If condition judgment in shell language

1、if Basic syntax :

if [ command ];then
    A statement executed under this condition 
elif [ command ];then
    A statement executed under this condition 
else
    A statement executed under this condition 
fi

2、 String judgment

When the string batch is small , Variables must use Double quotes .

[ -z "str" ]  If str The length of is zero , It is true  , That is to judge whether it is empty , Empty is true ;
[ -n "str" ]  If str The length of is non-zero , It is true  , That is, judge whether it is non empty , Not empty is true ;
[ "str1" = "str2" ]  If two strings are the same , It is true ;
[ "str1" != "str2" ]  If the strings are different , It is true ;
[ "str1" ]　  If the string is not empty , It is true , And -n similar .

Example ：

if [ -z "$path" ]; then
    echo "head  Please specify  path  Value " 1>&2    
fi

if [ "$opt" != "-" ]&&[ "$opt" != "+" ]; then
	echo "opt Only value - or +" 1>&2
	exit 1;
fi

3、 Numerical judgment

Never use the greater than sign for numerical comparison 、 Less than no. , Numerical comparisons can only be made using -eq 、-gt etc. .

[ num1 -eq num2 ]      num1  and  num2  It is true that two numbers are equal  , =
[ num1 -ne num2 ]      num1  and  num2  Two numbers are not equal to true  ,!=
[ num1 -gt num2 ]      num1  Greater than  num1  It's true  , >
[ num1 -ge num2 ]      num1  Greater than or equal to num2  It's true , >=
[ num1 -lt num2 ]      num1  Less than n um2  It's true  , <
[ num1 -le num2 ]      num1  Less than or equal to  num2  It's true , <=

Example ：

if [ ${num1} -gt ${num2} ]; then  
	echo "[ Relative catalog ] The length of should be less than [ File path ] The length of "
	exit 1
fi

4、 Judgment of files and directories

[ -e file ]  If  file  There is , It is true .
[ -d DIR ]   If  file  There is , And it's a directory , It is true .
[ -f file ]  If  file  There is , And it's an ordinary file , It is true .

[ -r file ]  If  file  Exist and readable , It is true .
[ -w file ]  If  file Exists and is writable , It is true .
[ -x file ]  If  file  Existing and executable , It is true .

[ -b file ]  If  file  There is , And is a block special file , It is true .
[ -c file ]  If  file  There is , And it's a word special file , It is true .


[ -g file ]  If  file  Exists and has been set SGID, It is true .
[ -k file ]  If  file  The adhesive bit exists and has been set , It is true .
[ -p file ]  If  file  There is and is a name pipeline (F If O), It is true .


[ -s file ]  If  file  Exist and not of size 0, It is true .
[ -t FD ]  If the file descriptor  FD  Open and point to a terminal , It is true .
[ -u file ]  If  file  Exist and set SUID (set user ID), It is true .

[ -O file ]  If  file  Exists and is a valid user ID, It is true .
[ -G file ]  If  file  Exists and belongs to a valid user group , It is true .
[ -L file ]  If  file  Exists and is a symbolic connection , It is true .
[ -N file ]  If  file  There is  and has been mod, If ied since it was last read It is true .
[ -S file ]  If  file  Exists and is a socket , It is true .
[ file1 -ot file2 ]  If  file1  Than  file2  To the old ,  perhaps  file2  Exist and  file1  Do not save , It is true in .
[ file1 -ef file2 ]  If  file1  and  file2  Point to the same device and node number , It is true .

5、 Complex logical judgment

operation	explain
-a	And
-o	or
!	Not

Example 1： And Use

If a>b And a< c

if (( a > b )) && (( a < c ))

perhaps

if [[ $a > $b ]] && [[ $a < $c ]]

perhaps

if [ $a -gt $b -a $a -lt $c ]

in addition ,“||“ and ”&&” stay shell It can be used in , As shown below ：

if [ a>b && a < c ]

Example 2： or Use

If a>b or a < c

if (( a > b )) || (( a < c ))

perhaps

if [[ $a > $b ]] || [[ $a < $c ]]

perhaps

if [ $a -gt $b -o $a -lt $c ]

in addition ,“||“ and ”&&” stay shell It can be used in , As shown below ：

if [ a>b || a < c ]

5、 give an example

#!/bin/bash
#echo -n "please input your score:"
#read score
#echo "input score is $ score "

read -p "please input a score:" score
echo -e "your score [$score] is judging by sys now"

if [ "$score" -ge "0" ]&&[ "$score" -lt "60" ];then
    echo "sorry,you are lost!"
elif [ "$score" -ge "60" ]&&[ "$score" -lt "85" ];then
    echo "just soso!"
elif [ "$score" -le "100" ]&&[ "$score" -ge "85" ];then
    echo "good job!"
else
    echo "input score is wrong , the range is [0-100]!"
fi

6、 Conditional variable substitution :

Bash Shell You can perform conditional substitution of variables , That is, only when certain conditions occur , The replacement condition is placed in {} in .
(1) ${value:-word}

When the variable is undefined or the value is empty , The return value is word The content of , Otherwise, return the value of the variable .

(2) ${value:=word}

Similar to the former , Only if the variable is undefined or the value is empty , Back in word Value , At the same time word Assign a value to value

(3) ${value:?message}

If a variable is assigned a value , Normal replacement . otherwise , The message is message Send to standard error output .（ If this substitution appears in Shell In the program , Then the program will terminate ）

(4) ${value:+word}

If a variable is assigned a value , Only its value can be used word Replace , otherwise , No replacement .

(5) ${value:offset}, ${value:offset:length}

Extract substrings from variables , here offset and length It can be an arithmetic expression .length When not specified , The default is from offset Length from start to end .

path="C:\Users\admin\AppData\Roaming"
echo ${path:4}
 The result is  sers/admin/AppData/Roaming

path="C:\Users\admin\AppData\Roaming"
echo ${path:0:2}
 The result is  C:


path="C:\Users\admin\AppData\Roaming"
echo ${path:0-7}
Roaming

(6) ${#value}

The number of characters in the variable , length .

(7) ${value#pattern}, ${value##pattern}

Get rid of value China and pattern Matching parts , On the condition that value Start with pattern Match , # And ## The difference is that one is the shortest matching pattern , One is the longest matching pattern .

(8) ${value%pattern} , ${value%%pattern}

On (7) similar , Just from value The tail of pattern Match , % And %% The difference between # And ## equally

(9) ${value/pattern/string}, ${value//pattern/string}

Replace the contents of the variable , with pattern Replace the matching part with string The content of , / And // The difference is the same as above .

path="C:\Users\admin\AppData\Roaming"
path=${path//'\'//}
echo $path

The input result is

C:/Users/admin/AppData/Roaming