Return value in a Bash function

Question

I am working with a bash script and I want to execute a function to print a return value   function fun1      return 34   function fun2      local res   fun1    echo  res     When I execute fun2  it does not print  34   Why is this the case

User · Answer

Git Bash on Windows using arrays for multiple return values  BASH CODE      bin bash    A 6-element array used for returning   values from functions  declare -a RET ARR RET ARR 0   A  RET ARR 1   B  RET ARR 2   C  RET ARR 3   D  RET ARR 4   E  RET ARR 5   F    function FN MULTIPLE RETURN VALUES          give the positional arguments inputs       1 and  2 some sensible names     local out dex 1   1    output index    local out dex 2   2    output index       Echo for debugging     echo  running  FN MULTIPLE RETURN VALUES        Here  Calculate output values     local op var 1  Hello     local op var 2  World        set the return values     RET ARR   out dex 1    op var 1    RET ARR   out dex 2    op var 2     echo  FN MULTIPLE RETURN VALUES EXAMPLES   echo  -------------------------------------------  fn  FN MULTIPLE RETURN VALUES  out dex a 0 out dex b 1 eval  fn  out dex a  out dex b     lt --Call function a   RET ARR 0    amp  amp  echo  RET ARR 0    a   b   RET ARR 1    amp  amp  echo  RET ARR 1    b   echo   ----------------------------------------------   c  2  d  3  FN MULTIPLE RETURN VALUES  c  d    lt --Call function c res   RET ARR 2    amp  amp  echo  RET ARR 2    c res   d res   RET ARR 3    amp  amp  echo  RET ARR 3    d res   echo   ----------------------------------------------   FN MULTIPLE RETURN VALUES 4 5     lt ---Call function e   RET ARR 4    amp  amp  echo  RET ARR 4    e   f   RET ARR 5    amp  amp  echo  RET ARR 5    f   echo   ----------------------------------------------     read -p  Press Enter To Exit     EXPECTED OUTPUT   FN MULTIPLE RETURN VALUES EXAMPLES  ------------------------------------------- running  FN MULTIPLE RETURN VALUES RET ARR 0   Hello RET ARR 1   World  running  FN MULTIPLE RETURN VALUES RET ARR 2   Hello RET ARR 3   World  running  FN MULTIPLE RETURN VALUES RET ARR 4   Hello RET ARR 5   World  Press Enter To Exit

User · Answer

captures the text sent to stdout by the command contained within  return does not output to stdout     contains the result code of the last command   fun1       return 34    fun2       fun1   local res      echo  res

User · Answer

As an add-on to others  excellent posts  here s an article summarizing these techniques     set a global variable  set a global variable  whose name you passed to the function set the return code  and pick it up with      echo  some data  and pick it up with MYVAR   myfunction      Returning Values from Bash Functions

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The return statement sets the exit code of the function  much the same as exit will do for the entire script   The exit code for the last command is always available in the    variable   function fun1      return 34    function fun2      local res   fun1    echo       lt -- Always echos 0 since the  local  command passes     res   fun1    echo       lt -- Outputs 34

User · Answer

Another way to achive this is name references  requires Bash 4 3     function example     local -n VAR  1   VAR foo    example RESULT echo  RESULT

User · Answer

Although bash has a return statement  the only thing you can specify with it is the function s own exit status  a value between 0 and 255  0 meaning  quot success quot    So return is not what you want  You might want to convert your return statement to an echo statement - that way your function output could be captured using     braces  which seems to be exactly what you want  Here is an example  function fun1      echo 34    function fun2      local res   fun1    echo  res    Another way to get the return value  if you just want to return an integer 0-255  is     function fun1      return 34    function fun2      fun1   local res      echo  res    Also  note that you can use the return value to use boolean logic like fun1    fun2 will only run fun2 if fun1 returns a non-0 value   The default return value is the exit value of the last statement executed within the function

User · Answer

Functions in Bash are not functions like in other language  they re actually commands  So functions are used as if they were binaries or scripts fetched from your path   From the perspective of your program logic there should be really no difference    Shell commands are connected by pipes  aka streams   and not fundamental or user-defined data types  as in  real  programming languages  There is no such thing like a return value for a command  maybe mostly because there s no real way to declare it  It could occur on the man-page  or the --help output of the command  but both are only human-readable and hence are written to the wind    When a command wants to get input it reads it from its input stream  or the argument list  In both cases text strings have to be parsed   When a command wants to return something it has to echo it to its output stream  Another oftenly practiced way is to store the return value in dedicated  global variables  Writing to the output stream is clearer and more flexible  because it can take also binary data  For example  you can return a BLOB easily   encrypt         gpg -c -o-  1   encrypt data in filename to stdout  asks for a passphrase     encrypt public dat  gt  private dat   write function result to file   As others have written in this thread  the caller can also use command substitution     to capture the output   Parallely  the function would  return  the exit code of gpg  GnuPG   Think of the exit code as a bonus that other languages don t have  or  depending on your temperament  as a  Schmutzeffekt  of shell functions  This status is  by convention  0 on success or an integer in the range 1-255 for something else  To make this clear  return  like exit  can only take a value from 0-255  and values other than 0 are not necessarily errors  as is often asserted   When you don t provide an explicit value with return the status is taken from the last command in a Bash statement function command and so forth  So there is always a status  and return is just an easy way to provide it

User · Answer

The problem with other answers is they either use a global  which can be overwritten when several functions are in a call chain  or echo which means your function cannot output diagnostic info  you will forget your function does this and the  result   i e  return value  will contain more info than your caller expects  leading to weird bug   or eval which is way too heavy and hacky    The proper way to do this is to put the top level stuff in a function and use a local with bash s dynamic scoping rule  Example    func1          ret val hi    func2         ret val bye    func3         local ret val nothing     echo  ret val     func1     echo  ret val     func2     echo  ret val    func3   This outputs  nothing hi bye   Dynamic scoping means that ret val points to a different object depending on the caller  This is different from lexical scoping  which is what most programming languages use  This is actually a documented feature  just easy to miss  and not very well explained  here is the documentation for it  emphasis is mine        Variables local to the function may be declared with the local   builtin  These variables are visible only to the function and the   commands it invokes    For someone with a C C   Python Java C  javascript background  this is probably the biggest hurdle  functions in bash are not functions  they are commands  and behave as such  they can output to stdout stderr  they can pipe in out  they can return an exit code  Basically there is no difference between defining a command in a script and creating an executable that can be called from the command line   So instead of writing your script like this    top-level code  bunch of functions more top-level code   write it like this      define your main  containing all top-level code main    bunch of functions   call main main     where main   declares ret val as local and all other functions return values via ret val    See also the following Unix  amp  Linux question  Scope of Local Variables in Shell Functions    Another  perhaps even better solution depending on situation  is the one posted by ya teck which uses local -n

User · Answer

You could do  return it         eval   FUNCNAME 1   r val  quot   1 quot      and then use it in your functions like this  fun1        return it 34    fun2        fun1  echo  fun1 r val

User · Answer

I like to do the following if running in a script where the function is defined   POINTER    used for function return values  my function           do stuff     POINTER  my function return     my other function           do stuff     POINTER  my other function return     my function RESULT   POINTER   my other function RESULT   POINTER    I like this  becase I can then include echo statements in my functions if I want  my function         echo  - gt  my function          do stuff     POINTER  my function return      echo   lt - my function   POINTER

[bash] Return value in a Bash function

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