A variable modified inside a while loop is not remembered

Question

In the following program  if I set the variable  foo to the value 1 inside the first if statement  it works in the sense that its value is remembered after the if statement  However  when I set the same variable to the value 2 inside an if which is inside a while statement  it s forgotten after the while loop  It s behaving like I m using some sort of copy of the variable  foo inside the while loop and I am modifying only that particular copy  Here s a complete test program      bin bash  set -e set -u  foo 0 bar  hello    if      bar      hello     then     foo 1     echo  Setting   foo to 1   foo  fi  echo  Variable   foo after if statement   foo     lines  first line nsecond line nthird line   echo -e  lines   while read line do     if      line      second line         then     foo 2     echo  Variable   foo updated to  foo inside if inside while loop      fi     echo  Value of   foo in while loop body   foo  done  echo  Variable   foo after while loop   foo     Output        testbash sh   Setting  foo to 1  1   Variable  foo after if statement  1   Value of  foo in while loop body  1   Variable  foo updated to 2 inside if inside while loop   Value of  foo in while loop body  2   Value of  foo in while loop body  2   Variable  foo after while loop  1    bash --version   GNU bash  version 4 1 10 4 -release  i686-pc-cygwin

User · Answer

You are the 742342nd user to ask this bash FAQ. The answer also describes the general case of variables set in subshells created by pipes:

E4) If I pipe the output of a command into read variable, why doesn't the output show up in $variable when the read command finishes?

This has to do with the parent-child relationship between Unix processes. It affects all commands run in pipelines, not just simple calls to read. For example, piping a command's output into a while loop that repeatedly calls read will result in the same behavior.

Each element of a pipeline, even a builtin or shell function, runs in a separate process, a child of the shell running the pipeline. A subprocess cannot affect its parent's environment. When the read command sets the variable to the input, that variable is set only in the subshell, not the parent shell. When the subshell exits, the value of the variable is lost.

Many pipelines that end with read variable can be converted into command substitutions, which will capture the output of a specified command. The output can then be assigned to a variable:
grep ^gnu /usr/lib/news/active | wc -l | read ngroup
can be converted into
ngroup=$(grep ^gnu /usr/lib/news/active | wc -l)
This does not, unfortunately, work to split the text among multiple variables, as read does when given multiple variable arguments. If you need to do this, you can either use the command substitution above to read the output into a variable and chop up the variable using the bash pattern removal expansion operators or use some variant of the following approach.

Say /usr/local/bin/ipaddr is the following shell script:
#! /bin/sh
host `hostname` | awk '/address/ {print $NF}'
Instead of using
/usr/local/bin/ipaddr | read A B C D
to break the local machine's IP address into separate octets, use
OIFS="$IFS"
IFS=.
set -- $(/usr/local/bin/ipaddr)
IFS="$OIFS"
A="$1" B="$2" C="$3" D="$4"
Beware, however, that this will change the shell's positional parameters. If you need them, you should save them before doing this.

This is the general approach -- in most cases you will not need to set $IFS to a different value.

Some other user-supplied alternatives include:
read A B C D << HERE
    $(IFS=.; echo $(/usr/local/bin/ipaddr))
HERE
and, where process substitution is available,
read A B C D < <(IFS=.; echo $(/usr/local/bin/ipaddr))

User · Answer

Hmmm    I would almost swear that this worked for the original Bourne shell  but don t have access to a running copy just now to check   There is  however  a very trivial workaround to the problem   Change the first line of the script from      bin bash   to     bin ksh   Et voila  A read at the end of a pipeline works just fine  assuming you have the Korn shell installed

User · Answer

Though this is an old question and asked several times  here s what I m doing after hours fidgeting with here strings  and the only option that worked for me is to store the value in a file during while loop sub-shells and then retrieve it  Simple  Use echo statement to store and cat statement to retrieve  And the bash user must chown the directory or have read-write chmod access   write to file echo  quot 1 quot   gt  foo txt  while condition  do      if  condition   then          write again to file         echo  quot 2 quot   gt  foo txt           fi done   read from file echo  quot Value of   foo in while loop body    cat foo txt  quot

