unihist: 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/unihist - from package: uniutils

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

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