Diff for "BashFAQ/082"

Differences between revisions 4 and 18 (spanning 14 versions)

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

For several reasons:

It's easier to read. The character ` is difficult to read with small or unusual fonts.
It's easier to type. The physical key to produce the character may be located in an obscure place on keyboards, or may not be present at all (like in the standard italian keyboard).
The backtick is easily confused with a single quote. People who see $() don't normally press the wrong keys. On the other hand, some people who see `cmd` may mangle it into 'cmd' because they don't know what a backtick is.
It makes nesting command substitutions easier. Compare:
- ```
  x=$(grep $(dirname "$path") file)
  x=`grep \`dirname "$path"\` file`
```
It just gets uglier and uglier after two levels.

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 \\

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.

The only time backticks are preferred is when writing code for the oldest Bourne shells, which are not POSIx compliant. See POSIX standard and section "2.6.3 Command Substitution" for $().

-  ⇤ ← Revision 4 as of 2007-09-14 11:52:14 → 
  Size: 1499
  Editor: leon
  Comment: are they really surprising? maybe we can generalize to all quotes instead?
+   ← Revision 18 as of 2010-11-04 08:39:04 → ⇥
  Size: 2077
  Editor: finproxy
  Comment: Correct markup for URL
-Deletions are marked like this.
+Additions are marked like this.
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-[[Anchor(faq82)]]
+<<Anchor(faq82)>>
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+ * It's easier to read.  The character {{{`}}} is difficult to read with small or unusual fonts.

 * It's easier to type.  The physical key to produce the character may be located in an obscure place on keyboards, or may not be present at all (like in the standard italian keyboard).

 * The backtick is easily confused with a single quote. People who see {{{$()}}} don't normally press the wrong keys.  On the other hand, some people who see {{{`cmd`}}} may mangle it into {{{'cmd'}}} because they don't know what a backtick is.
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- * It's easier to read.
 * Newbies who see {{{$()}}} don't normally press the wrong keys.  On the other hand, newbies who see {{{`cmd`}}} often mangle it into {{{'cmd'}}} because they don't know what a backtick is.
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+  {{{
  $ 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 $bar" 
  foo is \, bar is \\
  }}}
-Line 18:
+Line 27:
+ * Nested quoting inside {{{$()}}} is far more convenient.
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+Line 29:
-  echo "`echo \\a`"      # prints a
  echo "`echo \\\a`"     # prints \a
  echo "`echo \\\\a`"    # prints \a}}}
+  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.
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-Inside {{{$()}}}, there are no such surprises.

Backslashes are no more no less surprising than elsewhere IMHO
{{{
  echo `echo \a` # prints a
  echo `echo \\a` # prints a
  echo `echo \\\a` # prints \a
  echo `echo \\\\a` # prints \a
  echo $(echo \a)  #prints a
  echo $(echo \\a) prints \a
  echo $(echo \\\a) prints \a
  echo $(echo \\\\a) prints \\a
}}}
the same sort of things happens without any quotes or within "".

I suspect the real advantage of $( ) here is that you don't need to take extra care of the quotes (\ ""), you just put them as usual, ie
echo "`echo \"foo    bar\"`" vs echo "$( echo "foo     bar")"  -- pgas  

The only time backticks are preferred is when writing code for the oldest Bourne shells, which do not know about {{{$()}}}.
+The only time backticks are preferred is when writing code for the oldest Bourne shells, which are not POSIx compliant. See [[http://www.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#tag_02_06_03|POSIX standard and section "2.6.3 Command Substitution"]] for {{{$()}}}.