File test operators

The test command includes the following FILE operators that allow you to test for particular types of files:

    -b FILE - True if the FILE exists and is a special block file.
    -c FILE - True if the FILE exists and is a special character file.
    -d FILE - True if the FILE exists and is a directory.
    -e FILE - True if the FILE exists and is a file, regardless of type (node, directory, socket, etc.).
    -f FILE - True if the FILE exists and is a regular file (not a directory or device).
    -G FILE - True if the FILE exists and has the same group as the user running the command.
    -h FILE - True if the FILE exists and is a symbolic link.
    -g FILE - True if the FILE exists and has set-group-id (sgid) flag set.
    -k FILE - True if the FILE exists and has a sticky bit flag set.
    -L FILE - True if the FILE exists and is a symbolic link.
    -O FILE - True if the FILE exists and is owned by the user running the command.
    -p FILE - True if the FILE exists and is a pipe.
    -r FILE - True if the FILE exists and is readable.
    -S FILE - True if the FILE exists and is a socket.
    -s FILE - True if the FILE exists and has nonzero size.
    -u FILE - True if the FILE exists, and set-user-id (suid) flag is set.
    -w FILE - True if the FILE exists and is writable.
    -x FILE - True if the FILE exists and is executable.

Comparison operators are operators that compare values and return true or false. When comparing strings in Bash you can use the following operators:

    -eq - equals, -ne - not equal to
    string1 = string2 and string1 == string2 - The equality operator returns true if the operands are equal.
        Use the = operator with the test [ command.
        Use the == operator with the [[ command for pattern matching.
    string1 != string2 - The inequality operator returns true if the operands are not equal.
    string1 =~ regex- The regex operator returns true if the left operand matches the extended regular expression on the right.
    string1 > string2 - The greater than operator returns true if the left operand is greater than the right sorted by lexicographical (alphabetical) order.
    string1 < string2 - The less than operator returns true if the right operand is greater than the right sorted by lexicographical (alphabetical) order.
    -z string - True if the string length is zero.
    -n string - True if the string length is non-zero.

Following are a few points to be noted when comparing strings:

    A blank space must be used between the binary operator and the operands.
    Always use double quotes around the variable names to avoid any word splitting or globbing issues.
    Bash does not segregate variables by “type”, variables are treated as integer or string depending on the context.
    
There are multiple ways to check if a string contains a substring:

    One approach is to use surround the substring with asterisk symbols * which means match all characters.

EX block:

if [ -z "$1" ];then
   echo "Please enter something!"
else
   echo "Thanks..."
fi