Why is this string matching condition in [[ ]] not true?












0














$ tpgid=$(ps  --no-headers -o tpgid -p 1)

$ echo $tpgid

-1

$ if [[ $tpgid == "-1" ]]; then

>     echo "yes"

> else

>     echo "no"

> fi

no


Why is the condition not true? Thanks.



$ printf "%s" "$tpgid" > /tmp/test/fff

$ hd /tmp/test/fff

00000000  20 20 20 2d 31                                    |   -1|

00000005





bash







share|improve this question




















  • 4




    always quote your variables ... then you'd see the spaces in front of the -1, as DopeGhoti pointed out
    – Jeff Schaller
    11 hours ago


















0














$ tpgid=$(ps  --no-headers -o tpgid -p 1)

$ echo $tpgid

-1

$ if [[ $tpgid == "-1" ]]; then

>     echo "yes"

> else

>     echo "no"

> fi

no


Why is the condition not true? Thanks.



$ printf "%s" "$tpgid" > /tmp/test/fff

$ hd /tmp/test/fff

00000000  20 20 20 2d 31                                    |   -1|

00000005





bash







share|improve this question




















  • 4




    always quote your variables ... then you'd see the spaces in front of the -1, as DopeGhoti pointed out
    – Jeff Schaller
    11 hours ago
















0












0








0







$ tpgid=$(ps  --no-headers -o tpgid -p 1)

$ echo $tpgid

-1

$ if [[ $tpgid == "-1" ]]; then

>     echo "yes"

> else

>     echo "no"

> fi

no


Why is the condition not true? Thanks.



$ printf "%s" "$tpgid" > /tmp/test/fff

$ hd /tmp/test/fff

00000000  20 20 20 2d 31                                    |   -1|

00000005





bash







share|improve this question















$ tpgid=$(ps  --no-headers -o tpgid -p 1)

$ echo $tpgid

-1

$ if [[ $tpgid == "-1" ]]; then

>     echo "yes"

> else

>     echo "no"

> fi

no


Why is the condition not true? Thanks.



$ printf "%s" "$tpgid" > /tmp/test/fff

$ hd /tmp/test/fff

00000000  20 20 20 2d 31                                    |   -1|

00000005





bash




bash






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited 12 hours ago

























asked 12 hours ago









Tim

26k74246455




26k74246455








  • 4




    always quote your variables ... then you'd see the spaces in front of the -1, as DopeGhoti pointed out
    – Jeff Schaller
    11 hours ago
















  • 4




    always quote your variables ... then you'd see the spaces in front of the -1, as DopeGhoti pointed out
    – Jeff Schaller
    11 hours ago










4




4




always quote your variables ... then you'd see the spaces in front of the -1, as DopeGhoti pointed out
– Jeff Schaller
11 hours ago






always quote your variables ... then you'd see the spaces in front of the -1, as DopeGhoti pointed out
– Jeff Schaller
11 hours ago












2 Answers
2






active

oldest

votes


















7














Even though [[ ... ]] is "smarter" than [ ... ] or test ..., it's still a better idea to explicitly use numerical comparison operators:



if [[ "$tpgid" -eq -1 ]]; then ...


Further, your hexdump:



$ hd /tmp/test/fff

00000000  20 20 20 2d 31                                    |   -1|


shows that $tpgid expands to " -1", not "-1"; -eq knows how to handle this, while == is rightly doing a string comparison:



$ if [[ "   -1" == -1 ]]; then echo truthy; else echo falsy; fi

falsy

$ if [[ "   -1" -eq -1 ]]; then echo truthy; else echo falsy; fi

truthy


In short, the string matching condition is not returning true because the strings in fact do not match.






share|improve this answer































    3














    Most likely is that $tpgid contains leading and/or trailing whitespace. Since the value is numeric, you might want to use an arithmetic expression:



    if (( tpgid == -1 )); then ...





    share|improve this answer





















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      2 Answers
      2






      active

      oldest

      votes








      2 Answers
      2






      active

      oldest

      votes









      active

      oldest

      votes






      active

      oldest

      votes









      7














      Even though [[ ... ]] is "smarter" than [ ... ] or test ..., it's still a better idea to explicitly use numerical comparison operators:



      if [[ "$tpgid" -eq -1 ]]; then ...


