bash if false returns true instead of false -- why

Question

Why does the following output True     bin sh if   false    then     echo  quot True quot  else     echo  quot False quot  fi  This will always output True even though the condition would seem to indicate otherwise  If I remove the brackets    then it works  but I do not understand why

User · Answer

as noted by   tripleee  this is tangential  at best  still  in case you arrived here searching for something like that  as i did   here is my solution having to deal with user acessible configuration files  i use this function   function isTrue         if     quot        quot       TRUE OUI Y O  ON   1-9       then return 0 fi     return 1        wich can be used like that if isTrue  quot  whatever quot   then    You can alter the  quot truth list quot  in the regexp  the one in this sample is french compatible and considers strings like  quot Yeah  yup  on 1  Oui y true to be  quot True quot   note that the      provides case insensivity

User · Answer

A Quick Boolean Primer for Bash The if statement takes a command as an argument  as do  amp  amp       etc    The integer result code of the command is interpreted as a boolean  0 null true  1 else false   The test statement takes operators and operands as arguments and returns a result code in the same format as if  An alias of the test statement is    which is often used with if to perform more complex comparisons  The true and false statements do nothing and return a result code  0 and 1  respectively   So they can be used as boolean literals in Bash  But if you put the statements in a place where they re interpreted as strings  you ll run into issues  In your case  if   foo    then        quot if the string  foo  is non-empty  return true quot  if foo  then            quot if the command foo succeeds  return true quot   So  if   true      then echo  quot This text will always appear  quot    fi  if   false     then echo  quot This text will always appear  quot    fi  if true        then echo  quot This text will always appear  quot    fi  if false       then echo  quot This text will never appear  quot     fi   This is similar to doing something like echo   foo  vs  echo  quot  foo quot   When using the test statement  the result depends on the operators used  if    quot  foo quot     quot  bar quot        true if the string values of  foo and  bar are equal if    quot  foo quot  -eq  quot  bar quot      true if the integer values of  foo and  bar are equal if   -f  quot  foo quot              true if  foo is a file that exists  by path  if    quot  foo quot                 true if  foo evaluates to a non-empty string if foo                     true if foo  as a command subroutine                             evaluates to true success  returns 0 or null   In short  if you just want to test something as pass fail  aka  quot true quot   quot false quot    then pass a command to your if or  amp  amp  etc  statement  without brackets  For complex comparisons  use brackets with the proper operators  And yes  I m aware there s no such thing as a native boolean type in Bash  and that if and   and true are technically  quot commands quot  and not  quot statements quot   this is just a very basic  functional explanation

User · Answer

Adding context to hopefully help provide a bit of additional clarity on this subject  To a BaSH newbie  it s sense of true false statements is rather odd  Take the following simple examples and their results  This statement will return  quot true quot   foo  quot   quot   if    quot  foo quot     then echo  quot true quot   else echo  quot false quot   fi  But this will return  quot false quot   foo  quot   quot   if    foo    then echo  quot true quot   else echo  quot false quot   fi  Do you see why  The first example has a quoted  quot  quot  string  This causes BaSH to treat it literally  So  in a literal sense  a space is not null  While in a non-literal sense  the 2nd example above   a space is viewed by BaSH  as a value in  foo  as  nothing  and therefore it equates to null  interpreted here as  false    These statements will all return a text string of  quot false quot   foo   if    foo    then echo  quot true quot   else echo  quot false quot   fi foo   if    quot  foo quot     then echo  quot true quot   else echo  quot false quot   fi foo  quot  quot   if    foo    then echo  quot true quot   else echo  quot false quot   fi foo  quot  quot   if    quot  foo quot     then echo  quot true quot   else echo  quot false quot   fi  Interestingly  this type of conditional will always return true  These statements will all return a result of  quot true quot   foo  quot  quot   if   foo    then echo  quot true quot   else echo  quot false quot   fi  Notice the difference  the   symbol has been omitted from preceding the variable name in the conditional  It doesn t matter what word you insert between the brackets  BaSH will always see this statement as true  even if you use a word that has never been associated with a variable in the same shell before  if   sooperduper    then echo  quot true quot   else echo  quot false quot   fi  Likewise  defining it as an undeclared variable ensures it will always return false  if    sooperduper    then echo  quot true quot   else echo  quot false quot   fi  As to BaSH it s the same as writing  sooperduper  quot  quot  if    sooperduper    then echo  quot true quot   else echo  quot false quot   fi   One more tip     Brackets vs No Brackets Making matters more confusing  these variations on the IF THEN conditional both work  but return opposite results  These return false  if    foo    then echo  quot true quot   else echo  quot false quot   fi if     foo    then echo  quot true quot   else echo  quot false quot   fi  However  these will return a result of true  if  foo  then echo  quot true quot   else echo  quot false quot   fi if   foo    then echo  quot true quot   else echo  quot false quot   fi if      foo    then echo  quot true quot   else echo  quot false quot   fi  And  of course this returns a syntax error  along with a result of  false    if foo  then echo  quot true quot   else echo  quot false quot   fi  Confused yet  It can be quite challenging to keep it straight in your head in the beginning  especially if you re used to other  higher level programming languages

User · Answer

You are running the    aka test  command with the argument  false   not running the command false  Since  false  is a non-empty string  the test command always succeeds  To actually run the command  drop the   command   if false  then    echo  True  else    echo  False  fi

User · Answer

Using true false removes some bracket clutter         bin bash        true or false bash       basename  0       bash     amp  amp  sourced true    sourced false   sourced  amp  amp  echo  SOURCED   sourced    echo  CALLED     Just an alternate way     sourced   amp  amp   echo  CALLED       echo  SOURCED    sourced  amp  amp  return    exit

User · Answer

I found that I can do some basic logic by running something like   A true B true if   A  amp  amp   B   then     C true else     C false fi echo  C

[bash] bash "if [ false ];" returns true instead of false -- why?

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