adb: 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/adb - from package: android-tools

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

On Debian possible default full paths are:

/usr/bin/adb - from package: android-tools-adb

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

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