Does “bash: command: command not found” return false?

# example you need wget and your PATH is okay then:
# bash/ksh/.. will return exit code 127 if command not found
# 
# redirect stdin and stderr to the /dev/null = if exist, 
# output is not interesting
wget --help >/dev/null 2>&1
stat=$?   # variable ? include last command exit status
echo "exit status:$stat"
if ((stat == 127 )) ; then # not exist/found
   echo "install wget"
   exit 1
fi
echo "wget exist, continue"

You can use also if before command, but that handle all exit codes which are not 0.

You can do any command and test exit code using if

# if command ; then 
#     works fine
# else
#     not so fine
# fi

# negative testing ! = if not exit code 0 then
if ! wget --help >/dev/null 2>&1 ; then
   # give err msg to the stderr and exit 
   echo "install wget / wget didn't work correctly" >&2
   exit 1
fi
echo "wget works fine"

Before test it with if, look first working exit code

wget --help
echo $?
# will echo 0 = ok. not 0 is not ok
# if command return not 0, then  you can't test using if, you need
# test using exit value = 127

If your intent is to check whether a particular command is available, you should do so instead of trying to execute it:

if command -v spamegg >/dev/null; then
  echo spamegg is available
else
  apt-get install spamegg
fi

Trying to execute spamegg to see if it's available is a bad approach. First, it obfuscates your code, making it look like spamegg is used to install something. Second (and more importantly), the command your're checking might exist but simply fail for some reason:

if grep >/dev/null 2>&1; then
  echo grep is available
else
  echo grep is not available
fi

This will output grep is not available even when it is.

IMHO, I don't think your approach is the best way to tackle this problem. The reason being is just because a command returns not found, doesn't mean the program isn't installed. It could simply indicate that the program is not located in any of your PATH locations.

Perhaps, a better way would be to this is actually check against the list of installed packages:

RHEL/CentOS:

grep PROGRAM_NAME <(rpm -qa --qf "%{NAME}\n")

Debian/Ubuntu:

grep PROGRAM_NAME <(dpkg --get-selections | awk '{ print $1}')