You can use a function written in bash script to use like any other Unix/Linux command. This function can take command line arguments and return a value as exit code similar to standard commands. In the following example, we try to identify if a directory exists with the given name. The function returns 1 if a directory exists. Otherwise returns 0. You can copy the following piece of code and keep in a script or paste it on the console.

function isdir()
{
        echo REQ isdir $1
        if [ -d "$1" ]; then
                return 1
        else
                return 0
        fi
}

export -f isdir

Now let us invoke this as a standard command and examine the return code.

[neo@techpulp ~]$ ls
dir1 dir2 file1 file2
[neo@techpulp ~]$ isdir dir1
REQ isdir dir1
[neo@techpulp ~]$ echo $?
1
[neo@techpulp ~]$ isdir dir99
REQ isdir dir99
[neo@techpulp ~]$ echo $?
0

If you are using this code in a script, you can use it as shown below.

function isdir()
{
        echo REQ isdir $1
        if [ -d "$1" ]; then
                return 1
        else
                return 0
        fi
}

export -f isdir

isdir dir1
if [ "$?" == "0" ]; then
        echo Please specify an existing directory
        exit 0
fi
echo Continuing.. please wait