igraph: 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/igraph - from package: python-igraph

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

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