[: 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/[ - from package: coreutils

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

On Debian possible default full paths are:

/usr/bin/[ - from package: coreutils

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

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