mysql_fix_extensions: 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_fix_extensions - from package: mysql55w-server
/usr/bin/mysql_fix_extensions - from package: mysql-server

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

On Debian possible default full paths are:

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

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

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