mysql_waitpid: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/mysql_waitpid - from package: mysql55w
/usr/bin/mysql_waitpid - from package: mysql

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/mysql_waitpid - from package: mariadb-client-10.0
/usr/bin/mysql_waitpid - from package: mysql-client-5.5

You may install the required package with command apt-get install {package}

Error in other languages:
mysql_waitpid: bevel nie gevind nie
mysql_waitpid: comandă negăsită
mysql_waitpid: comando não encontrado
mysql_waitpid: commande introuvable
mysql_waitpid: command not found
mysql_waitpid: käsku ei ole
mysql_waitpid: không tìm thấy lệnh
mysql_waitpid: komanda nerasta
mysql_waitpid: Kommando nicht gefunden.
mysql_waitpid: kommandot finns inte
mysql_waitpid: komut yok
mysql_waitpid: nie znaleziono polecenia
mysql_waitpid: níor aimsíodh an t-ordú
mysql_waitpid: no se encontró la orden
mysql_waitpid: no s'ha trobat l'ordre
mysql_waitpid: opdracht niet gevonden
mysql_waitpid: parancs nem található
mysql_waitpid: perintah tidak ditemukan
mysql_waitpid: príkaz nenájdený
mysql_waitpid: příkaz nenalezen
mysql_waitpid: команда не найдена
mysql_waitpid: командата не е открита
mysql_waitpid: 命令找不到
mysql_waitpid: コマンドが見つかりません