aligrep: 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/share/doc/mh-book/examples/mh/bin/aligrep - from package: mh-book

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

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