Diff for "BashFAQ/024"

Anchor(faq24)

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

The following command always prints "total number of lines: 0", although the variable linecnt has a larger value in the while loop:

    # Non-working example (except in ksh88/ksh93)
    linecnt=0
    cat /etc/passwd | while read line
    do
        linecnt=`expr $linecnt + 1`
    done
    echo "total number of lines: $linecnt"

The reason for this surprising behaviour is that a while/for/until loop runs in a SubShell when it's part of a pipeline. For the while loop above, a new subshell with its own copy of the variable linecnt is created (initial value, taken from the parent shell: "0"). 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 has not changed) is used in the echo command.

Different shells behave differently when using redirection or pipes with a loop:

• BourneShell creates a subshell when the input or output of a loop 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 example above actually works in ksh88 and ksh93!)

• 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).

To solve this, either use a method that works without a subshell, or make sure you do all processing inside that subshell (a bit of a kludge, but often easier to work with):

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

To avoid the subshell completely (not easily possible if the other part of the pipe is a command!), use redirection, which does not have this problem (at least for ["BASH"] and KornShell):

    # POSIX
    linecnt=0
    while read line ; do
        linecnt=$(($linecnt+1))
   done < /etc/passwd
   echo "total number of lines: $linecnt"

For ["BASH"], when the input of the pipe is a command rather than a file, you can use ProcessSubstitution:

    # Bash
    while read LINE; do
        echo "-> $LINE"
    done < <(grep PATH /etc/profile)

If you're reading from a plain file, a portable and common work-around is to redirect the standard input of the script using exec:

    # Bourne
    linecnt=0
    exec < /etc/passwd    # redirect standard input from the file /etc/passwd
    while read line       # "read" gets its input from the file /etc/passwd
    do
        linecnt=`expr $linecnt + 1`
    done
    echo "total number of lines: $linecnt"

This works as expected, and prints a line count for the file /etc/passwd. But the input is redirected from that file permanently. What if we need to read the original standard input sometime later again? In that case we have to save a copy of the original standard input file descriptor, which we later can restore:

    # Bourne
    exec 3<&0             # save original stdin file descriptor 0 as FD 3
    exec 0</etc/passwd    # redirect stdin from the file /etc/passwd

    linecnt=0
    while read line       # "read" gets its input from the file /etc/passwd
    do
        linecnt=`expr $linecnt + 1`
    done

    exec 0<&3             # restore saved stdin (FD 0) from FD 3
    exec 3<&-             # close the no-longer-needed FD 3

    echo "total number of lines: $linecnt"

Subsequent exec commands can be combined into one line, which is interpreted left-to-right:

    exec 3<&0
    exec 0</etc/passwd
    _...read redirected standard input..._
    exec 0<&3
    exec 3<&-

is equivalent to

    exec 3<&0 0</etc/passwd
    _...read redirected standard input..._
    exec 0<&3 3<&-

Another useful trick (using Bash syntax) is breaking a variable into words using read:

    # Bash
    echo "$foo" | read a b c      # this doesn't work
    read a b c <<< "$foo"         # but this does

Again, the pipeline causes the read command in the first example to run in a subshell, so its effect is never witnessed in the parent process. The second example does not create any subshells, so it works as we expect. The <<< operator is specific to bash (2.05b and later), and the input which follows it is usually called a "here string".

For more examples of how to break input into words, see [:BashFAQ/001:FAQ #1].