Diff for "BashFAQ/082"

Differences between revisions 3 and 8 (spanning 5 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 non-US keyboards.
The backtick is easily confused with a single quote. People who see $() don't normally press the wrong keys. On the other hand, some people who see `cmd` may mangle it into 'cmd' because they don't know what a backtick is.
It makes nesting command substitutions easier. Compare:
- ```
  x=$(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

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.

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 $().

-  ⇤ ← Revision 3 as of 2007-09-13 19:19:33 → 
  Size: 895
  Editor: BetterWorld
  Comment: Add example for non-obvious backslashing behaviour
+   ← 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.
-Deletions are marked like this.
+Additions are marked like this.
 Line 6:
+ * 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 non-US keyboards.

 * The backtick is easily confused with a single quote. People who see {{{$()}}} don't normally press the wrong keys.  On the other hand, some people who see {{{`cmd`}}} may mangle it into {{{'cmd'}}} because they don't know what a backtick is.
-Line 7:
+Line 13:
-Line 11:
+Line 16:
-Line 14:
+Line 18:
- * It's easier to read.
 * Newbies who see {{{$()}}} 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.
-Line 17:
+Line 19:
+  {{{
  $ echo "`echo \\a`" "$(echo \\a)"
  a \a
  $ echo "`echo \\\\a`" "$(echo \\\\a)"
  \a \\a}}}
-Line 18:
+Line 25:
+ * Nested quoting inside {{{$()}}} is far more convenient.
-Line 19:
+Line 27:
-  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.
-Line 23:
+Line 33:
-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  
 ''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