How to return a string value from a Bash function

Question

I d like to return a string from a Bash function   I ll write the example in java to show what I d like to do   public String getSomeString       return  tadaa      String variable   getSomeString      The example below works in bash  but is there a better way to do this   function getSomeString      echo  tadaa     VARIABLE   getSomeString

User · Answer

You could use a global variable   declare globalvar  some string   string        eval    1  some other string       ----------  end of function string  ----------  string globalvar  echo     globalvar      This gives   some other string

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Like bstpierre above  I use and recommend the use of explicitly naming output variables    function some func     OUTVAR ARG1      local  outvar  1    local  result   Use some naming convention to avoid OUTVARs to clash        some processing         eval   outvar    result   Instead of just    result     Note the use of quoting the    This will avoid interpreting content in  result as shell special characters  I have found that this is an order of magnitude faster than the result   some func  arg1   idiom of capturing an echo  The speed difference seems even more notable using bash on MSYS where stdout capturing from function calls is almost catastrophic   It s ok to send in a local variables since locals are dynamically scoped in bash   function another func     ARG      local result    some func result   1     echo result is  result

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Bash  since version 4 3  feb 2014     has explicit support for reference variables or name references  namerefs   beyond  eval   with the same beneficial performance and indirection effect  and which may be clearer in your scripts and also harder to  forget to  eval  and have to fix this error    declare  -aAfFgilnrtux   -p   name  value       typeset  -aAfFgilnrtux   -p   name  value         Declare variables and or give them attributes         -n Give each name the nameref attribute  making it a name reference      to another variable   That other variable is defined by the value      of name   All references and assignments to name  except for        changing the -n attribute itself  are performed on the variable      referenced by name s value   The -n attribute cannot be applied to      array variables      When used in a function  declare and typeset make each name local  as with the local command  unless the -g option is supplied      and also      PARAMETERS      A variable can be assigned the nameref attribute using the -n option to the      declare or local builtin commands  see the descriptions of declare and local      below  to create a nameref  or a reference to another variable   This allows      variables to be manipulated indirectly   Whenever the nameref variable is        referenced or assigned to  the operation is actually performed on the variable      specified by the nameref variable s value   A nameref is commonly used within      shell functions to refer to a variable whose name is passed as an argument to        the function   For instance  if a variable name is passed to a shell function      as its first argument  running        declare -n ref  1       inside the function creates a nameref variable ref whose value is the variable      name passed as the first argument   References and assignments to ref are      treated as references and assignments to the variable whose name was passed as         1   If the control variable in a for loop has the nameref attribute  the list      of words can be a list of shell variables  and a name reference will be        established for each word in the list  in turn  when the loop is executed       Array variables cannot be given the -n attribute   However  nameref variables      can reference array variables and subscripted array variables   Namerefs can be        unset using the -n option to the unset builtin   Otherwise  if unset is executed      with the name of a nameref variable as an argument  the variable referenced by        the nameref variable will be unset    For example  EDIT 2   thank you Ron  namespaced  prefixed  the function-internal variable name  to minimize external variable clashes  which should finally answer properly  the issue raised in the comments by Karsten       1   string  your variable to contain the return value function return a string          declare -n ret  1     local MYLIB return a string message  The date is       MYLIB return a string message    date      ret  MYLIB return a string message     and testing this example     return a string result  echo  result The date is 20160817   Note that the bash  declare  builtin  when used in a function  makes the declared variable  local  by default  and  -n  can also be used with  local    I prefer to distinguish  important declare  variables from  boring local  variables  so using  declare  and  local  in this way acts as documentation   EDIT 1 -  Response to comment below by Karsten  - I cannot add comments below any more  but Karsten s comment got me thinking  so I did the following test which WORKS FINE  AFAICT - Karsten if you read this  please provide an exact set of test steps from the command line  showing the problem you assume exists  because these following steps work just fine     return a string ret  echo  ret The date is 20170104    I ran this just now  after pasting the above function into a bash term - as you can see  the result works just fine

