Description of the IF, while, for statement for the shell of Linux

I. Basic conditions of judgment

1) Logical operators

-A expr1-a expr2 logic and

-O expr1-o expr2 logic or

! !EXPR1 Logical Non-

2) Numerical judgment

-eq num1-eq num2 is equal

-ne Num1-ne num2 is not equal

-GT NUM1-GT num2 is greater than

-ge Num1-ge num2 is greater than or equal to

-lt Num1-lt num2 is less than

-le Num1-le num2 is less than or equal to

3) string judgment

= STR1 = str2 strings are equal

! = str1! = STR2 strings are unequal

=~ str1 =~ str2 str1 contains str2, note that the entire condition is placed between "[[]]"

-n-n str1 string length is not equal to 0

-z-z str2 whether the string length equals 0

4) file judgment

-r-r filename file exists and is readable

-w-w filename file exists and is writable

-s-s filename file exists and is not 0 in length

-f-f filename file exists and is a normal file

-d-d filename file exists and is a directory

Second, if Judgment statement basic format:

1) If Judgment statement basic format 1:

If [judging condition]

Then

Commands

Else

Fi

Example:

#数值判断:

Read-p "Enter a number (0/1):" num

If [$num-eq 1]

Then

echo "true"

Else

echo "false"

Fi

#字符串判断:

Str1= "This is a string"

if [["$str 1" =~ "This"]

Then

echo "true"

Else

echo "false"

Fi

#文件判断:

If [-F./test1.sh]

Then

echo "true"

Else

echo "false"

Fi

2) If Judgment statement basic Format 2:

If [judging condition]; Then

Commands

elif [judging conditions]; Then

Commands

...

Else

Commands

Fi

#举例:

Read-p "Enter the score:" Score

If ["$score"-lt 0]; Then

echo "Enter a score in 0~100"

elif ["$score"-GT 100]; Then

echo "Enter a score in 0~100"

elif ["$score"-LT 60]; Then

echo "Fail Score"

elif ["$score"-LT 70]; Then

echo "Pass Score"

elif ["$score"-lt 80]; Then

echo "Fair score"

elif ["$score"-LT 90]; Then

echo "Good score"

Else

echo "Excellent score"

Fi

#注意:

1. There must be a space between if and []

2. There must be a space before and after the judging condition

3, then if and if in the same line to use;

4, the judgment condition contains the variable, must use the double quotation mark to cause

Three, while loop statement basic format

1) While loop statement basic format:

#while [Judging condition]

Min=1

max=10

While [$min-le $max]

Do

Echo $min

min= ' expr $min + 1 '

Done

#注意:

1. There must be a space between the while and []

2, before and after the conditions to determine the space

# while (judging condition))

I=1

while (($i <=12))

Do

if (($i%4==0))

Then

Echo $i

Fi

i=$ (($i + 1))

Done

#注意:

1. The internal structure of this type is similar to C language

2, Attention Assignment calculation: i=$ (($i + 1))

Four, for Loop statement basic format

1) For Loop statement basic format:

#数字段形式

For i in {1..10}

Do

Echo $i

Done

#详细列出 (not many items)

For i in ' a ' B ' C ' d '

Do

Echo $i

Done

#seq形式 starting from 1

For i in ' seq 10 '

Do

Echo $i

Done

#语法循环--kind of like C syntax.

for ((I=1; i<+10; i++));

Do

Echo $i

Done

#对存在的文件进行循环

For shname in ' LS *.sh '

Do

echo "$shname" | Awk-f. ' {print '} '

Done

Description of the IF, while, for statement for the shell of Linux