Diff for "BashFAQ/082"

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 $().