Diff for "BashFAQ/082"

Differences between revisions 8 and 14 (spanning 6 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.
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 do not know about $().

-  ⇤ ← Revision 8 as of 2007-10-10 13:00:31 → 
  Size: 2698
  Editor: GreyCat
  Comment: Fix up grammar, and REMOVE all political correctness.  Show the difference between `` and $() explicitly in the backslash example.
+   ← Revision 14 as of 2010-06-05 09:52:41 → ⇥
  Size: 1849
  Editor: p54B07555
  Comment:
-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 type.  The physical key to produce the character may be located in an obscure place on non-US keyboards.
+ * It's easier to type.  The physical key to produce the character may be located in an obscure place on keyboards.
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+Line 22:
-  \a \\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 33:
+Line 36:
-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