mrabs: 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/lib/mrtrix/bin/mrabs - from package: mrtrix

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

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