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All answers above ignore what has been stated in the man page of bash    All variables declared inside a function will be shared with the calling environment  All variables declared local will not be shared    Example code     bin bash  f         echo function starts     local WillNotExists  It still does       DoesNotExists  It still does       echo function ends    echo  DoesNotExists  Should print empty line echo  WillNotExists  Should print empty line f                    Call the function echo  DoesNotExists  Should print It still does  echo  WillNotExists  Should print empty line   And output    sh -x   x sh   echo    echo    f   echo function starts  function starts   local  WillNotExists It still does     DoesNotExists  It still does     echo function ends  function ends   echo It still  does    It still does    echo   Also under pdksh and ksh this script does the same

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In my programs  by convention  this is what the pre-existing  REPLY variable is for  which read uses for that exact purpose   function getSomeString     REPLY  tadaa     getSomeString echo  REPLY   This echoes  tadaa   But to avoid conflicts  any other global variable will do   declare result  function getSomeString     result  tadaa     getSomeString echo  result   If that isn   t enough  I recommend Markarian451   s solution

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bash pattern to return both scalar and array value objects    definition  url parse       parse  url  into   url host    url port          local        inject caller  url  argument in local scope    local url host       url path         calculate  url    components    declare -p    url      return only  url    object fields to the caller     invocation  main       invoke url parser and inject  url    results in local scope    eval    url parse url http   host path     parse  url     echo  host  url host path  url path    use  url    components

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You could also capture the function output      bin bash function getSomeString          echo  tadaa      return var   getSomeString  echo  return var   Alternative syntax  return var  getSomeString  echo  return var   Looks weird  but is better than using global variables IMHO  Passing parameters works as usual  just put them inside the braces or backticks

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There is no better way I know of  Bash knows only status codes  integers  and strings written to the stdout

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They key problem of any  named output variable  scheme where the caller can pass in the variable name  whether using eval or declare -n  is inadvertent aliasing  i e  name clashes  From an encapsulation point of view  it s awful to not be able to add or rename a local variable in a function without checking ALL the function s callers first to make sure they re not wanting to pass that same name as the output parameter   Or in the other direction  I don t want to have to read the source of the function I m calling just to make sure the output parameter I intend to use is not a local in that function    The only way around that is to use a single dedicated output variable like REPLY  as suggested by Evi1M4chine  or a convention like the one suggested by Ron Burk   However  it s possible to have functions use a fixed output variable internally  and then add some sugar over the top to hide this fact from the caller  as I ve done with the call function in the following example  Consider this a proof of concept  but the key points are   The function always assigns the return value to REPLY  and can also return an exit code as usual From the perspective of the caller  the return value can be assigned to any variable  local or global  including REPLY  see the wrapper example   The exit code of the function is passed through  so using them in e g  an if or while or similar constructs works as expected  Syntactically the function call is still a single simple statement    The reason this works is because the call function itself has no locals and uses no variables other than REPLY  avoiding any potential for name clashes  At the point where the caller-defined output variable name is assigned  we re effectively in the caller s scope  technically in the identical scope of the call function   rather than in the scope of the function being called      bin bash function call       var func  args        REPLY      1           2    eval    1        REPLY  return        function greet       case   1  in     us  REPLY  hello        nz  REPLY  kia ora           return 123     esac    function wrapper       call REPLY greet         function main       local a b c d   call a greet us   echo  a   a          call b greet nz   echo  b   b          call c greet de   echo  c   c          call d wrapper us   echo  d   d          main   Output   a  hello   0  b  kia ora   0  c     123  d  hello   0

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Implement a generic return stack for functions  STACK    push       STACK     quot   1  quot      pop       export  1  quot   STACK    STACK    -1   quot    unset  STACK    STACK    -1        Usage  my func       push  quot Hello world  quot    push  quot Hello world2  quot    my func   pop MESSAGE2   pop MESSAGE1 echo   MESSAGE1    MESSAGE2

