Error in shell script for detecting packages on system - linux

Question

I wrote a shell script for detecting whether a package is installed or not. My script should write its name and status if it's installed. I can't figure out any problem with my code but when I run it, it doesn't execute the commands under if [ $? == 0 ] condition.
#!/bin/bash
if [ "$1" == "" ]; then
echo "Please hold the line."
else
dpkg -s $# &> /dev/null
fi
if [ $? == 1 ]; then
echo -e "Package \033[0;31mNOT\033[0m found." >&2
else
if [ $? == 0 ]; then
for i in $#; do
dpkg -s $i | grep Package
dpkg -s $i | grep Status
done
fi
fi
But the most weird thing to me is that it works if I add an echo after if statement. Looks like that:
#!/bin/bash
if [ "$1" == "" ]; then
echo "Please hold the line."
else
dpkg -s $# &> /dev/null
fi
if [ $? == 1 ]; then
echo -e "Package \033[0;31mNOT\033[0m found." >&2
else
echo hi
if [ $? == 0 ]; then
for i in $#; do
dpkg -s $i | grep Package
dpkg -s $i | grep Status
done
fi
fi
So if I add an echo -n to right position in my code it will work as I want. But I just want to know what is wrong with first one?

Answer 1

I think in general you could be more deliberate about your return code handling. You are making assumptions about what $? is referring to that may not be valid depending on your program flow, and regardless, make the program harder to read and understand.
#!/bin/bash
dpkg -s $# &> /dev/null
installed=$?
if [ $installed -eq 0 ]; then
for i in $#; do
dpkg -s $i | grep Package
dpkg -s $i | grep Status
done
else
echo -e "Package \033[0;31mNOT\033[0m found." >&2
fi

Answer 2

$? is the return status of the last executed command. 0 is successful, 1 or anything else is an error. Note:
dpkg -s python &> /dev/null # returns 0 (OK, true)
# $? equals 0 now
[ #? == 1 ] # false # returns 1 (error)
# $? equals 1 now
[ #? == 0 ] # false # returns 1 (error)
When you put echo, it works:
dpkg -s python &> /dev/null # returns 0 (OK, true)
# $? equals 0 now
[ #? == 1 ] # false # returns 1 (error)
# $? equals 1 now
echo hi # returns 0 (OK)
# $? equals 0 now
[ #? == 0 ] # true # returns 0 (OK)
You could save $? to a variable, but you don't really need the if inside the else since you already checked if #? == 1 so just put your code inside else:
#!/bin/bash
if [ "$1" == "" ]; then
echo "Please hold the line."
else
dpkg -s $# &> /dev/null
fi
if [ $? == 1 ]; then
echo -e "Package \033[0;31mNOT\033[0m found." >&2
else
for i in $#; do
dpkg -s $i | grep Package
dpkg -s $i | grep Status
done
fi
If you are worried of other possible return statuses of $? (greater than one). You could rewrite your script to
#!/bin/bash
if [ "$1" == "" ]; then
echo "Please hold the line."
else
dpkg -s $# &> /dev/null
fi
if [ $? == 0 ]; then
for i in $#; do
dpkg -s $i | grep Package
dpkg -s $i | grep Status
done
else
echo -e "Package \033[0;31mNOT\033[0m found." >&2
fi