ppmshift: 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/ppmshift - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmshift - from package: netpbm

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

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