Diff for "BashFAQ/024"

I set variables in a loop. Why do they disappear after the loop terminates? Or, why can't I pipe data to read?

The problem

Each command of a pipeline of at least two commands - where "command" can be any of: a simple or compound command, or pipeline - is executed asynchronously in a subshell. Or more simply, in most shells, each chunk of code separated by a pipe operator, including compound commands (which includes while/for/until loops) are forked off and executed at the same time in separate SubShell processes, which like all subshells, each have their own isolated environment and variable scope.

Non-working example:

    # Works only in ksh88/ksh93
    typeset -i linecnt=0
    printf '%s\n' foo bar | while read -r line

    do
        linecnt=$((linecnt+1))
    done
    printf 'total number of lines: %s\n' "$linecnt" # prints 0

The reason for this potentially surprising behaviour, as described above, is that each SubShell introduces a new variable context and environment. The while loop above is executed in a new subshell with its own copy of the variable linecnt created with the initial value of '0' taken from the parent shell. This copy then is used for counting. When the while loop is finished, the subshell copy is discarded, and the original variable linecnt of the parent (whose value hasn't changed) is used in the echo command.

Different shells exhibit different behaviors in this situation:

• BourneShell creates a subshell when the input or output of anything (loops, case etc..) but a simple command is redirected, either by using a pipeline or by a redirection operator ('<', '>').

• BASH creates a new process only if the loop is part of a pipeline.

• KornShell creates it only if the loop is part of a pipeline, but not if the loop is the last part of it. The read example above actually works in ksh88 and ksh93! (but not mksh)

• POSIX specifies the bash behaviour, but as an extension allows any or all of the parts of the pipeline to run without a subshell (thus permitting the KornShell behaviour, as well).

More broken stuff:

    # Bash 4
    # The problem also occurs without a loop
    printf '%s\n' foo bar | mapfile -t line  
    printf 'total number of lines: %s\n' "${#line[@]}" # prints 0

    f() {
        if [[ -t 0 ]]; then
            echo "$1"
        else
            read -r var
        fi
    };

    f 'hello' | f
    echo "$var" # prints nothing

Again, in both cases the pipeline causes read or some containing command to run in a subshell, so its effect is never witnessed in the parent process.

• It should be stressed that this issue isn't specific to loops. It's a general property of all pipes, though the "while/read" loop might be considered the cannonical example that crops up over and over when people read the help or manpage description of the read builtin and notice that it accepts data on stdin. They might recall that data redirected into a compound command is available throughout that command, but not understand why all the fancy process substitutions and redirects they run across in places like FAQ #1 are necessary. Naturally they proceed to put their funstuff directly into a pipeline, and confusion insues.

Workarounds

• If the input is a file, a simple redirect will suffice:

      # POSIX
      while read -r line; do linecnt=$(($linecnt+1)); done < file
      echo $linecnt

  Unfortunately, this doesn't work with a Bourne shell; see sh(1) from the Heirloom Bourne Shell for a workaround.

• Use command grouping and do everything in the subshell:

      # POSIX
      linecnt=0
      cat /etc/passwd | {
      while read -r line ; do
          linecnt=$((linecnt+1))
      done
      echo "total number of lines: $linecnt"
      }

  This doesn't really change the subshell situation, but if nothing from the subshell is needed in the rest of your code then destroying the local environment after you're through with it could be just what you want anyway.

• Use ProcessSubstitution (Bash only):

      # Bash
      while read -r line; do
          ((linecnt++))
      done < <(grep PATH /etc/profile)
      echo "total number of lines: $linecnt"

  This is essentially identical to the first workaround above. We still redirect a file, only this time the file happens to be a named pipe temporarily created by our process substitution to transport the output of grep.

• Use a named pipe:

      # POSIX
      mkfifo mypipe
      grep PATH /etc/profile > mypipe &
      while read -r line;do
          linecnt=$(($linecnt+1))
      done < mypipe
      echo "total number of lines: $linecnt"

• Use a coprocess (ksh, even pdksh, bash 4, oksh, mksh..):

      # ksh
      grep PATH /etc/profile |&
      while read -r -p line; do
          linecnt=$((linecnt+1))
      done
      echo "total number of lines: $linecnt"

• Use a HereString (Bash only):

       read -ra words <<< 'hi ho hum'
       printf 'total number of words: %d' "${#words[@]}"

  The <<< operator is specific to bash (2.05b and later), however it is a very clean and handy way to specify a small string of literal input to a command.

• With a POSIX shell, or for longer multi-line data, you can use a here document instead:

      # Bash
      declare -i linecnt
      while read -r; do
          ((linecnt++))
      done <<EOF
      hi
      ho
      hum
      EOF
      printf 'total number of lines: %d' "$linecnt"

• Use lastpipe (Bash 4.2)

       # Bash 4.2
       set +m
       shopt -s lastpipe
  
       printf '%s\n' hi{,,,,,} | while read -r "lines[x++]"; do :; done
       printf 'total number of lines: %d' "${#lines[@]}"

  Bash 4.2 introduces the aforementioned ksh-like behavior to Bash. The one caveat is that job control must not be enabled, thereby limiting its usefulness in an interactive shell.

For more related examples of how to read input and break it into words, see FAQ #1.

CategoryShell