The 12th chapter of the Linux command line and Shell scripting Encyclopedia uses structured commands

Many programs impose some logical control processes on the commands in shell scripts.

Structured commands allow you to change the order in which programs are executed. It's not necessarily in sequence.

12.1 using the If-then statement

The following format:

if command

Then

Commands

Fi

The IF statement will allow the command after the IF, if the command's exit code of 0 (which represents success), the command in the then section will be executed. Otherwise, it is not executed.

Example:

#!/bin/bash

# If then test

If PWD

Then

echo "PWD Success"

Fi

# This is the error cmd, no run then commands

If Thiserrorcmd

Then

echo "Com Success"

Fi

echo "This End"

The then section can use more than one command. The bash shell treats these commands as a block, either not executed or all executed.

Example:

#!/bin/bash

# If then test

Testuser=xcy

If grep $testuser/etc/passwd

Then

echo "This is first command"

echo "This is second command"

echo "I can even put in other commands besides Echo:"

Ls-a/home/$testuser/.b*

Fi

12.2 If–then-else Statements

Another set of commands:

if command

Then

Commands

Else

Commands

Fi

You can also follow more than one command after the else.

Example:

#!/bin/bash

# If then test

Testuser=hshsh

If grep $testuser/etc/passwd

Then

echo "This is first command"

echo "I can even put in other commands besides Echo:"

Ls-a/home/$testuser/.b*

Else

echo "Error:this is first command"

echo "Error:not Find User $testuser"

Fi

12.3 Nested IF

Can do this

if command

Then

If Commands

Then

Commands

Else

Commands

Fi

Else

Commands

Fi

This nesting problem is that the code is difficult to read and it's hard to figure out the logic flow . Here's another way to do this:

If Command1

Then

Command1s

Elif Command2

Then

Command2s

Elif Command3

Then

Command3s

Else

Commands

Fi

Example:

#!/bin/bash

# If-then-elif test, can changed TestUser testuser1

Testuser=xcy

Testuser1=xcy

If grep $testuser/etc/passwd

Then

echo "Step0:i can even put in other commands besides Echo:"

Ls-a/home/$testuser/.b*

elif grep $testuser 1/etc/passwd

Then

echo "Step1:i can even put in other commands besides Echo:"

Ls-a/home/$testuser 1/.b*

Else

echo "Not find user $testuser and $testuser 1"

Fi

12.4 Test Command

Test provides a way of testing different conditions in a if-then statement.

If the conditions listed in the test command are true, the test command exits and returns exit status code 0. Then the if then will execute smoothly.

If it does not, the test command exits and returns a non-0 exit status code. The If-then statement will no longer be executed.

The format is as follows:

if test condition

Then

Commands

Fi

When you have a condition, test tests the condition.

(1) If you do not write the condition section, you will exit with a non-0 exit status code.

You can also write this:

Note that there must be spaces on both sides of the condition .

if [condition]

Then

Commands

Fi

Test three types of conditions can be judged:

Numerical comparison

string comparison

File comparison

12.4.1 Numerical comparison

Here is the table for the numeric comparison command: You cannot use floating-point numbers in the test command

N1–eq N2 equals = =

N1–ge n2 equivalent to >=

N1–GT n2 equivalent to >

N1–le n2 equivalent to <=

N1–lt N2 equivalent to <

N1–ne n2 equals! =

12.4.2 string comparison

STR1 = str2 is the same

Str1! = str2 is not the same

STR1 < str2 str1 is less than str2

str1 > STR2 str1 is greater than str2

-N str1 length is not 0

-Z str1 length is 0

1. String equality

Just use =,! = is all right.

2. String order: There are two considerations

(1) The greater than sign must be escaped, otherwise it will be considered a redirect

(2) is greater than and less than the order and sort are used differently.

The uppercase letters in the comparison test are smaller than lowercase letters. The sort command happens

Example:

#!/bin/bash

# Test Command Test

if test

Then

echo "Step1 true"

Else

echo "Step1 false"

Fi

Num=8

if [$num-ge 8] # and you can pick up –le-eq . Wait a whole lot.

