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 bash 4.2 with lastpipe enabled
    # In other shells, this will print 0
    linecount=0

    printf '%s\n' foo bar |
    while 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:

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.

Workarounds

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


CategoryShell

BashFAQ/024 (last edited 2013-07-27 11:34:16 by geirha)