mjhjmtu

os_version=$(cat /etc/issue |grep Ubuntu)
if $os_version
then
echo found
else
echo notfound
fi

When I tried it on an Ubuntu machine it says
./test: line 2: Ubuntu: command not found
notfound

Can you please help me understand were I went wrong

This works for me, But I want to assign it to a variable

if cat /etc/issue |grep Ubuntu
then
echo found
else
echo notfound
fi

The if statement runs a command, and checks its exit status.
Using $os_version as a command works by expanding it, and running the resulting command line. So if the variable contains Ubuntu 18.04.1 LTS n l, it'll try to to run a command called Ubuntu with the arguments 18.04.1, LTS, etc.

You probably want to use

if [ -n "$os_version" ]; then
 ...
fi

to check if the variable is empty or not ([ -n "$var" ] is true if it's not empty, while [ -z "$var" ] if the variable is empty).

Alternatively, you could use the grep within the if statement itself as you did in the edit, and set a variable there:

distro=unknown
if grep -q Ubuntu < /etc/issue; then
 distro=ubuntu
fi
# ... later
if [ "$distro" = ubuntu ]; then
 # do something Ubuntu-specific
fi

grep -q Ubuntu /etc/issue && echo found || echo not found