gdal_sieve.py: 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/gdal_sieve.py - from package: gdal-python

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

On Debian possible default full paths are:

/usr/bin/gdal_sieve.py - from package: python-gdal

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

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