Returning a boolean from a Bash function

Question

I want to write a bash function that check if a file has certain properties and returns true or false  Then I can use it in my scripts in the  if   But what should I return   function myfun        return 0  else return 1  fi     then I use it like this   if myfun filename txt  then       of course this doesn t work  How can this be accomplished

User · Answer

I found the shortest form to test the function output is simply  do something            -e  1      e g  test file exists    do something  myfile txt       echo  File doesn t exist    exit 1

User · Answer

Use 0 for true and 1 for false   Sample      bin bash  isdirectory       if   -d   1      then       0   true     return 0    else       1   false     return 1   fi     if isdirectory  1  then echo  is directory   else echo  nopes   fi   Edit  From  amichair s comment  these are also possible  isdirectory       if   -d   1      then     true   else     false   fi     isdirectory         -d   1

User · Answer

Following up on  Bruno Bronosky and  mrteatime  I offer the suggestion that you just write your boolean return  backwards    This is what I mean   foo         if     1      bar     then         true  return     else         false  return     fi      That eliminates the ugly two line requirement for every return statement

User · Answer

Be careful when checking directory only with option -d   if variable  1 is empty the check will still be successfull  To be sure  check also that the variable is not empty       bin bash  is directory         if    -d  1     amp  amp     -n  1      then         return 0     else         return 1     fi       Test if is directory  1   then     echo  Directory exist  else     echo  Directory does not exist    fi

User · Answer

It might work if you rewrite this function myfun        return 0  else return 1  fi   as this function myfun        return  else false  fi    That is if false is the last instruction in the function you get false result for whole function but return interrupts function with true result anyway  I believe it s true for my bash interpreter at least

User · Answer

myfun          -d   1      if myfun  path   then     echo yes fi   or myfun  path   amp  amp  echo yes

User · Answer

Use the true or false commands immediately before your return  then return with no parameters   The return will automatically use the value of your last command   Providing arguments to return is inconsistent  type specific and prone to error if you are not using 1 or 0   And as previous comments have stated  using 1 or 0 here is not the right way to approach this function      bin bash  function test for cat       if    1    cat         then         true         return     else         false         return     fi    for i in cat hat  do     echo    i        if test for cat    i        then         echo  - True      else         echo  - False      fi done   Output     bash bash return sh  cat  - True hat  - False

User · Answer

I encountered a point  not explictly yet mentioned   which I was stumbling over   That is  not how to return the boolean  but rather how to correctly evaluate it     I was trying to say if   myfunc    then      but that s simply wrong   You must not use the brackets  if myfunc  then     is the way to do it   As at  Bruno and others reiterated  true and false are commands  not values   That s very important to understanding booleans in shell scripts   In this post  I explained and demoed using boolean variables  https   stackoverflow com a 55174008 3220983   I strongly suggest checking that out  because it s so closely related    Here  I ll provide some examples of returning and evaluating booleans from functions   This   test    false                                                   if test  then echo  it is   fi                                    Produces no echo output   i e  false returns false   test    true                                                    if test  then echo  it is   fi                                    Produces   it is                                                            i e  true returns true        And  test    x 1                                                    if test  then echo  it is   fi                                    Produces   it is                                                                              Because 0  i e  true  was returned implicitly   Now  this is what was screwing me up     test    true                                                    if   test    then echo  it is   fi                                Produces   it is                                                                              AND  test    false                                                    if   test    then echo  it is   fi                                ALSO produces   it is                                                                              Using the brackets here produced a false positive   I infer the  outer  command result is 0    The major take away from my post is  don t use brackets to evaluate a boolean function  or variable  like you would for a typical equality check e g  if   x -eq 1    then

User · Answer

Why you should care what I say in spite of there being a 250  upvote answer  It s not that 0   true and 1   false  It is  zero means no failure  success   and non-zero means failure  of type N    While the selected answer is technically  true  please do not put return 1   in your code for false  It will have several unfortunate side effects    Experienced developers will spot you as an amateur  for the reason below   Experienced developers don t do this  for all the reasons below   It is error prone    Even experienced developers can mistake 0 and 1 as false and true respectively  for the reason above    It requires  or will encourage  extraneous and ridiculous comments  It s actually less helpful than implicit return statuses    Learn some bash  The bash manual says  emphasis mine      return  n       Cause a shell function to stop executing and return the value n to its caller  If n is not supplied  the return value is the exit status of the last command executed in the function    Therefore  we don t have to EVER use 0 and 1 to indicate True and False  The fact that they do so is essentially trivial knowledge useful only for debugging code  interview questions  and blowing the minds of newbies   The bash manual also says     otherwise the function   s return status is the exit status of the last command executed   The bash manual also says          Expands to the exit status of the most recently executed foreground pipeline    Whoa  wait  Pipeline  Let s turn to the bash manual one more time      A pipeline is a sequence of one or more commands separated by one of the control operators         or      amp        Yes  They said 1 command is a pipeline  Therefore  all 3 of those quotes are saying the same thing       tells you what happened last  It bubbles up    My answer  So  while  Kambus demonstrated that with such a simple function  no return is needed at all  I think   was unrealistically simple compared to the needs of most people who will read this   Why return   If a function is going to return its last command s exit status  why use return at all  Because it causes a function to stop executing   Stop execution under multiple conditions  01  function i should    02      uname    uname -a   03 04           uname     Darwin     amp  amp  return 05 06      if      uname     Ubuntu     then 07          release    lsb release -a   08               release     LTS    09          return 10      fi 11 12      false 13    14 15  function do it    16      echo  Hello  old friend   17    18 19  if i should  then 20    do it 21  fi   What we have here is     Line 04 is an explicit -ish  return true because the RHS of  amp  amp  only gets executed if the LHS was true  Line 09 returns either true or false matching the status of line 08  Line 13 returns false because of line 12   Yes  this can be golfed down  but the entire example is contrived    Another common pattern    Instead of doing this    some command if       -eq 1     then     echo  some command failed  fi    Do this    some command status    if     exit  status   then     echo  some command failed  fi   Notice how setting a status variable demystifies the meaning of      Of course you know what    means  but someone less knowledgeable than you will have to Google it some day  Unless your code is doing high frequency trading  show some love  set the variable   But the real take-away is that  if not exist status  or conversely  if exit status  can be read out loud and explain their meaning  However  that last one may be a bit too ambitious because seeing the word exit might make you think it is exiting the script  when in reality it is exiting the        subshell        If you absolutely insist on using return 1 for false  I suggest you at least use return 255 instead  This will cause your future self  or any other developer who must maintain your code to question  why is that 255   Then they will at least be paying attention and have a better chance of avoiding a mistake

User · Answer

For code readability reasons I believe returning true false should    be on one line be one command be easy to remember mention the keyword return followed by another keyword  true or false    My solution is return   true  or return   false  as shown   is directory         if   -d    1      then         return   true      else         return   false      fi

[bash] Returning a boolean from a Bash function

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