Shell Basic Statement

Shell operators and basic statements

1. Operators

Operator	Describe	Example
File comparison Operators
-e filename	True if filename exists	[-e/var/log/syslog]
-D filename	True if filename is a directory	[-d/tmp/mydir]
-F filename	True if filename is a regular file	[-f/usr/bin/grep]
-L filename	True if filename is a symbolic link	[-l/usr/bin/grep]
-R filename	True if filename is readable	[-r/var/log/syslog]
-W filename	True if filename is writable	[-w/var/mytmp.txt]
-X filename	True if filename is executable	[-l/usr/bin/grep]
Filename1-nt filename2	If filename1 is newer than filename2, it is true	[/tmp/install/etc/services-nt/etc/services]
Filename1-ot filename2	If filename1 is older than filename2, it is true	[/boot/bzimage-ot Arch/i386/boot/bzimage]
String comparison operators (note the use of quotation marks, which is a good way to prevent whitespace from disturbing the code)
-Z String	True if string length is zero	[-Z "$myvar"]
-N String	True if string length is nonzero	[-N "$myvar"]
string1 = string2	True if string1 is the same as string2	["$myvar" = "One of the three"]
String1! = string2	True if string1 is different from string2	["$myvar"! = "one of the three"]
Arithmetic comparison operators
Num1-eq num2	Equals	[3-eq $mynum]
Num1-ne num2	Not equal to	[3-ne $mynum]
Num1-lt num2	Less than	[3-lt $mynum]
Num1-le num2	Less than or equal to	[3-le $mynum]
NUM1-GT num2	Greater than	[3-gt $mynum]
Num1-ge num2	Greater than or equal to	[3-ge $mynum]

Arithmetic operators

+-*/% indicates subtraction and take-rest operations

+ = = *=/= with C language meaning

Bitwise operators

> >>= means move one operation left and right

& &= | |= indicates bitwise AND, bit, or operation

~ ! Represents a non-operational

^ ^= represents XOR or manipulation

Relational operators

= = = = = Greater than, less than, greater than or equal, less than equals, equals, not equal to operation

&& | | Logical AND logical OR operational

Logical Expressions

Logical Non! The opposite of the conditional expression

if [! expression]

if [!-D $num] If there is no directory $num

Logic and –a Side- by-side of conditional expressions

If [expression 1–a-expression 2]

logical OR-o a conditional expression or

If [expression 1–o-expression 2]

2. Basic statements

Dellines=$[${filelines}-3] #使用 "" to the value of the variable filelines-3 to Dellines

1) for Loop

Example 1:

/bin/bash

For I in $ (seq) #将1-10 Number one assignment to variable i

Do echo $i #输出i

Done

Example 2:

#!/bin/bash

For ((i=1;i<=10;i++)) #在1-10, starting from 1, once plus 1

Do Echo $i

Done

Example 3:

#!/bin/bash

For i in a B c #指定循环内容

Do Echo $i

Done

Example 4:

#!/bin/bash

For i in ' ls ' #将ls命令查看到的文件名一次赋给i (under current directory)

Do Echo $i

Done

Example 5: Find out 1-100 numbers that can be divisible by 3

"1" uses the For

#!/bin/bash

Clear #清屏

For ((i=1;i<100;i++)) or for i in ' seq 100 ' or for I in {1..100}

Do if ((i%3==0)) #i除以3等于0

Then

Echo $i

Continue #继续下次循环

Fi

Done

"2" using while

#!/bin/bash

Clear

I=1

while (($i <100)) #条件i <100

Do if ((i%3==0)) #i除以3等于0

Then

Echo $i

Fi

i=$ (($i + 1)) #i加1

Done

2) While statement

While expression

Do

Command

Command

```

Done

Example 1:

#!/bin/bash

Clear

I=1

while (($i <=5)) #条件: i<=5

Do

Echo $i

Let "i++" #i加1

Done

3) If statement

If condition Then Command Else Command Fi

If condition

Then

Command

Elif conditions

Command

Fi

Example 1:

echo "Input your choice:" Read ANS

Note: "Read variable" reads the value of ANS from the keyboard

if [$ANS] Then Echo No empty Else Echo empth Fi

Note: If [$ANS] determines that the string variable is non-null, equivalent to if [-N $ANS]

Grep-w

[email protected] mnt]# cat 1.txt

1

12

123

[[email protected] mnt]# grep 1 1.txt

1

12

123

[Email protected] mnt]# grep-w 1 1.txt

1

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Shell Basic Statement