脚本放在 /home/user/mysql_backup.sh
crontab
# crontab -l
# m h dom mon dow command
28 16 * * * /home/user/mysql_backup.sh
脚本如下
代码如下：

#!/bin/sh
# mysql_backup.sh: backup mysql databases and keep newest 5 days backup.
#
# Last updated: 20 March 2006
# ----------------------------------------------------------------------
# This is a free shell script under GNU GPL version 2.0 or above
# Copyright (C) 2006 Sam Tang
# Feedback/comment/suggestions : http://www.real-blog.com/
# ----------------------------------------------------------------------
# your mysql login information
# db_user is mysql username
# db_passwd is mysql password
# db_host is mysql host
# -----------------------------
db_user="root"
db_passwd="password"
db_host="localhost"
# the directory for story your backup file.
backup_dir="/home/mybackup"
# date format for backup file (dd-mm-yyyy)
time="$(date +"%d-%m-%Y")"
# mysql, mysqldump and some other bin's path
MYSQL="/usr/local/mysql/bin/mysql"
MYSQLDUMP="/usr/local/mysql/bin/mysqldump"
MKDIR="/bin/mkdir"
RM="/bin/rm"
MV="/bin/mv"
GZIP="/bin/gzip"
# check the directory for store backup is writeable
test ! -w $backup_dir && echo "Error: $backup_dir is un-writeable." && exit 0
# the directory for story the newest backup
test ! -d "$backup_dir/backup.0/" && $MKDIR "$backup_dir/backup.0/"
# get all databases
all_db="$($MYSQL -u $db_user -h $db_host -p$db_passwd -Bse 'show databases')"
for db in $all_db
do
$MYSQLDUMP -u $db_user -h $db_host -p$db_passwd $db | $GZIP -9 > "$backup_dir/backup.0/$time.$db.gz"
done
# delete the oldest backup
test -d "$backup_dir/backup.5/" && $RM -rf "$backup_dir/backup.5"
# rotate backup directory
for int in 4 3 2 1 0
do
if(test -d "$backup_dir"/backup."$int")
then
next_int=`expr $int + 1`
$MV "$backup_dir"/backup."$int" "$backup_dir"/backup."$next_int"
fi
done
exit 0;

备注：
mysql是以mysql用户身份运行的,对/home /mybackup不可写也会失败
chmod 777 /home/mybackup问题解决了