问为什么源命令不能在使用包含cd命令的shell脚本的crontab中工作？

EN

cd /home/parthipan/test/wireshark/$a/$b

10 01 * * * . /home/parthipan/test/purging.sh

#!/bin/bash
a=$( date -d "`date` -60 days" "+%Y%m%d" | cut -b 1-4 )
b=$( date -d "`date` -60 days" "+%Y%m%d" | cut -b 5-6 )
c=$( date -d "`date` -60 days" "+%Y%m%d" | cut -b 7-8 )
cd /home/parthipan/test/wireshark/$a/$b
d=$( ll | awk -F " " '{print $9}' | awk -v value=$c '$1 < value' | sed "/^$/d" | xargs )
echo "The Days archiving are: $d"
e=$( ll | awk -F " " '{print $9}' | awk -v value=$c '$1 < value' | sed "/^$/d" | wc -l )
f=$( date -d "`date` - 61 days" "+%Y%m%d" | cut -b 5-8 )
if (( $e > 0 ))
        then
                tar --no-overwrite-dir -czvf backup_$f.tar.gz $d
                mv backup_$f.tar.gz /home/parthipan/test/logs_backup
                #rm -rf $d
                logger Data has been purged

May  1 00:50:01 parthipan-nadar systemd: Started Session 24 of user root.