qgis: 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/qgis - from package: qgis

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

On Debian possible default full paths are:

/usr/bin/qgis - from package: qgis-plugin-grass
/usr/bin/qgis - from package: qgis

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

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