How to modify a global variable within a function in bash

Question

I m working with this   GNU bash  version 4 1 2 1 -release  x86 64-redhat-linux-gnu    I have a script like below      bin bash  e 2  function test1       e 4   echo  hello     test1  echo   e    Which returns   hello 4   But if I assign the result of the function to a variable  the global variable e is not modified      bin bash  e 2  function test1       e 4   echo  hello     ret   test1   echo   ret  echo   e    Returns   hello 2   I ve heard of the use of eval in this case  so I did this in test1   eval  e 4    But the same result   Could you explain me why it is not modified  How could I save the echo of the test1 function in ret and modify the global variable too

User · Accepted Answer

When you use a command substitution  i e   the        construct   you are creating a subshell  Subshells inherit variables from their parent shells  but this only works one way  A subshell cannot modify the environment of its parent shell  Your variable  e is set within a subshell  but not the parent shell  There are two ways to pass values from a subshell to its parent  First  you can output something to stdout  then capture it with a command substitution  myfunc         echo  quot Hello quot     var  quot   myfunc  quot   echo  quot  var quot   The above outputs  Hello  For a numerical value in the range of 0 through 255  you can use return to pass the number as the exit status  mysecondfunc         echo  quot Hello quot      return 4    var  quot   mysecondfunc  quot  num var     echo  quot  var - num is  num var quot   This outputs  Hello - num is 4

User · Answer

Maybe you can use a file  write to file inside function  read from file after it  I have changed e to an array  In this example blanks are used as separator when reading back the array      bin bash  declare -a e e 0   first  e 1   secondddd   function test1       e 2   third   e 1   second   echo    e       gt   tmp tempout  echo hi    ret   test1   echo   ret   read -r -a e  lt   tmp tempout echo    e      echo    e 0    echo    e 1    echo    e 2      Output   hi first second third first second third

User · Answer

I had a similar problem  when I wanted to automatically remove temp files I had created  The solution I came up with was not to use command substitution  but rather to pass the name of the variable  that should take the final result  into the function  E g       bin bash  remove later    new tmp file         file   mktemp      remove later   remove later  file      eval  1  file   remove tmp files         rm  remove later   trap remove tmp files EXIT  new tmp file tmpfile1 new tmp file tmpfile2   So  in your case that would be      bin bash  e 2  function test1       e 4   eval  1  hello     test1 ret  echo   ret  echo   e    Works and has no restrictions on the  return value

User · Answer

It s because command substitution is performed in a subshell  so while the subshell inherits the variables  changes to them are lost when the subshell ends   Reference      Command substitution  commands grouped with parentheses  and asynchronous commands are invoked in a subshell environment that is a duplicate of the shell environment

User · Answer

What you are doing  you are executing test1     test1    in a sub-shell  child shell    and Child shells cannot modify anything in parent   You can find it in bash manual  Please Check  Things results in a subshell here

User · Answer

You can always use an alias   alias next  printf  blah  02d   count count    count 1

User · Answer

A solution to this problem  without having to introduce complex functions and heavily modify the original one  is to store the value in a temporary file and read   write it when needed   This approach helped me greatly when I had to mock a bash function called multiple times in a bats test case   For example  you could have     Usage read value path to tmp file function read value     cat    1        Usage  set value path to tmp file the value function set value     echo    2    gt     1      ----    Original code   function test1       e 4   set value    tmp file      e     echo  hello        Create the temp file   Note that tmp file is available in test1 as well tmp file   mktemp     Your logic e 2   Store the value set value    tmp file      e      Run test1 test1    Read the value modified by test1 e   read value    tmp file    echo   e    The drawback is that you might need multiple temp files for different variables  And also you might need to issue a sync command to persist the contents on the disk between one write and read operations

