llvm-diff: 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/llvm-diff - from package: llvm

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

On Debian possible default full paths are:

/usr/bin/llvm-diff - from package: llvm
/usr/lib/llvm-3.4/bin/llvm-diff - from package: llvm-3.4
/usr/lib/llvm-3.5/bin/llvm-diff - from package: llvm-3.5

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

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