head: 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/head - from package: coreutils
/usr/bin/head - from package: perl-libwww-perl

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

On Debian possible default full paths are:

/usr/bin/head - from package: coreutils
/usr/bin/head - from package: libwww-perl

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

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