User · Answer

This needs bash 4 1 if you use  fd  or local -n       The rest should work in bash 3 x I hope   I am not completely sure due to printf  q - this might be a bash 4 feature    Summary  Your example can be modified as follows to archive the desired effect     Add following 4 lines   passback     while   1 -lt       do printf   q  q     1      1    shift  done  return  1    passback      passback               capture       out         2   3 lt  amp -    2    gt  amp 3    ret     printf   q  q     1    out     3 gt  amp 1  echo   exit  ret      capture     eval     capture            e 2    Add following line  called  Annotation  function test1      passback e    function test1       e 4   echo  hello       Change following line to  capture ret test1   echo   ret  echo   e    prints as desired   hello 4   Note that this solution    Works for e 1000  too  Preserves    if you need      The only bad sideffects are    It needs a modern bash  It forks quite more often  It needs the annotation  named after your function  with an added    It sacrifices file descriptor 3    You can change it to another FD if you need that    In  capture just replace all occurances of 3 with another  higher  number      The following  which is quite long  sorry for that  hopefully explains  how to adpot this recipe to other scripts  too   The problem  d     let x    date   Y m d- H M S     x 0 d1   d  d2   d  d3   d  d4   d  echo  x  d1  d2  d3  d4   outputs  0 20171129-123521 20171129-123521 20171129-123521 20171129-123521   while the wanted output is  4 20171129-123521 20171129-123521 20171129-123521 20171129-123521   The cause of the problem  Shell variables  or generally speaking  the environment  is passed from parental processes to child processes  but not vice versa   If you do output capturing  this usually is run in a subshell  so passing back variables is difficult   Some even tell you  that it is impossible to fix   This is wrong  but it is a long known difficult to solve problem   There are several ways on how to solve it best  this depends on your needs   Here is a step by step guide on how to do it   Passing back variables into the parental shell  There is a way to pass back variables to a parental shell   However this is a dangerous path  because this uses eval   If done improperly  you risk many evil things   But if done properly  this is perfectly safe  provided that there is no bug in bash    passback     while   0 -lt       do printf   q  q     1      1    shift  done     d     let x    d   date   Y m d- H M S    passback x d     x 0 eval  d  d1  d eval  d  d2  d eval  d  d3  d eval  d  d4  d echo  x  d1  d2  d3  d4   prints  4 20171129-124945 20171129-124945 20171129-124945 20171129-124945   Note that this works for dangerous things  too   danger     danger       passback danger    eval  danger     bin echo     echo   danger    prints     bin echo     This is due to printf   q   which quotes everything such  that you can re-use it in a shell context safely   But this is a pain in the a    This does not only look ugly  it also is much to type  so it is error prone   Just one single mistake and you are doomed  right   Well  we are at shell level  so you can improve it   Just think about an interface you want to see  and then you can implement it   Augment  how the shell processes things  Let s go a step back and think about some API which allows us to easily express  what we want to do   Well  what do we want do do with the d   function   We want to capture the output into a variable  OK  then let s implement an API for exactly this     This needs a modern bash 4 3  see  help declare  if  -n  is present    we get rid of it below anyway     capture VARIABLE command args   capture     local -n output   1  shift output               Now  instead of writing  d1   d    we can write  capture d1 d   Well  this looks like we haven t changed much  as  again  the variables are not passed back from d into the parent shell  and we need to type a bit more   However now we can throw the full power of the shell at it  as it is nicely wrapped in a function   Think about an easy to reuse interface  A second thing is  that we want to be DRY  Don t Repeat Yourself   So we definitively do not want to type something like  x 0 capture1 x d1 d capture1 x d2 d capture1 x d3 d capture1 x d4 d echo  x  d1  d2  d3  d4   The x here is not only redundant  it s error prone to always repeate in the correct context   What if you use it 1000 times in a script and then add a variable   You definitively do not want to alter all the 1000 locations where a call to d is involved   So leave the x away  so we can write    passback     while   0 -lt       do printf   q  q     1      1    shift  done     d     let x    output   date   Y m d- H M S    passback output x     xcapture     local -n output   1   eval         2         x 0 xcapture d1 d xcapture d2 d xcapture d3 d xcapture d4 d echo  x  d1  d2  d3  d4   outputs  4 20171129-132414 20171129-132414 20171129-132414 20171129-132414   This already looks very good    But there still is the local -n which does not work in oder common bash 3 x   Avoid changing d    The last solution has some big flaws    d   needs to be altered It needs to use some internal details of xcapture to pass the output    Note that this shadows  burns  one variable named output  so we can never pass this one back   It needs to cooperate with  passback   Can we get rid of this  too   Of course  we can   We are in a shell  so there is everything we need to get this done   If you look a bit closer to the call to eval you can see  that we have 100  control at this location    Inside  the eval we are in a subshell  so we can do everything we want without fear of doing something bad to the parental shell   Yeah  nice  so let s add another wrapper  now directly inside the eval    passback     while   0 -lt       do printf   q  q     1      1    shift  done       DO NOT USE   xcapture          2    gt   gt  printf   q  q     1     cat      passback x        DO NOT USE     DO NOT USE  xcapture     eval     xcapture            d     let x    date   Y m d- H M S     x 0 xcapture d1 d xcapture d2 d xcapture d3 d xcapture d4 d echo  x  d1  d2  d3  d4   prints  4 20171129-132414 20171129-132414 20171129-132414 20171129-132414                                                       However  this  again  has some major drawback    The  DO NOT USE  markers are there  because there is a very bad race condition in this  which you cannot see easily    The  gt  printf     is a background job   So it might still execute while