unshare: 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/unshare - from package: util-linux-ng

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

On Debian possible default full paths are:

/usr/bin/unshare - from package: util-linux

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

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