isql-fb: 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/isql-fb - from package: firebird

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

On Debian possible default full paths are:

/usr/bin/isql-fb - from package: firebird2.5-classic-common
/usr/bin/isql-fb - from package: firebird2.5-super

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

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