Shell Scripting Learning

Regular expressions
^ Start tag

$ end Tag
[A-z]
[A-z]
[0-9]
. Single arbitrary character
* Number of previous character repetitions >=0
+ Number of previous character repetitions >=1
？ Number of previous character repetitions 0 or 1
{N,m} The number of times the preceding character repeats N to M

[A-z] [0-9] [0-9] B12 B1a

# grep ' ^r.*n$ '/etc/passwd

Sed ' match conditions/actions performed '/etc/passwd
Or
cat/etc/passwd | Sed ' match conditions/actions performed '

Replace sed ' s/old/new/g '

Ifconfig | grep ' inet addr: '
|grep-v ' 127.0.0.1 ' | Sed ' s/inet addr://g ' | Sed ' s/bcast.*//g '

Awk
# awk-f: ' {print '} '/etc/passwd take the first column with a colon delimiter

---------------------------------------------------------------------------------------
Environment variable System--Multiple user set commands to view environment variables
--User environment variables (/root/.bash_profile)
--System environment variables (/etc/profile)

Positional variables
-typically used in conjunction with scripts
--$0 Script Name
--$1, $, $, $4...$9 position parameters

Pre-defined variables
--$? Represents the program exit representative (typically 0 for execution success, not 0 for execution failure)
echo $?
--$# represents the number of arguments for the current shell
--$* (an entire string) [email protected] (each parameter counts as a string) represents all parameters
--$$ PID of the current process

Custom variables
--The syntax format is: Name=[value]
--Note:
Variable equals cannot have spaces on either side
Variable is case sensitive
--Use ($ variable name) to invoke the value of the variable after it is defined

Arithmetic operations
-- + - * / %
--$ ((expression))
--$[expression]
--expr expression
a=2 (duplicate assignment will overwrite)
B=3
# echo $ (($a + $b)) or $ (a+b)
# echo $[$a + $b]
# expr 5 + 2 Note there are spaces
# expr $a + $b

Built-in test judgment succeeded/established--failure/not established
--test testing An expression
--[test expression] the most common
--Note: there must be a space between the test expression and the brackets

Numerical comparison
-eq equals True
-ne is not equal to true
-GT is greater than true
-ge is true if it is greater than or equal
-lt is less than true
-le is less than or equal to true

Example 1:
[Email protected] ~]# echo $a $b
2 3
[Email protected] ~]# test $a-lt $b
[[email protected] ~]# echo $?
0 successes
[Email protected] ~]# test $a-gt $b
[[email protected] ~]# echo $?
1 failure

Example 2:
[Email protected] ~]# echo $a $b
2 3
[Email protected] ~]# [$a-lt $b]
[[email protected] ~]# echo $?
0
[Email protected] ~]# [$a-gt $b]
[[email protected] ~]# echo $?
1

&& Logic and
--CMD1 && cmd2cmd1 successfully executed CMD2
|| Logical OR
--CMD1 | | Cmd2cmd1 fails to execute cmd2.
; No logical relationship
--cmd1; CMD2 cmd1 after execution, execute CMD2

String test (strings are enclosed in double quotes)
= Equals is True
! = is not equal is true
-Z String string length 0 is True
-N string string length not 0 is true

File test
-e File name True if the file exists
-D file name determines whether the directory
-f filename If file exists and is normal file true
-r file name True if the file exists and is readable
-W file name True if the file exists and is writable
-X file name if file exists and executable is true
-S file name True if the file exists and has at least one character
Linux also offers non-(! ), or (-O), and (-a) three logical operators,
Used to connect test conditions in order of precedence:! Highest,-a second,-o lowest

Date (man + command, viewing Help)
# date ' +%y%m%d%h%m '
# date-d ' 7 day ago ' +%y%m%d%h%m '
# date-s "2016-04-09 15:30:13" Modify system time (temporary)

Take string ${variable name: offset:length}
[[email protected] ~]# a= "201604091527"
[Email protected] ~]# echo $a
201604091527
[[email protected] ~]# echo ${a:2:3} skip two fetch 3
160

Take string echo ${variable name%.*}
[Email protected] ~]# a= "123.log"

[[email protected] ~]# echo ${a%.*}
123

Find File
-type-name
-size (file size)-mtime (file modification time)
Script Case:

#!/bin/bash
# #号表示注释
# The Shell script executes from the top down
Date
grep ' Root '/etc/passwd

Execute script:
# chmod +x 1.sh
#/tmp/1.sh

. /tmp/1.sh Execute File/tmp/1.sh

Control statements
#!/bin/bash
For i in {1..50}
Do
Mkdir/usr/local/src/dir$i
Done

#!/bin/bash
For i in ' seq ' #反引号代表命令预先执行
Do
Mkdir/usr/local/src/dir$i
Done

#!/bin/bash
For ((i=1;i<=10;i++));d o
sum=$ ((sum+i))
Done
Echo $SUM

#!/bin/bash
I=1
While [$i-le 10]
Do
sum=$ ((sum+i))
I=$[I+1]
Done
Echo $SUM

#!/bin/bash
While Read-r line
Do
echo $line: HELLO
Done </etc/passwd

Username:hello
Root:hello
Adm:hello

#!/bin/bash
While Read-r line
Do
Echo ' echo $line | Awk-f: ' {print $} ': HELLO
Done </etc/passwd

#!/bin/bash
if [-d/tmp/123];then
Ls/tmp/123
Else
Mkdir/tmp/123
Fi

Top Free

#!/bin/bash
Case $ in
Top
Top
;;
Free
Free
;;
df
Df
;;
*)
echo "USAGE:$0{TOP|FREE|DF}"
Esac

If for and case--control statement

Shell functions: Some commands that are repeatedly called can usually be placed inside a function
#!/bin/bash
Sum () {
echo $ (($1+$2))
}
Sum 5 6

Scheduled Tasks

Usage: at [TIME] specifies the time to execute a specific command

Example:
At 4:17# specifies that the scheduled task be performed 4:17 A.M. the same day
at> cp/etc/passwd/tmp# Scheduled Task content
At> <EOT> #输入完成后, press Ctrl+d to end recurring scheduled Tasks crontab

1) Service Crond status
Chkconfig--list | grep Crond
Crontab-e
0 2 * * 3 cp/etc/passwd/tmp Weekly 32-point backup
5 1 10,25 * * rm-rf/tmp/* 1:5 A.M., 10th and 25th per month
*/10 * * * * ntpdate time server IP performs time synchronization every 10 minutes
0 1-6 * * * daily from 1 o'clock in the morning to 6.

2) Restart the Crond Services service Crond restart

Tail-f Dynamic View

crontab [u user] [-l|-r|-e]

-L: List execution tasks

-U: Specifies a user, without the-u option, for the current user

-e: Make a scheduled task

Shell Scripting Learning