sha1sum: 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/sha1sum - from package: coreutils

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

On Debian possible default full paths are:

/usr/bin/sha1sum - from package: coreutils
/usr/lib/plan9/bin/sha1sum - from package: 9base

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

Error in other languages:
sha1sum: bevel nie gevind nie
sha1sum: comandă negăsită
sha1sum: comando não encontrado
sha1sum: commande introuvable
sha1sum: command not found
sha1sum: käsku ei ole
sha1sum: không tìm thấy lệnh
sha1sum: komanda nerasta
sha1sum: Kommando nicht gefunden.
sha1sum: kommandot finns inte
sha1sum: komut yok
sha1sum: nie znaleziono polecenia
sha1sum: níor aimsíodh an t-ordú
sha1sum: no se encontró la orden
sha1sum: no s'ha trobat l'ordre
sha1sum: opdracht niet gevonden
sha1sum: parancs nem található
sha1sum: perintah tidak ditemukan
sha1sum: príkaz nenájdený
sha1sum: příkaz nenalezen
sha1sum: команда не найдена
sha1sum: командата не е открита
sha1sum: 命令找不到
sha1sum: コマンドが見つかりません