7 Structured Command

1. Using the If then statement

if-then syntax

One of the most used statements in scripts is the If ~ then statement. In this syntax, if the execution result of the commands defined after the if (exit code 0) is normally executed, then the command in the then clause is executed. It is not performed. The basic syntax is as follows.

​

- Usage syntax

if command

then

commands

fi

​

if command; then

commands

fi

​

So, let's create an example that lists the user's bash shell files if there is a corresponding user in /etc/passwd.

​

$touch test10

$gedit test10

#! /bin/bash

#if then statement Executing multiple commands

testuser=hyowon

if grep $testuser /etc/passwd

then

echo "The bash files for user $testuser are:"

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

fi

#

​

and if you run​

$ ./test10

hyowon:x:1000:1000:hyowon,,,:/home/hyowon:/bin/bash

The bash files for user hyowon are:

/home/hyowon/.bash_history /home/hyowon/.bash_logout /home/hyowon/.bashrc

​

You can see that the then statement is executed normally. However, if you test with a value that does not exist in the passwd file in testuser, there is no output as shown below.

$ ./test10

$

​

elseif syntax

You can think of the elseif statement as an extension of the if statement. I think that the fact that you can use multiple conditional ifs is better than if statements. And the usage syntax is as follows. Like the if statement, when the exit code of the elseif statement is 0 (when the command execution result is normal), the then statement is executed.

​

if command1

then

commands

elif command2

then

more commands

fi

​

2. Using the Test command

In the if-then-elseif statement, the exit code determines whether or not to execute the command, but there may be cases where you want to execute the then command as the result of the conditional clause comparison. As for the usage syntax, you can give the condition to be tested after the test command as shown below. The test command checks the following conditional clause and returns an exit code of 0 if true and 1 if false. The disadvantage of test command is that test command cannot handle floating point. If you use it to handle floating point like the bc command, you won't get the desired result because test doesn't support floating point. Also, instead of the test command, it is possible to use conditional clauses by grouping them with brackets.

​

The conditions that can be compared with the test are the following three cases.

- number comparison

- string comparison

- Compare files

​

construction

test condition or if [ condition ]

==> A space must be inserted between the bracket and the Condition.

then

commands

fi

​

Numeric comparison example

$touch test11

$gedit test11

#Number and Variable Comparison Example

val1=15

val2=20

if [ $val1 -gt 5 ]

then

echo "The test value $val1 is greater than 5"

fi

if [ $val1 -eq $val2 ]

then

echo "The values are equal"

else

echo "The values are different"

fi

​

Then let's run it.

$ ./test11

The test value 15 is greater than 5

The values are different

​

The table below is the comparison operator table.

​String comparison

The test command can also compare strings. Let's create a simple example as below.

​

#! /bin/bash

# test string comparison example

tempuser=hyowon

​

if [ $USER = $tempuser ]

==> Compare whether the currently logged in user and tempuser name are the same

then

echo "Welcome $tempuser"

fi

#

​

let's run

$ ./test13

Welcome hyowon

​

Below are the operators that can be used for string comparison.

​

string order

When performing string comparison, you must pay attention to the following two points.

​

1) A backslash must be entered before the > and < comparison operators so that it is not recognized as a redirect.

​

2) The result of >, < is not the same as sort naming. The comparison operator does not compare in ASCII code, but sort sorts in the order of system location language setting.

​

Below is an example.

$ cat test9b

#!/bin/bash

# testing string sort order

val1=Testing

val2=testing

if [ $val1 \> $val2 ]

then

echo "$val1 is greater than $val2"

else

echo "$val1 is less than $val2"

fi

$ ./test9b

Testing is less than testing

$ sort testfile

testing

Testing

$

​

String length comparison

Let's create an example string length comparison.

#! /bin/bash

#String length comparison example

val1=test

val2=' '

​

if [ -n $val1 ]

then

echo "The string 'val1' is not empty"

else

echo "The string '$val1' is empty"

fi

​

if [ -z $val2 ]

then

echo "The string ’$val2’ is empty"

else

echo "The string ’$val2’ is not empty"

fi

​

if [ -z $val3 ]

then

echo "The string ’$val3’ is empty"

else

echo "The string ’$val3’ is not empty"

fi

​

​

let's run

$ ./test14

The string 'test' is not empty

The string ’ ’ is empty

The string ’’ is empty

​

The table below shows operators that compare the lengths of strings.

​

Compare files

The test command can also compare files. First, let's look at an example of directory comparison. The -d option checks whether a directory exists or not.

​

$touch test15

$gedit test15

#! /bin/bash

#example

if [ -d $HOME ]

then

echo "Your Home directory exists"

cd $HOME

ls

else

echo "Threr's problem with your HOME directory"

fi

​

let's run

$ ./test15

Your Home directory exists

1234.mkv

13826761_MotionElements_energetic-action-sport-rock-30sec-powerful-aggressive-background-music_preview-2.mp3

13826761_MotionElements_energetic-action-sport-rock-30sec-powerful-aggressive-background-music_preview.mp3

2020-10-10 15-25-47.mkv

NPKI

Steam

VirtualBox VMs

age.txt

deja-dup

dir.Iu6oW4

dir.d03tYD

​

​

This time, we'll write a script that examines directories and files, creates them if they don't exist, and adds information to them.

