
if [ "$1" != -v ]; then
Otherwise, when $1 is completely empty, your test becomes:
[ != -v ]
instead of
[ "" != -v ]
...and != is not a unary operator (that is, one capable of taking only a single argument).

Answer 2

Or for what seems like rampant overkill, but is actually simplistic ... Pretty much covers all of your cases, and no empty string or unary concerns.
In the case the first arg is '-v', then do your conditional ps -ef, else in all other cases throw the usage.
#!/bin/sh
case $1 in
'-v') if [ "$1" = -v ]; then
echo "`ps -ef | grep -v '\['`"
else
echo "`ps -ef | grep '\[' | grep root`"
fi;;
*) echo "usage: $0 [-v]"
exit 1;; #It is good practice to throw a code, hence allowing $? check
esac
If one cares not where the '-v' arg is, then simply drop the case inside a loop. The would allow walking all the args and finding '-v' anywhere (provided it exists). This means command line argument order is not important. Be forewarned, as presented, the variable arg_match is set, thus it is merely a flag. It allows for multiple occurrences of the '-v' arg. One could ignore all other occurrences of '-v' easy enough.
#!/bin/sh
usage ()
{
echo "usage: $0 [-v]"
exit 1
}
unset arg_match
for arg in $*
do
case $arg in
'-v') if [ "$arg" = -v ]; then
echo "`ps -ef | grep -v '\['`"
else
echo "`ps -ef | grep '\[' | grep root`"
fi
arg_match=1;; # this is set, but could increment.
*) ;;
esac
done
if [ ! $arg_match ]
then
usage
fi
But, allow multiple occurrences of an argument is convenient to use in situations such as:
$ adduser -u:sam -s -f -u:bob -trace -verbose
We care not about the order of the arguments, and even allow multiple -u arguments. Yes, it is a simple matter to also allow:
$ adduser -u sam -s -f -u bob -trace -verbose