Linux-based Shell programming (2)-conditional judgment, arithmetic operations, testing

What if the condition is judged in bash?

Condition test Type

• Integer test
• Character test
• File test

Expressions for conditional tests

There are three kinds

[ expression ]  --方括号与表达式之间一定要有一个空格[[ expression ]] test expression

Integer test

-eq: 测试两个整数是否相等  例：$A -eq $B-ne: 测试两个整数是否不等  不等为真，相等为假-gt: 测试一个数是否大于另一个数；大于，为真；否则，为假；-lt: 测试一个数是否小于另一个数；小于，为真；否则，为假；-ge: 大于或等于-le: 小于或等于

The logical relationship between commands

逻辑与： &&    第一个条件为假时，第二条件不用再判断；    第一个条件为真时，第二条件必须得判断；逻辑或： ||    第一个条件为假时，第二条件必须得判断；    第一个条件为真时，第二条件不用再判断；

Example: Comparing two number sizes

[root @iZ28g26851kZ  ~]# a=< Span class= "Hljs-number" >12  [Root @iZ28g26851kZ  ~]# b= 15  [Root @iZ28g26851kZ  ~]# [ $A -gt  $B ] && echo " a relatively large " | | echo  "b relatively large"  b larger [root @iZ28g26851kZ  ~]# A=20  [Root @iZ28g26851kZ  ~]# [  $A -gt  $B ] && echo " a relatively large " | | echo  "b relatively large"  a larger [Root @iZ28g26851kZ  ~]#  

If a is greater than B is [ $A -gt $B ] true, it needs to be executed (judged) echo "A比较大"
Finally [ $A -gt $B ] && echo "A比较大" true, so no more executionecho "B比较大"

If statement--– single branch

if 判断条件 ;then    statement1    ...fi

--– Dual Branch

if 判断条件 ;then    statement1    ...else    statement2    ...fi

--– Multi-Branch

if 判断条件 ;then    statement1    ...elif 判断条件2 ; then    statement2    ...elif 判断条件3 ; then    statement3    ...else    statement4    ...fi

Example: To determine whether the user User1 exists, there is a display user already exists, does not exist the user is created, the password and user name are the same

adduser.sh

#!/bin/bash  name=user1if  ID  $NAME  &>/dev/null; then  echo   "user already exists"  else  echo   "current Total user ' wc-l/etc/passwd | Cut-d " "-f1 "People"  echo  " user does not exist, creating user " useradd  $NAME   echo   $NAME  | passwd--stdin  $NAME  &>/dev/null echo   "current Total user ' wc-l/etc/passwd | Cut-d "  fi  

Execution results

[root@iZ28g26851kZ ~]# ./adduser.sh 当前一共有用户30人用户不存在,正在创建用户当前一共有用户31人[root@iZ28g26851kZ ~]# ./adduser.sh 用户已存在[root@iZ28g26851kZ ~]# 

Arithmetic operations

As I said before, all the variables in the shell are strings, so the arithmetic operation has to be handled by the command.

Let

Let arithmetic expression
Cases:

[root@iZ28g26851kZ ~]# b=23[root@iZ28g26851kZ ~]# a=12[root@iZ28g26851kZ ~]# let c=$a+$b[root@iZ28g26851kZ ~]# echo $c35[root@iZ28g26851kZ ~]# 

$[Arithmetic expression]

Cases:

[root@iZ28g26851kZ ~]# c=$[$a+$b*2][root@iZ28g26851kZ$c58[root@iZ28g26851kZ

$ (arithmetic expression)

Cases:

[root@iZ28g26851kZ ~]# c=$(($a*2+$b*2))[root@iZ28g26851kZ$c70[root@iZ28g26851kZ

Expr

Expr is generally used for integer operations, which are automatically rounded and removed directly after the decimal point.
And there is a space between the operand and the operation symbol, and if there is no space, it will look like this

[root@iZ28g26851kZ ~]# expr 10/310/3[root@iZ28g26851kZ ~]# 

Output as is, so you have to add a space
Cases:

[root@iZ28g26851kZ ~]# expr 10 / 33[root@iZ28g26851kZ ~]# expr 10 + 313[root@iZ28g26851kZ ~]# expr 10 - 37[root@iZ28g26851kZ ~]# expr 10 * 3expr: syntax error

Can see, add, subtract, except, there is no problem, but the multiplication operation actually said grammatical error,
Okay, here's expr. * When matching characters are used, you need to escape

[root@iZ28g26851kZ ~]# expr 10 \* 330

OK, so it's done.

File test

-e FILE: 测试文件是否存在-f FILE: 测试文件是否为普通文件-d FILE: 测试指定路径是否为目录-r FILE: 测试当前用户对指定文件是否有读取权限-w FILE: 测试当前用户对指定文件是否有写入权限-x FILE: 测试当前用户对指定文件是否有执行权限

Example: Determine if a file exists, the number of rows is displayed

#!/bin/bashFILENAME=/etc/passwdif-e$FILENAMEthen   echo"文件存在"   echo"文件一共有` wc -l $FILENAME | cut -d"" -f1`行"else   echo"文件不存在"fi

How to exit Scripts halfway

Use Exit #即可 where you want to exit, #号为一个退出状态 (a value of 0-255).

0:表示脚本执行成功1-255:表示执行失败也可以什么都不返回，脚本会默认返回上一条结果的执行状态

Cases:
exittest.sh

#!/bin/bashecho"第一行"echo"第二行"exit18echo"第三行"

Execution results

[root@iZ28g26851kZ ~]# ./exittest.sh 第一行第二行[root@iZ28g26851kZ ~]# echo $?18[root@iZ28g26851kZ ~]# 

Show the process of executing a script

Sometimes, when a script is too long and cannot be quickly found after an error, you can use it bash -x xxx.sh to execute the script, which displays the process of executing the script.
Example: Judging whether a file exists

filetest.sh

#!/bin/bash#FILENAME=/etc/passwdif-e$FILENAME ];then  echo"文件存在"else  echo"文件不存在"fi

Perform

[[email protected] ~]# bash -x filetest.sh ‘[‘-e‘]‘echo $‘\346\226\207\344\273\266\345\255\230\345\234\250‘文件存在[[email protected] ~]# 

This makes it easy to check the script.

Character Test Equality comparison

stringstring ]

Remember, [space string space = = Space string space]
No one can not do oh ~ ~
Cases:

[root@iZ28g26851kZ"asd""asdd" ][root@iZ28g26851kZ$?1[root@iZ28g26851kZ"asd""asd" ][root@iZ28g26851kZ$?0[root@iZ28g26851kZ

Unequal comparison

stringstring ]

Cases:

[root@iZ28g26851kZ"asd""asd" ][root@iZ28g26851kZ$?1[root@iZ28g26851kZ"asd""asdd" ][root@iZ28g26851kZ$?0[root@iZ28g26851kZ

Linux-based Shell programming (2)-conditional judgment, arithmetic operations, testing