我这里拿php作为一个例子，我的php安装在/usr/local/webserver/php下，没有把php加入环境变量时，你在命令行执行
#查看当前php的版本信息
[root@CentOS ~]# php -v
会提示你此命令不存在。
下面详细说说linux下修改环境变量的方法
方法一：在/etc/profile文件中添加变量【对所有用户生效（永久的）】
用VI在文件/etc/profile文件中增加变量，该变量将会对Linux下所有用户有效，并且是“永久的”。
[root@CentOS ~]# vi /etc/profile
在文件末尾加上如下两行代码
PATH=/usr/local/webserver/php/bin:$PATH
export PATH
# /etc/profile
# System wide environment and startup programs, for login setup
# Functions and aliases go in /etc/bashrc
# It's NOT a good idea to change this file unless you know what you
# are doing. It's much better to create a custom.sh shell script in
# /etc/profile.d/ to make custom changes to your environment, as this
# will prevent the need for merging in future updates.
pathmunge () {
case ":${PATH}:" in
*:"$1":*)
;;
*)
if [ "$2" = "after" ] ; then
PATH=$PATH:$1
else
PATH=$1:$PATH
fi
esac
}
if [ -x /usr/bin/id ]; then
if [ -z "$EUID" ]; then
# ksh workaround
EUID=`id -u`
UID=`id -ru`
fi
USER="`id -un`"
LOGNAME=$USER
MAIL="/var/spool/mail/$USER"
fi
# Path manipulation
if [ "$EUID" = "0" ]; then
pathmunge /sbin
pathmunge /usr/sbin
pathmunge /usr/local/sbin
else
pathmunge /usr/local/sbin after
pathmunge /usr/sbin after
pathmunge /sbin after
fi
HOSTNAME=`/bin/hostname 2>/dev/null`
HISTSIZE=1000
if [ "$HISTCONTROL" = "ignorespace" ] ; then
export HISTCONTROL=ignoreboth
else
export HISTCONTROL=ignoredups
fi
export PATH USER LOGNAME MAIL HOSTNAME HISTSIZE HISTCONTROL
# By default, we want umask to get set. This sets it for login shell
# Current threshold for system reserved uid/gids is 200
# You could check uidgid reservation validity in
# /usr/share/doc/setup-*/uidgid file
if [ $UID -gt 199 ] && [ "`id -gn`" = "`id -un`" ]; then
umask 002
else
umask 022
fi
for i in /etc/profile.d/*.sh ; do
if [ -r "$i" ]; then
if [ "${-#*i}" != "$-" ]; then
. "$i"
else
. "$i" >/dev/null 2>&1
fi
fi
done
unset i
unset pathmunge
PATH=/usr/local/webserver/php/bin:$PATH
export PATH
要是刚才的修改马上生效，需要执行以下代码
[root@CentOS ~]# source /etc/profile
这时再查看系统环境变量，就能看见刚才加的东西已经生效了
[root@CentOS ~]# echo $PATH
/usr/local/webserver/php/bin:/usr/lib/qt-3.3/bin:/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin
现在就能直接使用php命令了（而不是像之前写很长一串/usr/local/webserver/php/bin/php -v），例如查看当前php的版本
[root@CentOS ~]# php -v
PHP 5.3.8 (cli) (built: Jun 27 2012 14:28:20)
Copyright (c) 1997-2011 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2011 Zend Technologies
方法二：在用户目录下的.bash_profile文件中增加变量【对单一用户生效（永久的）】
用VI在用户目录下的.bash_profile文件中增加变量，改变量仅会对当前用户有效，并且是“永久的”。具体操作和方法1一样，这里就不在列举代码了。
方法三：直接运行export命令定义变量【只对当前shell（BASH）有效（临时的）】
在shell的命令行下直接使用[export变量名=变量值]定义变量，该变量只在当前的shell（BASH）或其子shell（BASH）下是有效的，shell关闭了，变量也就失效了，再打开新shell时就没有这个变量，需要使用的话还需要重新定义。
例如
export PATH=/usr/local/webserver/php/bin:$PATH