User · Answer

This is an interesting question and touches on a very basic concept in Bourne shell and subshell   Here I provide a solution that is different from the previous solutions by doing some kind of filtering  I will give an example that may be useful in real life  This is a fragment for checking that downloaded files conform to a known checksum  The checksum file look like the following  Showing just 3 lines    49174 36326 dna align feature txt gz 54757     1 dna txt gz 55409  9971 exon transcript txt gz   The shell script      bin sh         failcnt 0   this variable is only valid in the parent shell  variable xx captures all the outputs from the while loop xx   cat   checkfile    while read -r line  do     num1   echo  line   awk   print  1        num2   echo  line   awk   print  2        fname   echo  line   awk   print  3        if   -f   fname     then         res   sum  fname          filegood   sum  fname   awk -v na  num1 -v nb  num2 -v fn  fname    if  na     1  amp  amp  nb     2    print  TRUE     else   print  FALSE                if     filegood     FALSE     then             failcnt   expr  failcnt   1    only in subshell             echo   fname BAD  failcnt          fi     fi done   tail -1    I am only interested in the final result   you can capture a whole bunch of texts and do further filtering failcnt   xx   BAD     I am only interested in the number   this variable is in the parent shell echo failcnt  failcnt if    failcnt -gt 0    then     echo  failcnt files failed else     echo download successful fi   The parent and subshell communicate through the echo command   You can pick some easy to parse text for the parent shell   This method does not break your normal way of thinking  just that you have to do some post processing  You can use grep  sed  awk  and more for doing so

User · Answer

How about a very simple method        call your while loop in a function       - set your value inside  nonsense  but shows the example       - return your value inside       capture your value outside      set outside      display outside          bin bash       set -e       set -u       No idea why you need this  not using here      foo 0     bar  hello       if      bar      hello         then         foo 1         echo  Setting    foo to  foo      fi      echo  Variable   foo after if statement   foo       lines  first line nsecond line nthird line       function my while loop            echo -e  lines   while read line     do         if      line      second line             then         foo 2  return 2          echo  Variable   foo updated to  foo inside if inside while loop          fi          echo -e  lines   while read line do     if      line      second line         then     foo 2                echo  Variable   foo updated to  foo inside if inside while loop      return 2      fi        Code below won t be executed since we returned from function in  if  statement       We aready reported the  foo var beint set to 2 anyway     echo  Value of   foo in while loop body   foo   done        my while loop  foo           echo  Variable   foo after while loop   foo        Output      Setting   foo 1     Variable  foo after if statement  1     Value of  foo in while loop body  1     Variable  foo after while loop  2      bash --version      GNU bash  version 3 2 51 1 -release  x86 64-apple-darwin13      Copyright  C  2007 Free Software Foundation  Inc

User · Answer

I use stderr to store within a loop  and read from it outside  Here var i is initially set and read inside the loop as 1    reading lines of content from 2 files concatenated   inside loop  write value of var i to stderr  before iteration    outside  read var i from stderr  has last iterative value  f  tmp file1 g  tmp file2 i 1 cat  f  g     while read -r s  do   echo  s  gt   dev null     some work   echo  i  gt  2   let i   done  read -r i  lt  2 echo  i  Or use the heredoc method to reduce the amount of code in a subshell  Note the iterative i value can be read outside the while loop  i 1 while read -r s  do   echo  s  gt   dev null   let i   done  lt  lt EOT   cat  f  g  EOT let i-- echo  i

User · Answer

echo -e  lines   while read line          done   The while loop is executed in a subshell  So any changes you do to the variable will not be available once the subshell exits   Instead you can use a here string to re-write the while loop to be in the main shell process  only echo -e  lines will run in a subshell   while read line do     if      line      second line         then         foo 2         echo  Variable   foo updated to  foo inside if inside while loop      fi     echo  Value of   foo in while loop body   foo  done  lt  lt  lt     echo -e   lines      You can get rid of the rather ugly echo in the here-string above by expanding the backslash sequences immediately when assigning lines  The        form of quoting can be used there   lines   first line nsecond line nthird line  while read line  do         done  lt  lt  lt    lines

User · Answer

UPDATED 2  Explanation is in Blue Moons s answer   Alternative solutions   Eliminate echo  while read line  do     done  lt  lt EOT first line second line third line EOT   Add the echo inside the here-is-the-document  while read line  do     done  lt  lt EOT   echo -e  lines  EOT   Run echo in background   coproc echo -e  lines while read -u   COPROC 0   line  do      done   Redirect to a file handle explicitly  Mind the space in  lt   lt      exec 3 lt   lt  echo -e   lines  while read -u 3 line  do     done   Or just redirect to the stdin   while read line  do     done  lt   lt  echo -e   lines    And one for chepner  eliminating echo    arr   first line   second line   third line    for  i 0 i lt    arr       i     line   arr i             Variable  lines can be converted to an array without starting a new sub-shell  The characters   and n has to be converted to some character  e g  a real new line character  and use the IFS  Internal Field Separator  variable to split the string into array elements  This can be done like   lines  first line nsecond line nthird line  echo   lines  OIFS   IFS  IFS    n  arr    lines    n    n      Conversion IFS   OIFS  echo    arr       Length     arr     set grep  arr   Result is  first line nsecond line nthird line first line second line third line  Length  3 arr   0   first line   1   second line   2   third line

[bash] A variable modified inside a while loop is not remembered

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