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There are two syntaxes for command substitution: with dollar-parentheses and with backticks. Running top -p $(pidof init) and top -p `pidof init` gives the same output. Are these two ways of doing the same thing, or are there differences?

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  • 19
    See also: BashFAQ/082. Jan 13 '11 at 16:38
  • 46
    For a second there I thought this was a jQuery question. Jan 13 '11 at 20:42
  • The result might depend on the shell - some support both.
    – artdanil
    Feb 15 '11 at 22:33
382
+50

The old-style backquotes ` ` do treat backslashes and nesting a bit different. The new-style $() interprets everything in between ( ) as a command.

echo $(uname | $(echo cat))
Linux

echo `uname | `echo cat``
bash: command substitution: line 2: syntax error: unexpected end of file
echo cat

works if the nested backquotes are escaped:

echo `uname | \`echo cat\``
Linux

backslash fun:

echo $(echo '\\')
\\

echo `echo '\\'`
\

The new-style $() applies to all POSIX-conformant shells.
As mouviciel pointed out, old-style ` ` might be necessary for older shells.

Apart from the technical point of view, the old-style ` ` has also a visual disadvantage:

  • Hard to notice: I like $(program) better than `program`
  • Easily confused with a single quote: '`'`''`''`'`''`'
  • Not so easy to type (maybe not even on the standard layout of the keyboard)

(and SE uses ` ` for own purpose, it was a pain writing this answer :)

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    The only thing I would add, is that I call '(' a paren, not a bracket (which is '['). Jan 13 '11 at 18:09
  • @Kendall: and here I thought '{' was the left bracket for all those years...
    – SamB
    Jan 13 '11 at 20:32
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    @Sam: { } is usually called "curly brackets" or "braces" en.wikipedia.org/wiki/Braces_(punctuation)#Braces Jan 13 '11 at 21:35
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    I also refer to '{' as curly braces. Though it seems odd you need to add the qualifier "curly" if you call the other things brackets... I guess it's just because they actually curl. Jan 14 '11 at 4:23
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    @slim I don't know on US/UK keyboards, but on spanish keyboards ` is a dead key, so I have to type either a double-backtick (something I usually forget I can even do) or backtick then space, which is a pain.
    – Darkhogg
    Apr 4 '14 at 16:39
42

Obvious difference I observe is that you cannot nest backticks while you can nest $(). Maybe both exist for legacy reasons. Similarly, the . and source commands are synonyms.

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    Some Bourne-derived shells don't recognize source. Dash is one example. Jan 13 '11 at 16:40
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    That's not true. You can nest backtick to any level, just more painfully. Note that both $(...) and `...` are standard (the latter being deprecated) while . is standard but not source Oct 25 '12 at 10:17
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    Correction, only in (t)csh can they not be nested. (t)csh don't support $(...) though. They do support source (and not .) though. Aug 1 '14 at 12:28
  • "you cannot nest backticks" is only accurate if we imply "without any other addition or modification, i.e. without adding any escape characters". But I guess (also based on the downvotes and on the previous, upvoted comments) this is not how most people read it. A clarifying "without adding proper escaping at each nesting level" would significantly improve this answer's clarity.
    – fra-san
    Oct 1 '20 at 11:59
31

$() does not work with old Bourne shell. But it has been years decades since I worked with old Bourne shell.

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  • Old as in 1970's and early 1980's, correct? Oct 1 '19 at 19:15
9

To add to what others said here, you can use the backticks to simulate inline comments:

echo foo `# I'm a comment!` bar

The output is: foo bar.

See the following for more information: https://stackoverflow.com/a/12797512 (Note also the comments below that post.)

6

Another note, $() will use more system resource than using backticks, but is slightly faster.

In Mastering Unix shell scripting, Randal K. Michael had done a test in a chapter named "24 Ways to Process a File Line-by-Line".

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    This claim is nonsense. There is no reason why it should be faster as it is just using a different notation for the parser.
    – schily
    Sep 6 '15 at 13:17
  • @schily: Maybe, I only quote from the book, you can read it for more details.
    – cuonglm
    Sep 6 '15 at 15:19
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    I would tend to agree with @schily...why would it take more resources?
    – Wildcard
    Mar 7 '16 at 23:21
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    @Wildcard, I suppose it's because $() makes your script one byte bigger than if it used ` (assuming you don't nest them and don't use backslashes within). As to which would be faster to parse, that would vary between shells and would be irrelevant as negligible compared to the cost of creating a pipe and forking of process which command substitution entails. Dec 2 '16 at 9:45
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The $() syntax will not work with the old bourne shell.
With newer shells ` ` and $() are equivalent but $() is much more convenient to use when you need to nest multiple commands.

For instance :

echo $(basename $(dirname $(dirname /var/adm/sw/save )))

is easier to type and debug than :

echo `basename \`dirname \\\`dirname /var/adm/sw/save \\\`\``
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    While $() may look nice, it is a pain when implementing a related parser because it requires a dual recursive parser.
    – schily
    Sep 6 '15 at 13:18
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    @schily On the other side, what would be a shell without a good parser.
    – Emmanuel
    Sep 8 '15 at 15:58
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    The problem is that you need to know where the string ends before you call the parser. This is relatively simple with the backticks, but it is hard with brackets as they are used for various purposes in the shell. So you need the parser twice and in a way that does not exist in the Bourne Shell.
    – schily
    Sep 8 '15 at 20:04

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