linux中backticks反引号的作用

This is a backtick. A 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.

Btw, if you ever wanted to use a backtick literally, e.g. in a string, you can escape it by placing a backslash (\) before it.

shareimprove this answer

edited May 2 at 21:25

Jacob Kopczynski

31

answered Dec 22 '11 at 14:43

rozcietrzewiacz

27.9k37089

• 27
  This explains backticks pretty well, but using $(your expression)is a better way to do the same thing as $() allows you to nest expressions. for instance: cd $(dirname $(type -P touch)) will cd you into the directory containing the touch command – Khaja Minhajuddin Dec 27 '11 at 1:10
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  @KhajaMinhajuddin You're definitely right about nesting - the above mentioned question covers it in detail. But even though I think it is a good practise to use $() in most situations, it does not make backticks a worse thing. For practical purposes, one has to admit that they are much faster to type on the command line (2 keystrokes compared to at least 5, including Shift). – rozcietrzewiacz Oct 23 '12 at 13:14 
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  @rozcietrzewiacz Your latter remark is probably true for most keyboards but $( ) is definitely easier to type than ` ` at least on a French keyboard. – jlliagre May 19 '13 at 20:38
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  And another +1 for using backticks in the ASCII art ;) – Ignorant Oct 5 '17 at 19:25 
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  I wrote excape instead of escape and know I cannot fix that 1-letter mistake!