Diff for "BashFAQ/024"

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

In most shells, each command of a pipeline is executed in a separate SubShell. Non-working example:

# Works only in ksh88/ksh93, or zsh or bash 4.2 with lastpipe enabled
# In other shells, this will print 0
linecount=0

printf '%s\n' foo bar |
while IFS= read -r line
do
    linecount=$((linecount + 1))
done

echo "total number of lines: $linecount"

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 linecount 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 linecount 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, Yash and PDKsh-derived shells create a new process only if the loop is part of a pipeline.

• KornShell and Zsh 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, ksh93, zsh! (but not MKsh or other PDKsh-derived shells)

• 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 canonical 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 ensues.

Workarounds

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

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

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

  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/Zsh/Ksh93 only):

  # Bash/Ksh93/Zsh
  while IFS= read -r line
  do
      ((linecount++))
  done < <(grep PATH /etc/profile)
  
  echo "total number of lines: $linecount"

  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 IFS= read -r line
  do
      linecount=$((linecount + 1))
  done < mypipe
  
  echo "total number of lines: $linecount"

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

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

  # bash>4
  coproc grep PATH /etc/profile
  
  while IFS= read -r line
  do
      linecount=$((linecount + 1))
  done <&"${COPROC[0]}"
  
  echo "total number of lines: $linecount"

• Use a HereString (Bash/Zsh/Ksh93 only, though the example uses the Bash-specific read -a (Ksh93 and Zsh using read -A instead)):

  # Options:
  # -r Backslash does not act as an escape character for the word separators or line delimiter.
  # -a The words are assigned to sequential indices of the array "words"
  
  read -ra words <<< 'hi ho hum'
  printf 'total number of words: %d\n' "${#words[@]}"

  The <<< operator is available in Bash (2.05b and later), Zsh (where it was first introduced inspired from a similar operator in the Unix port of the rc shell), Ksh93 and Yash.

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

  # POSIX
  linecount=0
  while IFS= read -r; do
      linecount=$((linecount+1))
  done <<EOF
  hi
  ho
  hum
  EOF
  
  printf 'total number of lines: %d\n' "$linecount"

• Use lastpipe (Bash 4.2)

  # Bash 4.2
  # +m: Disable monitor mode (job control). Background processes display their
  #     exit status upon completion when in monitor mode (we don't want that).
  set +m
  shopt -s lastpipe
  
  x=0
  printf '%s\n' hi{,,,,,} | while IFS= read -r "lines[x++]"; do :; done
  printf 'total number of lines: %d\n' "${#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