the  passback x is running  You can see this yourself if you add a sleep 1  before printf or  passback   xcapture a d  echo then outputs x or a first  respectively   The  passback x should not be part of  xcapture  because this makes it difficult to reuse that recipe  Also we have some unneded fork here  the   cat    but as this solution is  DO NOT USE  I took the shortest route    However  this shows  that we can do it  without modification to d    and without local -n    Please note that we not neccessarily need  xcapture at all  as we could have written everyting right in the eval   However doing this usually isn t very readable  And if you come back to your script in a few years  you probably want to be able to read it again without much trouble   Fix the race  Now let s fix the race condition   The trick could be to wait until printf has closed it s STDOUT  and then output x   There are many ways to archive this    You cannot use shell pipes  because pipes run in different processes  One can use temporary files  or something like a lock file or a fifo   This allows to wait for the lock or fifo  or different channels  to output the information  and then assemble the output in some correct sequence    Following the last path could look like  note that it does the printf last because this works better here     passback     while   0 -lt       do printf   q  q     1      1    shift  done      xcapture       printf   q  q     1          2   3 lt  amp -   passback x  gt  amp 3      3 gt  amp 1     xcapture     eval     xcapture            d     let x    date   Y m d- H M S     x 0 xcapture d1 d xcapture d2 d xcapture d3 d xcapture d4 d echo  x  d1  d2  d3  d4   outputs  4 20171129-144845 20171129-144845 20171129-144845 20171129-144845   Why is this correct     passback x directly talks to STDOUT  However  as STDOUT needs to be captured in the inner command  we first  save  it into FD3  you can use others  of course  with  3  amp 1  and then reuse it with  gt  amp 3  The        2   3 lt  amp -   passback x  gt  amp 3  finishes after the  passback  when the subshell closes STDOUT  So the printf cannot happen before the  passback  regardless how long  passback takes  Note that the printf command is not executed before the complete commandline is assembled  so we cannot see artefacts from printf  independently how printf is implemented    Hence first  passback executes  then the printf   This resolves the race  sacrificing one fixed file descriptor 3  You can  of course  choose another file descriptor in the case  that FD3 is not free in your shellscript   Please also note the 3 lt  amp - which protects FD3 to be passed to the function   Make it more generic   capture contains parts  which belong to d    which is bad  from a reusability perspective   How to solve this   Well  do it the desparate way by introducing one more thing  an additional function  which must return the right things  which is named after the original function with   attached   This function is called after the real function   and can augment things  This way  this can be read as some annotation  so it is very readable     passback     while   0 -lt       do printf   q  q     1      1    shift  done     capture       printf   q  q     1          2   3 lt  amp -    2    gt  amp 3      3 gt  amp 1    capture     eval     capture            d       passback x    d     let x    date   Y m d- H M S     x 0 capture d1 d capture d2 d capture d3 d capture d4 d echo  x  d1  d2  d3  d4   still prints  4 20171129-151954 20171129-151954 20171129-151954 20171129-151954   Allow access to the return-code  There is only on bit missing   v   fn  sets    to what fn returned   So you probably want this  too  It needs some bigger tweaking  though     This is all the interface you need    Remember  that this burns FD 3   passback     while   1 -lt       do printf   q  q     1      1    shift  done  return  1    passback      passback               capture       out         2   3 lt  amp -    2    gt  amp 3    ret     printf   q  q     1    out     3 gt  amp 1  echo   exit  ret      capture     eval     capture              Here is your function  annotated with which sideffects it has  fails      passback x y    fails     x  1  y 69  echo FAIL  return 23       And now the code which uses it all x 0 y 0 capture wtf fails 42 echo     x  y  wtf   prints  23 42 69 FAIL   There is still a lot room for improvement    passback   can be elmininated with passback     set --            while   1 -lt       do printf   q  q     1      1    shift  done  return  1     capture   can be eliminated with capture     eval      out         2   3 lt  amp -    2    gt  amp 3    ret     printf   q  q     1    out     3 gt  amp 1  echo   exit  ret        The solution pollutes a file descriptor  here 3  by using it internally  You need to keep that in mind if you happen to pass FDs  Note thatbash 4 1 and above has  fd  to use some unused FD   Perhaps I will add a solution here when I come around   Note that this is why I use to put it in separate functions like  capture  because stuffing this all into one line is possible  but makes it increasingly harder to read and understand Perhaps you want to capture STDERR of the called function  too  Or you want to even pass in and out more than one filedescriptor from and to variables  I have no solution yet  however here is a way to catch more than one FD  so we can probably pass back the variables this way  too    Also do not forget   This must call a shell function  not an external command      There is no easy way to pass environment variables out of external commands     With LD PRELOAD  it should be possible  though     But this then is something completely different    Last words  This is not the only possible solution   It is one example to a solution   As always you have many ways to express things in the shell  So feel free to improve and find something better   The solution presented here is quite far from being perfect    It was nearly not testet at all  so please forgive typos  There is a lot of room for improvement  see above  It uses many features from modern bash  so probably is hard to port to other shells  And there might be some quirks I haven t thought about    However I think it is quite easy to use    Add just 4 lines of  library   Add just 1 line of  annotation  for your shell function  Sacrifices just one file descriptor temporarily  And each step should be easy to understand even years later

[bash] How to modify a global variable within a function in bash?

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