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The most straightforward and robust solution is to use command substitution  as other people wrote   assign         local x     x  Test      echo   x     x   assign    This assigns string  Test  to x   The downside is performance as this requires a separate process   The other technique suggested in this topic  namely passing the name of a variable to assign to as an argument  has side effects  and I wouldn t recommend it in its basic form  The problem is that you will probably need some variables in the function to calculate the return value  and it may happen that the name of the variable intended to store the return value will interfere with one of them   assign         local x     x  Test      eval   1   x     assign y   This assigns string  Test  to y  as expected  assign x   This will NOT assign anything to x in this scope            because the name  x  is declared as local inside the function   You might  of course  not declare internal variables of the function as local  but you really should always do it as otherwise you may  on the other hand  accidentally overwrite an unrelated variable from the parent scope if there is one with the same name   One possible workaround is an explicit declaration of the passed variable as global   assign         local x     eval declare -g  1     x  Test      eval   1   x      If name  x  is passed as an argument  the second row of the function body will overwrite the previous local declaration  But the names themselves might still interfere  so if you intend to use the value previously stored in the passed variable prior to write the return value there  be aware that you must copy it into another local variable at the very beginning  otherwise the result will be unpredictable  Besides  this will only work in the most recent version of BASH  namely 4 2  More portable code might utilize explicit conditional constructs with the same effect   assign         if     1    x     then       local x     fi     x  Test      eval   1   x      Perhaps the most elegant solution is just to reserve one global name for function return values and use it consistently in every function you write

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agt agtsoft   temp  cat   fc     bin sh  fcall  function fcall   local res p  1  shift  fname     eval   p  res      fcall   function f1       res     1  2  2      function f2       local a      eval   fcall  fname f1  a 2 3      echo f2  a     a 3  f2  echo after a  a  res  res  agt agtsoft   temp    fc f2 10 after a 3  res

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To illustrate my comment on Andy s answer  with additional file descriptor manipulation to avoid use of  dev tty      bin bash  exec 3 gt  amp 1  returnString         exec 4 gt  amp 1  gt  amp 3     local s  1     s   s   some default string       echo  writing to stdout      echo  writing to stderr   gt  amp 2     exec  gt  amp 4-     echo   s     my string   returnString       echo  my string     my string     Still nasty  though

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The options have been all enumerated  I think  Choosing one may come down to a matter of the best style for your particular application  and in that vein  I want to offer one particular style I ve found useful  In bash  variables and functions are not in the same namespace  So  treating the variable of the same name as the value of the function is a convention that I find minimizes name clashes and enhances readability  if I apply it rigorously  An example from real life   UnGetChar  function GetChar           assume failure     GetChar        if someone previously  ungot  a char     if     -z   UnGetChar     then         GetChar   UnGetChar          UnGetChar          return 0                 success       else  if not at EOF     elif IFS  read -N1 GetChar   then         return 0             success     else         return 1             EOF     fi    function UnGetChar        UnGetChar   1      And  an example of using such functions   function GetToken           assume failure     GetToken        if at end of file     if   GetChar  then         return 1                EOF       if start of comment     elif      GetChar             then         while      GetChar        n      do             GetToken    GetChar              GetChar         done         UnGetChar   GetChar        if start of quoted string     elif     GetChar            then       et cetera   As you can see  the return status is there for you to use when you need it  or ignore if you don t  The  returned  variable can likewise be used or ignored  but of course only after the function is invoked   Of course  this is only a convention  You are free to fail to set the associated value before returning  hence my convention of always nulling it at the start of the function  or to trample its value by calling the function again  possibly indirectly    Still  it s a convention I find very useful if I find myself making heavy use of bash functions   As opposed to the sentiment that this is a sign one should e g   move to perl   my philosophy is that conventions are always important for managing the complexity of any language whatsoever

