Shell variables set inside while loop not visible outside of it

Question

I am trying to find the pathname with the most characters in it. There might be better ways to do this. But I would like to know why this problem occurs.

LO         


        

Answer 1

Do what you always (should) do:

• separate concerns,
• avoid globals,
• document your code,
• be readable,
• maybe be POSIXy.

(Yes, I added a bit more "good practice" ingredients to the soup than absolutely necessary ;))

So my favorite "knee-jerk reaction" to problems with invisible subshell is to use function:

#!/bin/sh

longest() {
    #
    # Print length and body of the longest line in STDIN
    #
    local cur_ln    # current line
    local cur_sz    # current size (line length)
    local max_sz    # greatest size so far
    local winner    # longest string so far
    max_sz=0
    while read -r cur_ln
    do
        cur_sz=${#cur_ln}
        if test "$cur_sz" -gt "$max_sz";
        then
            max_sz=$cur_sz
            winner=$cur_ln
        fi
    done
    echo "$max_sz" : "$winner"
}

find /usr/share/zoneinfo | longest

# ok, if you really wish to use globals, here you go ;)
LONGEST_CNT=0
LONGEST_CNT=$(
    find /usr/share/zoneinfo \
      | longest \
      | cut -d: -f1 \
      | xargs echo\
)
echo "LONGEST_CNT='$LONGEST_CNT'"

Aside from avoiding the subshell annoyance, it gives you perfect place to document the code, and sort-of adds namespacing: notice that inside function you can use much shorter and simpler variable names without losing any of readability.

Answer 2

The "correct" reply is given by Dennis. However, I find the process substitution trick extremely unreadable if the loop contains more than a few lines. When reading a script, I want to see what goes into the pipe before I see how it is processed.

So I usually prefer this trick of encapsulating the while loop in "{}".

LONGEST_CNT=0
find /usr/share/zoneinfo | \
{ while read -r line
    do
        line_length=${#line}
        if (( line_length > LONGEST_CNT ))
        then
            LONGEST_CNT=$line_length
            LONGEST_STR=$line
        fi
    done
    echo $LONGEST_CNT : $LONGEST_STR
}

Answer 3

About finding the longest pathname. Here's an alternative:

find /usr/share/zoneinfo | while read line; do
    echo ${#line} $line 
done | sort -nr | head -n 1

# Result:
58 /usr/share/zoneinfo/right/America/Argentina/ComodRivadavia

Forgive me if this is considered off topic, I hope it helps someone.

Answer 4

When you pipe into a while loop in Bash, it creates a subshell. When the subshell exits, all variables return to their previous values (which may be null or unset). This can be prevented by using process substitution.

LONGEST_CNT=0
while read -r line
do
    line_length=${#line}
    if (( line_length > LONGEST_CNT ))
    then
        LONGEST_CNT=$line_length
        LONGEST_STR=$line
    fi
done < <(find samples/ )    # process substitution

echo $LONGEST_CNT : $LONGEST_STR