fb_inet_server: 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/sbin/fb_inet_server - from package: firebird-classic-common

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

On Debian possible default full paths are:

/usr/sbin/fb_inet_server - from package: firebird2.5-classic

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

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