skipfish: 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/skipfish - from package: skipfish

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

On Debian possible default full paths are:

/usr/bin/skipfish - from package: skipfish

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

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