Then

echo "Step2 num >= 8"

Else

echo "Step2 num not >= 8"

Fi

Testuser=xcddy

if [$USER! = $testuser]

# if [$USER = $testuser]

Then

echo "step3, user not $testuser"

Else

echo "Step3, User is $testuser"

Fi

Str1=test1 # Small

Str2=test1 # Big

if [$str 1 \> $str 2] # need to be escaped

Then

echo "Step4, $str 1 > $str 2"

Else

echo "Step4, $str 1 <= $str 2"

Fi

str3=

If [-Z $str 3]

#if [-N $str 3]

Then

echo "Step5, $str 3 len is 0"

Else

echo "Step5, $str 3 len is not 0"

Fi

12.4.3 file Comparison

Most powerful, but also the most used form of comparison. Allow testing of the status of files and directories on the Liunx file system

Whether the-D file exists and is a directory

Whether the-e file exists

Whether the-f file exists and is a file

-R file exists and is readable

-s file exists not empty

Whether the-W file exists and is writable

-X file exists and is executable

Whether the-O file exists and is owned by the current user//uppercase O

The-G file exists and the default group is the same as the current user

File1–nt file2 file1 is newer than file2

File1–ot File2 file1 is older than file2

Example:

#!/bin/bash

Jump_dir=testdir

If [-D $jump _dir]

Then

echo "$jump _dir is exist.

CD $jump _dir

Echo-n "now PWD:"

Pwd

Else

echo "$jump _dir is not exist, should create Dir"

mkdir $jump _dir

echo "Change Dir to $jump _dir"

CD $jump _dir

Echo-n "now PWD:"

Pwd

Fi

Test_file=moduleok

If [-e $test _file]

Then

echo "$test _file is exist"

MV Moduleok Running

Sleep 3

MV Running Moduleok

Else

echo "$test _file isn't exist, touch this:"

Touch $test _file

MV Moduleok Running

Sleep 3

MV Running Moduleok

Fi

echo "Check up dir:"

Cd..

Echo-n "now PWD:"

Pwd

Run_test=test2

If [-X $run _test]

Then

echo "You can run the script: $run _test"

./$run _test

Else

echo "You is unable to execute the script: $run _test"

Fi

File1=test1

File2=test5

If [$file 1-nt $file 2]

Then

echo "$file 1 is new, $file 2 was old"

Else

echo "$file 1 is old, $file 2 is new"

Fi

12.5 Qualifying Tests

If-then allows you to combine tests using Boolean logic:

(1) [Condition1] && [condition2]: Use and Boolean operator to combine two conditions

(2) [Condition1] | | [Condition2]: Use or to combine two conditions

Advanced features of 12.6 If-then

Provides two advanced features that you can use in the If-then statement:

(1) Double brackets for mathematical expressions

(2) Two-side brackets for advanced string processing functions

12.6.1 using double brackets

The test command can only use simple arithmetic operations in comparisons.

Allows you to use advanced mathematical expressions in the comparison process. More mathematical symbols are provided.

val++, val--after increase and decrease

++val,--val first increase first minus

! Logical negation

-Position Negation

* * Power operation

<< left displacement >> right displacement

& Bit Boolean and | Bit Boolean or

&& Logic and | | Logical OR

The greater-than sign inside the double brackets does not need to be escaped

12.6.2 using both brackets

Note that not all shells support both brackets.

A regular expression can be defined in a pattern match to match a string

[[Expression]]

Expression uses the standard string comparison used in the test command, but it provides another feature not provided by the test command – pattern matching

Example:

#!/bin/bash

# (()) test

val=10

#val =5

if ((Val * * 2 > 90))

Then

echo "$val ^ 2 > 90"

((val2 = $val * * 2))

echo "Result: $val ^ 2 is $val 2"

Else

echo "$val ^ 2 < 90"

Fi

Str1=xcyhaha

if [[$str 1 = = xcy*]]

Then

echo "str1 = = xcy*, str1 is $str 1"

Else

echo "STR1! = xcy*, str1 is $str 1"

Fi

12.7 Case Command

With the case command, you don't have to write all the elif statements to keep checking the value of the same variable.

The case command takes a list format to examine multiple values for a single variable.

Syntax: Note the two semicolons that follow

Case variable in

Pattern1 | PATTERN2) commands1;;

PATTERN3) commands2;;

*) default commands;;

Esac

The case command compares the specified variable to a different pattern. If the variable and pattern match, then the shell executes the command specified for the change mode.

Multiple pattern modes can be split in a line by a vertical bar operation noon.

* Asterisks capture all values that do not match the known pattern.

The case command provides a clearer way to specify different options for each possible value of a variable.

Example:

#!/bin/bash

var=68

case $ var in

3)

        echo "var = 3" ;

4)

        echo "var = = 4"

5)

        echo "var = = 5"

7|6)

        echo "var = = 6|7" ;;

*)

        echo "var defaule" Span style= "COLOR: #ff0000" >;

Esac

The Linux command line and Shell script programming encyclopedia 12th use structured commands