linux command learn tips

awk强大的文件操作，从Alfred Aho，Peter Weinberger and Brain Kernighan
找出非系统产生的账号：

awk -F “:” ‘($3 >= 1000) { printf $1 “\n”}’ /etc/passwd
awk Pattern {ACTIONS;}

awk中仅有string和numeric，可以互相转换。
文件中每一行check每一种pattern（Regex，Boolean，Special）,In Boolean Search,进行fuzzy matching，” 23”==23

ack, awk, grep

git 25 tips useful But a little long

查看当前服务器的运行级别。

runlevel
who -r

找到/usr下面大于10M的文件，找到120天之前被修改过的文件. 找出90天内未被访问过的文件。

find /usr -size +10M

find /home -mtime +120

find /home \! -atime -90

alias 设置别名，方便以后使用。

alias ls=ls -al

工作流程控制的几个命令：

jobs [-lp] [job]
bg [job]
fg [job]
kill [-signal] job..
wait [job..]

%number: refer to job by number
%string: job whose name begins with string
%?string: job whose name contains string
%+ or %%: current job
%-: previous job

bg      show all jobs in the backend
fg %2   put the 2 in the front
jobs    show all jobs

ENV NAME	MEANING
$HISTCMD	the history number of the current command
$HISTSIZE	number of remembered commands
$HOME	login directory
$MAIL	absolute path of mailbox
$OLDPWD	absolute path of previous directory
$PATH	command search path
$PPID	process ID of parent
$PS1	primary prompt 1
$RANDOM	generate random number
$SECONDS	number of seconds since shell started
$SHELL	absolute path of shell
$UID	user ID of the current user
$$	process ID of current shell
$?	exit status of most recent statement
$#	number of the argruments
$x	all the arguments
$!	PID of the moust recent background job

shell if 语句，testing[]里面是判断，-a是and，-o是or，-e是equal，-z是zero，-n不是zero

if [ $DEBUG -eq 1]; then
elif [ -f /etc/passwd ]; then
else
fi