xml_grep: 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/xml_grep - from package: perl-XML-Twig

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

On Debian possible default full paths are:

/usr/bin/xml_grep - from package: xml-twig-tools

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

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