ysearch: 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/ysearch - from package: patchy

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

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