Diff for "BashFAQ/082"

Differences between revisions 7 and 19 (spanning 12 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 7 as of 2007-10-09 17:39:07 → 
  Size: 2733
  Editor: JariAalto
  Comment: Change "newbie" to "non-experienced people". Move "$(" an single quote explanation upward in bullets
+   ← Revision 19 as of 2010-11-04 08:39:36 → ⇥
  Size: 2077
  Editor: finproxy
  Comment: Fix typo in word POSIX
-Deletions are marked like this.
+Additions are marked like this.
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-[[Anchor(faq82)]]
+<<Anchor(faq82)>>
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+Line 5:
- * It's easier to read. The character ` is difficult to read with small fonts or on large display. The key may to access the character may be located in a obscure place in non-US keyboard.
+ * It's easier to read.  The character {{{`}}} is difficult to read with small or unusual fonts.
-Line 8:
+Line 7:
- * The key is easily confused with single quote. People who see {{{$()}}} don't normally press the wrong keys.  On the other hand, non-experienced people who see {{{`cmd`}}} maymangle it into {{{'cmd'}}} because they don't know what a backtick is.
+ * 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).
-Line 10:
+Line 9:
+ * The backtick is easily confused with a single quote. People who see {{{$()}}} don't normally press the wrong keys.  On the other hand, some people who see {{{`cmd`}}} may mangle it into {{{'cmd'}}} because they don't know what a backtick is.
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-  echo "`echo \\a`"      # prints a
  echo "`echo \\\a`"     # prints \a
  echo "`echo \\\\a`"    # prints \a}}}
 Inside {{{$()}}}, there are no such surprises.
+  $ 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 31:
+Line 35:
-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  
 ''Just for the record, \" inside {{{`backticks`}}} is only required in Korn shell and Bourne shell.  Bash and dash both treat''
 {{{
 echo "`echo "foo       bar"`"}}}
 ''exactly the same way they treat''
 {{{
 echo "$(echo "foo       bar")"}}}
 ''although this is an excellent point that I'm going to add to the bullet list up above.'' -- GreyCat

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