glibc_post_upgrade.i686: 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/sbin/glibc_post_upgrade.i686 - from package: glibc

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

Error in other languages:
glibc_post_upgrade.i686: bevel nie gevind nie
glibc_post_upgrade.i686: comandă negăsită
glibc_post_upgrade.i686: comando não encontrado
glibc_post_upgrade.i686: commande introuvable
glibc_post_upgrade.i686: command not found
glibc_post_upgrade.i686: käsku ei ole
glibc_post_upgrade.i686: không tìm thấy lệnh
glibc_post_upgrade.i686: komanda nerasta
glibc_post_upgrade.i686: Kommando nicht gefunden.
glibc_post_upgrade.i686: kommandot finns inte
glibc_post_upgrade.i686: komut yok
glibc_post_upgrade.i686: nie znaleziono polecenia
glibc_post_upgrade.i686: níor aimsíodh an t-ordú
glibc_post_upgrade.i686: no se encontró la orden
glibc_post_upgrade.i686: no s'ha trobat l'ordre
glibc_post_upgrade.i686: opdracht niet gevonden
glibc_post_upgrade.i686: parancs nem található
glibc_post_upgrade.i686: perintah tidak ditemukan
glibc_post_upgrade.i686: príkaz nenájdený
glibc_post_upgrade.i686: příkaz nenalezen
glibc_post_upgrade.i686: команда не найдена
glibc_post_upgrade.i686: командата не е открита
glibc_post_upgrade.i686: 命令找不到
glibc_post_upgrade.i686: コマンドが見つかりません