What's the different between ` and ' in programming, such as MySQL and shells

Question

phpMySQL use backticks ` to surround column names, and I've just see people use backticks in shell scripts.

Is that the same if they use ' ?

Answer 1

In MySQL, backticks could be used to enclose table and column names which are reserved words, like this:

SELECT  *
FROM    `table`

Single quotes just denote a string constant:

SELECT  *
FROM    `table`
WHERE   `name` = 'test'

In shells, backticks let you use the output of one command as an argument to another command:

echo `date`

will execute date and use its output as an argument to echo.

Single quotes let you use whitespaces, dollar signs and backslashes inside the arguments:

echo '$HOSTNAME'

will output the string $HOSTNAME as it is,

echo $HOSTNAME
echo "$HOSTNAME"

both will output the value of the environment variable HOSTNAME.

Answer 2

In shell, a pair of backticks will be replaced by the (stdout) output of the command they enclose.

X=`expr 3 \* 2 + 4` # expr evaluate arithmatic expressions. man expr for details.
echo “$X”