shrink_width: 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/bin/shrink_width - from package: libvips-tools

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

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