v.segment: 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/lib64/grass-6.4.4/bin/v.segment - from package: grass

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

On Debian possible default full paths are:

/usr/lib/grass64/bin/v.segment - from package: grass-core

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

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