“$（变量）”和“$（VARIABLE）”之间有什么区别？

Question

What's the difference? And why these are working:

echo $LOGNAME       #prints username
echo "$(logname)"   #prints username

but this isn't:

echo "$(LOGNAME)"   #prints command not found.

Answer 1

logname is a command.
LOGNAME is a variable.

$(logname) works because logname command exists.
$(LOGNAME) tries to run the command LOGNAME which does not exist.

Read the following useful guide

Answer 2

$LOGNAME is a variable. logname is a command. When you do

echo $LOGNAME

you are echoing the variable, whereas when you do

echo "$(logname)"

you are echoing the result of executing the command. It happens to be the case that the output is the same.

If you do env | grep LOGNAME env | grep LOGNAME , you will see that $LOGNAME is an environment variable and if you do which logname you will see the path to the executable. However, if you do which LOGNAME , you will see that there is no output. echo $? shows that the exit status of the command is 1, which means that no executable could be found.

Coincidentally, you can do the same thing with $PWD and pwd .