Shell script: variable scope in functions

Question

I wrote a quick shell script to emulate the situation of xkcd #981 (without hard links, just symlinks to parent dirs) and used a recursive function to create all the directories. Unfortunately this script does not provide the desired result, so I think my understanding of the scope of variable $count is wrong.

How can I properly make the function use recursion to create twenty levels of folders, each containing 3 folders (3^20 folders, ending in soft links back to the top)?

#!/bin/bash
echo "Generating folders:"
toplevel=$PWD
count=1
GEN_DIRS() {
for i in 1 2 3
do
        dirname=$RANDOM
        mkdir $dirname
        cd $dirname
        count=$(expr $count + 1)
        if [ $count < 20 ] ; then
                GEN_DIRS
        else
                ln -s $toplevel "./$dirname"
        fi
done
}
GEN_DIRS
exit

Jonathan Leffler · Accepted Answer · 2013-08-29 01:05:57Z

2

Try this (amended version of the script) — it seems to work for me. I decline to test to 20 levels deep, though; at 8 levels deep, each of the three top-level directories occupies some 50 MB on a Mac file system.

#!/bin/bash
echo "Generating folders:"
toplevel=$PWD
GEN_DIRS()
{
    cur=${1:?}
    max=${2:?}
    for i in 1 2 3
    do
        dirname=$RANDOM
        if [ $cur -le $max ]
        then
            (
            echo "Directory: $PWD/$dirname"
            mkdir $dirname
            cd $dirname
            GEN_DIRS $((cur+1)) $max
            )
        else
            echo "Symlink:   $PWD/$dirname"
            ln -s $toplevel "./$dirname"
        fi
    done
}

GEN_DIRS 1 ${1:-4}

Lines 6 and 7 are giving names to the positional parameters ($1 and $2) passed to the function — the ${1:?} notation simply means that if you omit to pass a parameter $1, you get an error message from the shell (or sub-shell) and it exits.

The parentheses on their own (lines 13 and 18 above) mean that the commands in between are run in a sub-shell, so changes in directory inside the sub-shell do not affect the parent shell.

The condition on line 11 now uses arithmetic (-le) instead of string < comparisons; this works better for deep nesting (because the < is a lexicographic comparison, so level 9 is not less than level 10). It also means that the [ command is OK to use instead of the [[ command (although [[ would also work, I prefer the old-fashioned notation).

edited Aug 29, 2013 at 1:05

answered Aug 28, 2013 at 21:16

Jonathan Leffler

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4 Comments

cliffroared Over a year ago

Lines six/seven are setting cur and max to one and four I take it? Also, why the paren's around 14-19? Great solution, thanks!

cliffroared Over a year ago

Unfortunately, not creating all dirs right now. When called with 1 and 20, it creates 4 levels deep, and only one symlink in the final dir instead of 3 :/

Jonathan Leffler Over a year ago

The 4 levels was hard-coded in the call to GEN_DIRS. The 1 vs 3 symlinks was a misunderstanding of the requirement. Both issues are now fixed. With the script called gendirs.sh, running bash gendirs.sh creates 4 levels, and bash gendirs.sh 8 generates 8 levels, etc.

cliffroared Over a year ago

Best answer because it works, I understand it, and I've learned something from it. Thanks!

konsolebox · Accepted Answer · 2013-08-28 21:20:39Z

0

I end up creating a script like this:

#!/bin/bash

echo "Generating folders:"

toplevel=$PWD

level=0
maxlevel=4

function generate_dirs {
    pushd "$1" >/dev/null || return
    (( ++level ))
    for i in 1 2 3; do
        dirname=$RANDOM
        if (( level < maxlevel )); then
            echo "$PWD/$dirname"
            mkdir "$dirname" && generate_dirs "$dirname"
        else
            echo "$PWD/$dirname (link to top)"
            ln -sf "$toplevel" "$dirname"
        fi
    done
    popd >/dev/null 
    (( --level ))
}

generate_dirs .

exit

answered Aug 28, 2013 at 21:20

konsolebox

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3 Comments

cliffroared Over a year ago

Works perfectly, but I can't say I understand it all. Can you explain line 11? You are deleting the argument you called the function with? And then why the last two lines of the function?

cliffroared Over a year ago

Also, twenty would take a lot longer than I thought :P

konsolebox Over a year ago

@CliffordSR See help pushd for more info. ` >/dev/null` sends output to null to make it quiet. And || return ends the function if it fails to change to that directory. The argument is a directory. The code is safe. If something fails for unknown cause it wouldn't go on an infinite loop or create stuffs on a directory that is not supposed to be it.

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