Shell Programming (ii)-IF judgment and special usage, file directory attribute judgment, case judgment

[TOC]

Shell Programming (ii) One, the logical judgment in Shell Script 1.1 judgment Statement if1.1.1 format 1:

if 判断语句；then    commandfi

Example 1

# vim if01.sh          //判断数值大小第一种方法用[]，注意前后空格#!/bin/basha=5if [ $a -gt 3 ]then   echo okfi[[email protected] ~]# sh if01.shok

• []-GT: Greater Than,
• []-lt: Less Than,
• []-ge: greater than or equal to,
• []-le: Less than or equal to,
• []-eq: Equals,
• []-ne: Not equal to

1.1.2 Format 2:

格式：if 判断语句；then    commandelse   commandfi

[[email protected] ~]# vim if02.sh#!/bin/basha=2if [ $a -gt 3 ]then   echo okelse   echo nookfi

View the execution process

[[email protected] ~]# sh -x if02.sh+ a=2+ ‘[‘ 2 -gt 3 ‘]‘+ echo nooknook

1.1.3 Format 3:

if 判断语句1;then   commandelif 判断语句2;then   commandelse   commandfi

Example:

#!/bin/bashread -p "请输入考试分数:" aif [ $a -lt 60 ]then   echo "重考，未通过考试"elif [ $a -gt 60 ] && [ $a -lt 85 ]then   echo "通过考试，成绩良好"else   echo "通过考试，成绩优秀！ "fi

[[email protected] ~]# sh if03.sh请输入考试分数:80通过考试，成绩良好[[email protected] ~]# sh if03.sh请输入考试分数:97通过考试，成绩优秀！ [[email protected] ~]# sh if03.sh请输入考试分数:54重考，未通过考试

Second, the file directory attribute judgment

• [] [-F file] to determine whether it is a normal file, and there is
• [] [-D file] Determines if it is a directory, and there is
• [] [-e file] Determines whether a file or directory exists
• [] [-R File] to determine whether the document is readable
• [] [-W file] Determines whether the file is writable
• [] [-X file] Determines whether the file is executable

-e filename 如果 filename存在，则为真 [ -e /var/log/syslog ]-d filename 如果 filename为目录，则为真 [ -d /tmp/mydir ]-f filename 如果 filename为常规文件，则为真 [ -f /usr/bin/grep ]-L filename 如果 filename为符号链接，则为真 [ -L /usr/bin/grep ]-r filename 如果 filename可读，则为真 [ -r /var/log/syslog ]-w filename 如果 filename可写，则为真 [ -w /var/mytmp.txt ]-x filename 如果 filename可执行，则为真 [ -L /usr/bin/grep ]

• Example 1

  #!/bin/bashf="/tmp/xavilinux"if [ -f $f ]thenecho $f existelsetouch $ffi

  Execution process

[[email protected] ~]# sh -x 1.sh+ f=/tmp/xavilinux+ ‘[‘ -f /tmp/xavilinux ‘]‘+ touch /tmp/xavilinux[[email protected] ~]# sh -x 1.sh+ f=/tmp/xavilinux+ ‘[‘ -f /tmp/xavilinux ‘]‘+ echo /tmp/xavilinux exist/tmp/xavilinux exist

• Example 2

#!/bin/bashf="/tmp/xavilinux"if [ -r $f ]then   echo $f readablefi

[[email protected] shell]# sh file02.sh/tmp/xavilinux readable

* Common syntax

[[email protected] shell]# vim file.sh#!/bin/bashf="/tmp/xavilinux" [ -f $f ] && rm -f $f等同于以下：if [ -f $f ]then   rm -f $ffi

Third, if special usage

If [-Z "$a"]?? This indicates what happens when the value of variable A is empty

Develop good habits, be sure to add "double quotation marks" to the value of the judgment; if "file" can be omitted

If [-n ' $a '] means that the value of variable A is not empty

[[email protected] ~]# if [ -n iftest1.sh ]; then echo ok; fiok[[email protected] ~]# if [ -n "$b" ]; then echo $b; else echo[[email protected] ~]# if [ -n "$b" ]; then echo $b; else echo "b is null"; fib is null

If Grep-q ' 123 ' 1.txt; Then?? What happens if the 1.txt contains a ' 123 ' row

Grep-w W means word, word
GREP-WQ plus Q do not filter users, directly display the results

[[email protected] ~]# grep -w ‘xavi‘ /etc/passwdxavi:x:1000:1000:xavi,xavi‘s office,62580558,62589906:/home/xavi:/bin/bashxavidsf:x:1001:1001:xavi:/home/xavidsf:/bin/bash[[email protected] ~]# if grep -w ‘xavi‘ /etc/passwd; then echo "xavi exist"; fixavi:x:1000:1000:xavi,xavi‘s office,62580558,62589906:/home/xavi:/bin/bashxavidsf:x:1001:1001:xavi:/home/xavidsf:/bin/bashxavi exist[[email protected] ~]# if grep -wq ‘xavi‘ /etc/passwd; then echo "xavi exist"; fixavi exist

if [!-e file]; Then what happens when the file doesn't exist?

if (($a <1)); Then ... Equivalent to if [$a-lt 1]; Then ...
Symbols such as <,>,==,!=,>=,<= cannot be used in []

Iv. Case Judgment 4.1 format

case  变量名 in value1)  command  ;;value2)  command  ;;*)  commond  ;;esac

If one of the value in the case is the same, we can write this:

在case程序中，可以在条件中使用 |，表示或的意思，  比如2|3)         command      ;;

Script case

#!/bin/bashread -p "Please input a number: " n     //让用户输入一个数字if [ -z "$n" ]              //判断用户有没有输入then    echo "Please input a number."    exit 1fin1=`echo $n|sed ‘s/[0-9]//g‘`       //检查用户输入的是不是全部是数字，不是数字就置空if [ -n "$n1" ]then echo "Please just input a number, without any other words." exit 1fiif [ $n -lt 60 ] && [ $n -ge 0 ]        //经过如上的筛选，我们来判断输入数字属于哪个范围，并且把值交给tagthen    tag=1elif [ $n -ge 60 ] && [ $n -lt 80 ]then    tag=2elif [ $n -ge 80 ]  && [ $n -lt 90 ]then    tag=3elif [ $n -ge 90 ] && [ $n -le 100 ]then    tag=4else     tag=0  //大于100的情况ficase $tag in        //根据如上得到的值，进行判断    1)    echo "you didn‘t pass the exam!"        ;;    2)        echo "good!"        ;;    3)        echo "very good!"        ;;    4)        echo "perfect!!!"        ;;    *)        echo "Pls input a number range 0-100."        ;; esac

Shell Programming (ii)-IF judgment and special usage, file directory attribute judgment, case judgment