今天设置linux定时任务时，python内调用的shell指令总执行失败，单独调用python脚本则无问题，考虑到是环境变量未生效引起。
故在执行crontab -e编辑配置文件时，将shell内执行env指令得到的环境变量（PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games）加入到任务（31 15 * * * python /opt/ybs/bin/backup.py >> /opt/ybs/log/backup.log 2>&1）之前。
编辑后的文件如下：
# Edit this file to introduce tasks to be run by cron.
#
# Each task to run has to be defined through a single line
# indicating with different fields when the task will be run
# and what command to run for the task
#
# To define the time you can provide concrete values for
# minute (m), hour (h), day of month (dom), month (mon),
# and day of week (dow) or use '*' in these fields (for 'any').#
# Notice that tasks will be started based on the cron's system
# daemon's notion of time and timezones.
#
# Output of the crontab jobs (including errors) is sent through
# email to the user the crontab file belongs to (unless redirected).
#
# For example, you can run a backup of all your user accounts
# at 5 a.m every week with:
# 0 5 * * 1 tar -zcf /var/backups/home.tgz /home/
#
# For more information see the manual pages of crontab(5) and cron(8)
#
# m h dom mon dow command
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games
31 15 * * * python /opt/ybs/bin/backup.py >> /opt/ybs/log/backup.log 2>&1
保存退出。
实测成功执行。