What does ` (backquote/backtick) mean in bash?

Question

I came across the following command

sudo chown `id -u` /somedir

And I wonder what is the meaning of the ` symbol. I noticed for instance that while the command above works well the one below does not

sudo chown 'id -u' /somedir

Answer 1

This is a backtick. Backtick is not a quotation sign, it has a very special meaning. Everything you type between backticks is evaluated (executed) by the shell before the main command (like chown in your examples), and the output of that execution is used by that command, just as if you'd type that output at that place in the command line.

So, what

sudo chown `id -u` /somedir

effectively runs (depending on your user ID) is:

sudo chown 1000 /somedir
  \    \     \     \
   \    \     \     `-- the second argument to "chown" (target directory)
    \    \     `-- your user ID, which is the output of "id -u" command
     \    `-- "chown" command (change ownership of file/directory)
      `-- the "run as root" command; everything after this is run with root privileges

Have a look at this question to learn why, in many situations, it is not a good idea to use backticks.

Answer 2

This symbol mean that whatever inside it, is interpreted as the result of that command.

for example:

$ls /home
one two
$pwd
/usr/three
$cp `pwd` /home
$ls /home
one two three

The above resutl will be that the three directory will be copied to my /home directory.

Answer 3

The backtick ` runs the contents of the enclosed string, so something like this

file `which hostname`

will find out the path to the hostname command, and then tell you how it was built.

The command that you put in your question runs id -u to get the effective user id, and then changes the ownership of /somedir to that user.