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You can echo a string  but catch it by piping     the function to something else   You can do it with expr  though ShellCheck reports this usage as deprecated

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The way you have it is the only way to do this without breaking scope  Bash doesn t have a concept of return types  just exit codes and file descriptors  stdin out err  etc

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You could have the function take a variable as the first arg and modify the variable with the string you want to return      bin bash set -x function pass back a string         eval   1  foo bar rab oof      return var    pass back a string return var echo  return var   Prints  foo bar rab oof    Edit  added quoting in the appropriate place to allow whitespace in string to address  Luca Borrione s comment   Edit  As a demonstration  see the following program  This is a general-purpose solution  it even allows you to receive a string into a local variable      bin bash set -x function pass back a string         eval   1  foo bar rab oof      return var    pass back a string return var echo  return var  function call a string func          local lvar         pass back a string lvar      echo  lvar   lvar  locally     call a string func echo  lvar   lvar  globally    This prints     return var    pass back a string return var   eval  return var     foo bar rab oof         return var  foo bar rab oof    echo foo bar rab oof foo bar rab oof   call a string func   local lvar    pass back a string lvar   eval  lvar     foo bar rab oof         lvar  foo bar rab oof    echo  lvar     foo bar rab oof     locally  lvar  foo bar rab oof  locally   echo  lvar          globally  lvar    globally   Edit  demonstrating that the original variable s value is available in the function  as was incorrectly criticized by  Xichen Li in a comment      bin bash set -x function pass back a string         eval  echo in pass back a string  original  1 is    1      eval   1  foo bar rab oof      return var  original return var  pass back a string return var echo  return var  function call a string func          local lvar  original lvar       pass back a string lvar      echo  lvar   lvar  locally     call a string func echo  lvar   lvar  globally    This gives output     return var  original return var    pass back a string return var   eval  echo in pass back a string  original return var is  return var     echo in pass back a string  original return var is original return var in pass back a string  original return var is original return var   eval  return var     foo bar rab oof         return var  foo bar rab oof    echo foo bar rab oof foo bar rab oof   call a string func   local  lvar original lvar    pass back a string lvar   eval  echo in pass back a string  original lvar is  lvar     echo in pass back a string  original lvar is original lvar in pass back a string  original lvar is original lvar   eval  lvar     foo bar rab oof         lvar  foo bar rab oof    echo  lvar     foo bar rab oof     locally  lvar  foo bar rab oof  locally   echo  lvar          globally  lvar    globally

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Addressing Vicky Ronnen s head up  considering the following code   function use global       eval   1  changed using a global var      function capture output       echo  always changed     function test inside a func       local  myvar  local starting value      echo  3    myvar       use global   myvar      echo  4    myvar        myvar    capture output       echo  5    myvar     function only difference       local  myvar  local starting value      echo  7    myvar       local use global   myvar      echo  8    myvar       local  myvar    capture output       echo  9    myvar     declare myvar  global starting value  echo  0   myvar   use global  myvar  echo  1   myvar   myvar    capture output   echo  2   myvar   test inside a func echo  6    myvar    this was local inside the above function  only difference    will give  0  global starting value 1  changed using a global var 2  always changed 3  local starting value 4  changed using a global var 5  always changed 6   7  local starting value 8  local starting value 9  always changed   Maybe the normal scenario is to use the syntax used in the test inside a func function  thus you can use both methods in the majority of cases  although capturing the output is the safer method always working in any situation  mimicking the returning value from a function that you can find in other languages  as Vicky Ronnen correctly pointed out

