what is the difference between PWD , pwd , ${PWD} , ${pwd} , $(PWD) , $(pwd)

I am using it inside docker bind mount and it giving different result.

>Solution :

PWD , pwd , ${PWD} , ${pwd} , $(PWD) , $(pwd)

I assume that you are typing the above at a shell prompt.

• PWD is the name of a non-existent command. That fails with "command not found".
• pwd is the name of a shell builtin command. That outputs the current directory.
• ${PWD} expands to the value of the PWD shell variable which contains the pathname of the current directory. It then attempts to execute it which fails because a directory is not executable.
• ${pwd} expands a non-existent shell variable. That gives an empty string and is ignored.
• $(PWD) attempts to get the output from running PWD, and run that as a command. It fails because the PWD command does not exist.
• $(pwd) runs the existing command pwd. Then it takes the output of that and attempts to run it as a command. It fails because the output is a directory pathname, and directories are not executable.