clamav-unofficial-sigs.sh: 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/clamav-unofficial-sigs.sh - from package: clamav-unofficial-sigs

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

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