如何设置环境变量

如题：

初学对照教程使用命令 ，当用到 useradd 命令的时候 提示 bash: useradd: command not found
请教别人说是环境变量的问题，百度了下，说是要 修改 profile文件 或者.bashrc文件 ，于是找到
这两个文件，可是不知道应该 怎么修改，望指教
profile :[root@localhost etc]# cat profile
# /etc/profile

# System wide environment and startup programs, for login setup
# Functions and aliases go in /etc/bashrc

pathmunge () {
if ! echo $PATH | /bin/egrep -q "(^|:)$1($|:)" ; then
if [ "$2" = "after" ] ; then
PATH=$PATH:$1
else
PATH=$1:$PATH
fi
fi
}

# ksh workaround
if [ -z "$EUID" -a -x /usr/bin/id ]; then
EUID=`id -u`
UID=`id -ru`
fi

# Path manipulation
if [ "$EUID" = "0" ]; then
pathmunge /sbin
pathmunge /usr/sbin
pathmunge /usr/local/sbin
fi

# No core files by default
ulimit -S -c 0 > /dev/null 2>&1

if [ -x /usr/bin/id ]; then
USER="`id -un`"
LOGNAME=$USER
MAIL="/var/spool/mail/$USER"
fi

HOSTNAME=`/bin/hostname`
HISTSIZE=1000

if [ -z "$INPUTRC" -a ! -f "$HOME/.inputrc" ]; then
INPUTRC=/etc/inputrc
fi

export PATH USER LOGNAME MAIL HOSTNAME HISTSIZE INPUTRC

for i in /etc/profile.d/*.sh ; do
if [ -r "$i" ]; then
. $i
fi
done

unset i
unset pathmunge