      Further, your hexdump:



      $ hd /tmp/test/fff
      
00000000  20 20 20 2d 31                                    |   -1|


      shows that $tpgid expands to " -1", not "-1"; -eq knows how to handle this, while == is rightly doing a string comparison:



      $ if [[ "   -1" == -1 ]]; then echo truthy; else echo falsy; fi
      
falsy
      
$ if [[ "   -1" -eq -1 ]]; then echo truthy; else echo falsy; fi
      
truthy


      In short, the string matching condition is not returning true because the strings in fact do not match.






      share|improve this answer




























        7














        Even though [[ ... ]] is "smarter" than [ ... ] or test ..., it's still a better idea to explicitly use numerical comparison operators:



        if [[ "$tpgid" -eq -1 ]]; then ...


        Further, your hexdump:



        $ hd /tmp/test/fff
        
00000000  20 20 20 2d 31                                    |   -1|


        shows that $tpgid expands to " -1", not "-1"; -eq knows how to handle this, while == is rightly doing a string comparison:



        $ if [[ "   -1" == -1 ]]; then echo truthy; else echo falsy; fi
        
falsy
        
$ if [[ "   -1" -eq -1 ]]; then echo truthy; else echo falsy; fi
        
truthy


        In short, the string matching condition is not returning true because the strings in fact do not match.






        share|improve this answer


























          7












          7








          7






          Even though [[ ... ]] is "smarter" than [ ... ] or test ..., it's still a better idea to explicitly use numerical comparison operators:



          if [[ "$tpgid" -eq -1 ]]; then ...


          Further, your hexdump:



          $ hd /tmp/test/fff
          
00000000  20 20 20 2d 31                                    |   -1|


          shows that $tpgid expands to " -1", not "-1"; -eq knows how to handle this, while == is rightly doing a string comparison:



          $ if [[ "   -1" == -1 ]]; then echo truthy; else echo falsy; fi
          
falsy
          
$ if [[ "   -1" -eq -1 ]]; then echo truthy; else echo falsy; fi
          
truthy


          In short, the string matching condition is not returning true because the strings in fact do not match.






          share|improve this answer














          Even though [[ ... ]] is "smarter" than [ ... ] or test ..., it's still a better idea to explicitly use numerical comparison operators:



          if [[ "$tpgid" -eq -1 ]]; then ...


          Further, your hexdump:



          $ hd /tmp/test/fff
          
00000000  20 20 20 2d 31                                    |   -1|


          shows that $tpgid expands to " -1", not "-1"; -eq knows how to handle this, while == is rightly doing a string comparison:



          $ if [[ "   -1" == -1 ]]; then echo truthy; else echo falsy; fi
          
falsy
          
$ if [[ "   -1" -eq -1 ]]; then echo truthy; else echo falsy; fi
          
truthy


          In short, the string matching condition is not returning true because the strings in fact do not match.







          share|improve this answer














          share|improve this answer



          share|improve this answer








          edited 11 hours ago

























          answered 12 hours ago









          DopeGhoti

          43.2k55382




          43.2k55382

























              3














              Most likely is that $tpgid contains leading and/or trailing whitespace. Since the value is numeric, you might want to use an arithmetic expression:



              if (( tpgid == -1 )); then ...





              share|improve this answer


























                3














                Most likely is that $tpgid contains leading and/or trailing whitespace. Since the value is numeric, you might want to use an arithmetic expression:



                if (( tpgid == -1 )); then ...





                share|improve this answer
























                  3












                  3








                  3






                  Most likely is that $tpgid contains leading and/or trailing whitespace. Since the value is numeric, you might want to use an arithmetic expression:



                  if (( tpgid == -1 )); then ...





                  share|improve this answer












                  Most likely is that $tpgid contains leading and/or trailing whitespace. Since the value is numeric, you might want to use an arithmetic expression:



                  if (( tpgid == -1 )); then ...






                  share|improve this answer












                  share|improve this answer



                  share|improve this answer










                  answered 12 hours ago









                  glenn jackman

                  50.3k570107




                  50.3k570107






























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