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As previously mentioned  the  correct  way to return a string from a function is with command substitution   In the event that the function also needs to output to console  as  Mani mentions above   create a temporary fd in the beginning of the function and redirect to console   Close the temporary fd before returning your string      bin bash   file   func return test sh returnString         exec 3 gt  amp 1  gt  dev tty     local s  1     s   s   some default string       echo  writing directly to console      exec 3 gt  amp -          echo   s     my string   returnString       echo  my string     my string     executing script with no params produces         func return test sh writing directly to console my string    some default string    hope this helps people  -Andy

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Although there were a lot of good answers  they all did not work the way I wanted them to  So here is my solution with these key points  Helping the forgetful programmer Atleast I would struggle to always remember error checking after something like this  var   myFunction  Allows assigning values with newline chars  n Some solutions do not allow for that as some forgot about the single quotes around the value to assign  Right way  eval  quot   returnVariable     value   quot  or even better  see the next point below  Using printf instead of eval Just try using something like this myFunction  quot date  amp  amp  var2 quot  to some of the supposed solutions here  eval will execute whatever is given to it  I only want to assign values so I use printf -v  quot   returnVariable  quot   quot  s quot   quot   value  quot  instead  Encapsulation and protection against variable name collision If a different user or at least someone with less knowledge about the function  this is likely me in some months time  is using myFunction I do not want them to know that he must use a global return value name or some variable names are forbidden to use  That is why I added a name check at the top of myFunction      if     quot   1  quot     quot returnVariable quot      then         echo  quot Cannot give the ouput to   quot returnVariable  quot  as a variable with the same name is used in myFunction    quot          echo  quot If that is still what you want to do please do that outside of myFunction    quot          return 1     fi  Note this could also be put into a function itself if you have to check a lot of variables  If I still want to use the same name  here  returnVariable  I just create a buffer variable  give that to myFunction and then copy the value returnVariable  So here it is  myFunction    myFunction         if     quot   1  quot     quot returnVariable quot      then         echo  quot Cannot give the ouput to   quot returnVariable  quot  as a variable with the same name is used in myFunction    quot          echo  quot If that is still what you want to do please do that outside of myFunction    quot          return 1     fi     if     quot   1  quot     quot value quot      then         echo  quot Cannot give the ouput to   quot value  quot  as a variable with the same name is used in myFunction    quot          echo  quot If that is still what you want to do please do that outside of myFunction    quot          return 1     fi     local returnVariable  quot   1  quot      local value               nHello World n                 echo  quot setting the returnVariable now    quot      printf -v  quot   returnVariable  quot   quot  s quot   quot   value  quot     Test cases  var1  quot I m not greeting  quot  myFunction var1       -eq 0     amp  amp  echo  quot myFunction    SUCCESS quot     echo  quot myFunction    FAILURE quot  printf  quot var1  n s n quot   quot   var1  quot     Output    setting the returnVariable now      myFunction    SUCCESS   var1                  Hello World                returnVariable  quot I m not greeting  quot  myFunction returnVariable       -eq 0     amp  amp  echo  quot myFunction    SUCCESS quot     echo  quot myFunction    FAILURE quot  printf  quot returnVariable  n s n quot   quot   returnVariable  quot     Output   Cannot give the ouput to  quot returnVariable quot  as a variable with the same name is used in myFunction      If that is still what you want to do please do that outside of myFunction      myFunction    FAILURE   returnVariable    I m not greeting   var2  quot I m not greeting  quot  myFunction  quot date  amp  amp  var2 quot        -eq 0     amp  amp  echo  quot myFunction    SUCCESS quot     echo  quot myFunction    FAILURE quot  printf  quot var2  n s n quot   quot   var2  quot     Output   setting the returnVariable now         myFunction  line     printf   date  amp  amp  var2   not a valid identifier   myFunction    FAILURE   var2    I m not greeting   myFunction var3       -eq 0     amp  amp  echo  quot myFunction    SUCCESS quot     echo  quot myFunction    FAILURE quot  printf  quot var3  n s n quot   quot   var3  quot     Output   setting the returnVariable now      myFunction    SUCCESS   var3                  Hello World

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