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

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 "true":

foo=" "; if [ "$foo" ]; then echo "true"; else echo "false"; fi

But this will return "false":

foo=" "; if [ $foo ]; then echo "true"; else echo "false"; fi

Do you see why? The first example has a quoted "" 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 "false":

foo=; if [ $foo ]; then echo "true"; else echo "false"; fi
foo=; if [ "$foo" ]; then echo "true"; else echo "false"; fi
foo=""; if [ $foo ]; then echo "true"; else echo "false"; fi
foo=""; if [ "$foo" ]; then echo "true"; else echo "false"; fi

Interestingly, this type of conditional will always return true:

These statements will all return a result of "true":

foo=""; if [ foo ]; then echo "true"; else echo "false"; 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 "true"; else echo "false"; fi

Likewise, defining it as an undeclared variable ensures it will always return false:

if [ $sooperduper ]; then echo "true"; else echo "false"; fi

As to BaSH it's the same as writing:

sooperduper="";if [ $sooperduper ]; then echo "true"; else echo "false"; 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 "true"; else echo "false"; fi
if [ ! foo ]; then echo "true"; else echo "false"; fi

However, these will return a result of true:

if $foo; then echo "true"; else echo "false"; fi
if [ foo ]; then echo "true"; else echo "false"; fi
if [ ! $foo ]; then echo "true"; else echo "false"; fi

And, of course this returns a syntax error (along with a result of 'false'):

if foo; then echo "true"; else echo "false"; 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

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

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

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

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

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

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