unstr: 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/unstr - from package: fortune-mod

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

On Debian possible default full paths are:

/usr/bin/unstr - from package: fortune-mod

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

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