i-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 Debian possible default full paths are:

/usr/bin/i-ls - from package: integrit

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

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