unifdef: 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/unifdef - from package: unifdef

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

On Debian possible default full paths are:

/usr/bin/unifdef - from package: unifdef

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

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