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Comment: corrected the single quote example for backticks
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Comment: mention "command substitution" in first paragraph to improve searchability on BashFAQ
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For several reasons: {{{`...`}}} is the legacy syntax for [[CommandSubstitution|command substitution]], required by only the very oldest of non-POSIX-compatible bourne-shells. There are several reasons to always prefer the {{{$(...)}}} syntax:
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 * It's easier to read. The character {{{`}}} is difficult to read with small or unusual fonts. === Important differences ===
 * Backslashes (\) inside backticks are handled in a non-obvious manner:
 {{{#!highlight bash
$ echo "`echo \\a`" "$(echo \\a)"
a \a
$ echo "`echo \\\\a`" "$(echo \\\\a)"
\a \\a
# Note that this is true for *single quotes* too!
$ foo=`echo '\\'`; bar=$(echo '\\'); echo "foo is $foo, bar is $bar"
foo is \, bar is \\
}}}
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 * It's easier to type. The physical key to produce the character may be located in an obscure place on non-US keyboards.

 * The backtick is easily confused with a single quote. People who see {{{$()}}} don't normally press the wrong keys. On the other hand, some people who see {{{`cmd`}}} may mangle it into {{{'cmd'}}} because they don't know what a backtick is.		  * Nested quoting inside {{{$()}}} is far more convenient.
 {{{#!highlight bash
echo "x is $(sed ... <<<"$y")"
}}}
 In this example, the quotes around {{{$y}}} are treated as a pair, because they are inside {{{$()}}}. This is confusing at first glance, because most C programmers would expect the quote before {{{x}}} and the quote before {{{$y}}} to be treated as a pair; but that isn't correct in shells. On the other hand,
 {{{#!highlight bash
echo "x is `sed ... <<<\"$y\"`"
}}}
 requires backslashes around the internal quotes in order to be portable. Bourne and Korn shells require these backslashes, while Bash and dash don't.
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  {{{
  x=$(grep $(dirname "$path") file)
  x=`grep \`dirname "$path"\` file`}}}
 It just gets uglier and uglier after two levels.		  {{{#!highlight bash
x=$(grep -F "$(dirname "$path")" file)
x=`grep -F "\`dirname \"$path\"\`" file`
}}}
 It just gets uglier and uglier after two levels. {{{$()}}} forces an entirely new context for quoting, so that everything within the command substitution is protected and can be treated as though it were on its own, with no special concern over quoting and escaping.
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 * Backslashes (\) inside backticks are handled in a non-obvious manner:
  {{{
  $ echo "`echo \\a`" "$(echo \\a)"
  a \a
  $ echo "`echo \\\\a`" "$(echo \\\\a)"
  \a \\a
  # Note that this is true for *single quotes* too!
  $ foo=`echo '\\'`; bar=$(echo '\\'); echo "foo is $foo, bar is $bar"
  foo is \, bar is \\
  }}}		 === Other advantages ===
 * The function of {{{$(...)}}} as being an expansion is visually clear. The syntax of a {{{$}}}-prefixed token is consistent with all other expansions that are parsed from within double-quotes, at the same time, from left-to-right. Backticks are the only exception. This improves human and machine readability, and consistent syntax makes the language more intuitive for readers.
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 * Nested quoting inside {{{$()}}} is far more convenient.
  {{{
  echo "x is $(echo "$y" | sed ...)"}}}
 In this example, the quotes around {{{$y}}} are treated as a pair, because they are inside {{{$()}}}. This is confusing at first glance, because most C programmers would expect the quote before {{{x}}} and the quote before {{{$y}}} to be treated as a pair; but that isn't correct in shells. On the other hand,
  {{{
  echo "x is `echo \"$y\" | sed ...`"}}}
 requires backslashes around the internal quotes in order to be portable. Bourne and Korn shells require these backslashes, while Bash and dash don't.		  * Per the above, people are (hopefully) accustomed to seeing double-quoted expansions and substitutions with the usual {{{"$..."}}} syntax. Quoting command substitutions is almost always the correct thing to do, yet the great majority of {{{`...`}}} specimens we find in the wild are left unquoted, perhaps because those who still use the legacy syntax are less experienced, or they don't associate it with the other expansions due to the different syntax. In addition, the {{{`}}} character is easily camouflaged when adjacent to {{{"}}} making it even more difficult to read, especially with small or unusual fonts.
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The only time backticks are preferred is when writing code for the oldest Bourne shells, which do not know about {{{$()}}}.  * The backtick is also easily confused with a single quote.

=== See also: ===
 * [[http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_03|POSIX standard section "2.6.3 Command Substitution"]]
 * [[http://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xcu_chap02.html#tag_23_02_06_03|POSIX rationale for including the $() syntax]]
 * CommandSubstitution
 * [[http://wiki.bash-hackers.org/syntax/expansion/cmdsubst|bash-hackers: command substitution]]

Why is $(...) preferred over `...` (backticks)?

`...` is the legacy syntax for command substitution, required by only the very oldest of non-POSIX-compatible bourne-shells. There are several reasons to always prefer the $(...) syntax:

Important differences

  • Backslashes (\) inside backticks are handled in a non-obvious manner: 
       1 $ echo "`echo \\a`" "$(echo \\a)"
   2 a \a
   3 $ echo "`echo \\\\a`" "$(echo \\\\a)"
   4 \a \\a
   5 # Note that this is true for *single quotes* too!
   6 $ foo=`echo '\\'`; bar=$(echo '\\'); echo "foo is $foo, bar is $bar"
   7 foo is \, bar is \\

  • Nested quoting inside $() is far more convenient. 

       1 echo "x is $(sed ... <<<"$y")"

    In this example, the quotes around $y are treated as a pair, because they are inside $(). This is confusing at first glance, because most C programmers would expect the quote before x and the quote before $y to be treated as a pair; but that isn't correct in shells. On the other hand, 

       1 echo "x is `sed ... <<<\"$y\"`"
    requires backslashes around the internal quotes in order to be portable. Bourne and Korn shells require these backslashes, while Bash and dash don't.
  • It makes nesting command substitutions easier. Compare: 
       1 x=$(grep -F "$(dirname "$path")" file)
   2 x=`grep -F "\`dirname \"$path\"\`" file`

    It just gets uglier and uglier after two levels. $() forces an entirely new context for quoting, so that everything within the command substitution is protected and can be treated as though it were on its own, with no special concern over quoting and escaping.

Other advantages

  • The function of $(...) as being an expansion is visually clear. The syntax of a $-prefixed token is consistent with all other expansions that are parsed from within double-quotes, at the same time, from left-to-right. Backticks are the only exception. This improves human and machine readability, and consistent syntax makes the language more intuitive for readers.

  • Per the above, people are (hopefully) accustomed to seeing double-quoted expansions and substitutions with the usual "$..." syntax. Quoting command substitutions is almost always the correct thing to do, yet the great majority of `...` specimens we find in the wild are left unquoted, perhaps because those who still use the legacy syntax are less experienced, or they don't associate it with the other expansions due to the different syntax. In addition, the ` character is easily camouflaged when adjacent to " making it even more difficult to read, especially with small or unusual fonts.

  • The backtick is also easily confused with a single quote.

See also:

BashFAQ/082 (last edited 2022-02-19 00:13:59 by larryv)