Why does ${$#} return same result as $$ in dash?












1















While trying to get last positional parameter set in /bin/dash , I've tried echo ${$#}. Surprisingly this did not result in an error, but into PID which is the same as $$ variable contents. Question, is why did that syntax work ? What is the syntax rule that shell applied here ?



Basically, what I did is



$ set 1 2 3 4 5

$ echo ${$#}

13819

$ echo $$

13819


Apparently, % character also get ignored in such construct



$ echo ${$%}

13819


But * and @ result in bad substitution error:



$ echo ${$*}

sh: 10: Bad substitution

$ echo ${$@}

sh: 11: Bad substitution





shell







share|improve this question























  • What are you expecting ${$*} and ${$@} to produce?

    – Kusalananda
    1 min ago
















1















While trying to get last positional parameter set in /bin/dash , I've tried echo ${$#}. Surprisingly this did not result in an error, but into PID which is the same as $$ variable contents. Question, is why did that syntax work ? What is the syntax rule that shell applied here ?



Basically, what I did is



$ set 1 2 3 4 5

$ echo ${$#}

13819

$ echo $$

13819


Apparently, % character also get ignored in such construct



$ echo ${$%}

13819


But * and @ result in bad substitution error:



$ echo ${$*}

sh: 10: Bad substitution

$ echo ${$@}

sh: 11: Bad substitution





shell







share|improve this question























  • What are you expecting ${$*} and ${$@} to produce?

    – Kusalananda
    1 min ago














1












1








1


1






While trying to get last positional parameter set in /bin/dash , I've tried echo ${$#}. Surprisingly this did not result in an error, but into PID which is the same as $$ variable contents. Question, is why did that syntax work ? What is the syntax rule that shell applied here ?



Basically, what I did is



$ set 1 2 3 4 5

$ echo ${$#}

13819

$ echo $$

13819


Apparently, % character also get ignored in such construct



$ echo ${$%}

13819


But * and @ result in bad substitution error:



$ echo ${$*}

sh: 10: Bad substitution

$ echo ${$@}

sh: 11: Bad substitution





shell







share|improve this question














While trying to get last positional parameter set in /bin/dash , I've tried echo ${$#}. Surprisingly this did not result in an error, but into PID which is the same as $$ variable contents. Question, is why did that syntax work ? What is the syntax rule that shell applied here ?



Basically, what I did is



$ set 1 2 3 4 5

$ echo ${$#}

13819

$ echo $$

13819


Apparently, % character also get ignored in such construct



$ echo ${$%}

13819


But * and @ result in bad substitution error:



$ echo ${$*}

sh: 10: Bad substitution

$ echo ${$@}

sh: 11: Bad substitution





shell




shell






share|improve this question













share|improve this question











share|improve this question




share|improve this question










asked 14 mins ago









Sergiy KolodyazhnyySergiy Kolodyazhnyy

10.2k32660




10.2k32660













  • What are you expecting ${$*} and ${$@} to produce?

    – Kusalananda
    1 min ago



















  • What are you expecting ${$*} and ${$@} to produce?

    – Kusalananda
    1 min ago

















What are you expecting ${$*} and ${$@} to produce?

– Kusalananda
1 min ago





What are you expecting ${$*} and ${$@} to produce?

– Kusalananda
1 min ago










1 Answer
1






active

oldest

votes


















2














This is $$ with an empty prefix removed:




${parameter#[word]}



Remove Smallest Prefix Pattern. The word shall be expanded to produce a pattern. The parameter expansion shall then result in parameter, with the smallest portion of the prefix matched by the pattern deleted. If present, word shall not begin with an unquoted #.




The same applies for % (suffix). @ and * are not parameter expansion modifiers, so they are errors. It would happen for $? or a hypothetical $= as well.





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    1 Answer
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    1 Answer
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    active

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    2














    This is $$ with an empty prefix removed:




    ${parameter#[word]}



    Remove Smallest Prefix Pattern. The word shall be expanded to produce a pattern. The parameter expansion shall then result in parameter, with the smallest portion of the prefix matched by the pattern deleted. If present, word shall not begin with an unquoted #.




    The same applies for % (suffix). @ and * are not parameter expansion modifiers, so they are errors. It would happen for $? or a hypothetical $= as well.





    share




























      2














      This is $$ with an empty prefix removed:




      ${parameter#[word]}



      Remove Smallest Prefix Pattern. The word shall be expanded to produce a pattern. The parameter expansion shall then result in parameter, with the smallest portion of the prefix matched by the pattern deleted. If present, word shall not begin with an unquoted #.




      The same applies for % (suffix). @ and * are not parameter expansion modifiers, so they are errors. It would happen for $? or a hypothetical $= as well.





      share


























        2












        2








        2







        This is $$ with an empty prefix removed:




        ${parameter#[word]}



        Remove Smallest Prefix Pattern. The word shall be expanded to produce a pattern. The parameter expansion shall then result in parameter, with the smallest portion of the prefix matched by the pattern deleted. If present, word shall not begin with an unquoted #.




        The same applies for % (suffix). @ and * are not parameter expansion modifiers, so they are errors. It would happen for $? or a hypothetical $= as well.





        share













        This is $$ with an empty prefix removed:




        ${parameter#[word]}



        Remove Smallest Prefix Pattern. The word shall be expanded to produce a pattern. The parameter expansion shall then result in parameter, with the smallest portion of the prefix matched by the pattern deleted. If present, word shall not begin with an unquoted #.




        The same applies for % (suffix). @ and * are not parameter expansion modifiers, so they are errors. It would happen for $? or a hypothetical $= as well.






        share











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        answered 4 mins ago









        Michael HomerMichael Homer

        48.3k8127167




        48.3k8127167






























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