What's the difference between $@ and $* in UNIX?

Question

Hi,

So I was just wondering what's the difference between $@ and $* in UNIX, as when echoed in a script they both seem to produce the same output.

Thanks.

Answer 1

Please see the bash man page under Special Parameters.

   Special Parameters
       The shell treats several parameters specially.  These parameters may only be referenced; assignment to them is not allowed.
       *      Expands to the positional parameters, starting from one.  When the expansion occurs within double quotes, it expands to a  sin-
              gle  word  with  the  value  of  each parameter separated by the first character of the IFS special variable.  That is, "$*" is
              equivalent to "$1c$2c...", where c is the first character of the value of the IFS variable.  If IFS is  unset,  the  parameters
              are separated by spaces.  If IFS is null, the parameters are joined without intervening separators.
       @      Expands  to  the  positional  parameters,  starting  from  one.  When the expansion occurs within double quotes, each parameter
              expands to a separate word.  That is, "$@" is equivalent to "$1" "$2" ...  If the double-quoted expansion occurs within a word,
              the  expansion  of  the  first  parameter is joined with the beginning part of the original word, and the expansion of the last
              parameter is joined with the last part of the original word.  When there are no positional parameters, "$@" and  $@  expand  to
              nothing (i.e., they are removed).

Answer 2

One difference is in how they handle the IFS variable on output.

#!/bin/sh
echo "unquoted asterisk " $*
echo "quoted asterisk $*"
echo "unquoted at " $@
echo "quoted at $@"
IFS="X"
echo "IFS is now $IFS"
echo "unquoted asterisk " $*
echo "quoted asterisk $*"
echo "unquoted at " $@
echo "quoted at $@"

If you run this like this: ./demo abc def ghi, you get this output:

unquoted asterisk abc def ghi
quoted asterisk abc def ghi
unquoted at abc def ghi
quoted at abc def ghi
IFS is now X
unquoted asterisk abc def ghi
quoted asterisk abcXdefXghi
unquoted at abc def ghi
quoted at abc def ghi

Notice that (only) the "quoted asterisk" line shows an X between each "word" after IFS is changed to "X". If the value of IFS contains multiple characters, only the first character is used for this purpose.

This feature can also be used for other arrays:

$ array=(123 456 789)
$ saveIFS=$IFS; IFS="|"
$ echo "${array[*]}"
123|456|789
$ IFS=$saveIFS