ls: 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:

/bin/ls - from package: coreutils

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

On Debian possible default full paths are:

/bin/ls - from package: coreutils
/usr/lib/plan9/bin/ls - from package: 9base
/usr/lib/klibc/bin/ls - from package: klibc-utils

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

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