​

#! /bin/bash

#After checking for the existence of a file under the directory, add it if it does not exist

​

if [ -e $HOME ]

then

echo "Direcroty exists. let's check file"

#checking file

if [ -e $HOME/testing ]

then

echo " file exists, append data to it"

date >> $HOME/testing

else

echo "creating file"

date > $HOME/testing

fi

else

echo "no Home directory"

fi

​

let's run

$ ./test16

Direcroty exists. let's check file

creating file

$ ./test16

Direcroty exists. let's check file

file exists, append data to it

$

​

The --e option tells whether files and directories exist. You should use the -f option to check if it is a file only. Let's make an example

​

#!/bin/bash

# check if a file

if [ -e $HOME ]

then

echo "The object exists, is it a file?"

if [ -f $HOME ]

then

echo "Yes, it’s a file!"

else

echo "No, it’s not a file!"

if [ -f $HOME/.bash history ]

then

echo "But this is a file!"

fi

fi

else

echo "Sorry, the object doesn’t exist"

fi

​

let's run

$ ./test17

The object exists, is it a file?

No, it’s not a file!

But this is a file!

$

​

File read permission check example

Let's create an example that checks if a file exists, and then checks if it has read permission.

.

#!/bin/bash

#Check if the file is readable

pwfile=/etc/shadow

#First check if the file exists

if [ -f $pwfile ]

then

# Check if it is readable

if [ -r $pwfile ]

then

tail $pwfile

else

echo "Sorry, I’m unable to read the $pwfile file"

fi

else

echo "Sorry, the file $file doesn’t exist"

fi

#

​

let's run

$ ./test18

Sorry, I’m unable to read the /etc/shadow file

$ ./test18

​

Check for empty files

The -s option checks whether the file is empty or not and returns the result. Let's make an example.

​

#!/bin/bash

# testing if a file is empty

file=t15test ==>Enter the value to be used as the file name in the file variable.

touch $file ==> Create a file with input values

​

​

if [ -s $file ] ==>Check if the file is empty

then

echo "The $file file exists and has data in it" ==> This statement will be executed.

else

echo "The $file exists and is empty"

fi

date > $file ===> Puts the result of date into the file value.

if [ -s $file ] ==>Checking the file is not empty

then

echo "The $file file has data in it"

else

echo "The $file is still empty" ==>

fi

#

​

let's run

$./test15

The t15test exists and is empty

The t15test file has data in it

$

​

반응형

Check file write permission

The -w option checks whether the file has write permission. Let's make an example

​

#!/bin/bash

# checking if a file is writeable

logfile=$HOME/t16test

touch $logfile

chmod u-w $logfile ==> Remove write permission.

now=`date +%Y%m%d-%H%M`

==> In the now variable, the execution result of date is formatted and saved.

​

if [ -w $logfile ]

==> If the file exists and you have write permission, then execute the then statement

​

then

echo "The program ran at: $now" > $logfile

echo "The fist attempt succeeded"

else

echo "The first attempt failed"

fi

​

chmod u+w $logfile

if [ -w $logfile ]

then

echo "The program ran at: $now" > $logfile

echo "The second attempt succeeded"

else

echo "The second attempt failed"

fi

​

let's run

$ ./test16

The first attempt failed

The second attempt succeeded

​

$ cat $HOME/t16test

The program ran at: 20070930-1602

$

​

Checking the execute permission of a file

$ cat test17

#!/bin/bash

# testing file execution

if [ -x test16 ]

then

echo "You can run the script:"

else

echo "Sorry, you are unable to execute the script"

fi

​

let's run

$ ./test17

You can run the script:

​

Checking Ownership of a File

-0 is an option to check if the file's owner is the current user. Let's make an example

​

#!/bin/bash

# check file ownsership

if [ -O /etc/passwd ]

then

echo "You’re the owner of the /etc/passwd file"

else

echo "Sorry, you’re not the owner of the /etc/passwd file"

fi

$ ./test18

Sorry, you’re not the owner of the /etc/passwd file

$ su

Password:

# ./test18

You’re the owner of the /etc/passwd file

#

​

Check the group's default group and make sure it's the same as the user's group.

#!/bin/bash

# check file group test

if [ -G $HOME/testing ]

then

echo "You’re in the same group as the file"

else

echo "The file is not owned by your group"

fi

$ ls -l $HOME/testing

-rw-rw-r-- 1 rich rich 58 2007-09-30 15:51 /home/rich/testing

$ ./test19

You’re in the same group as the file

​

$ chgrp sharing $HOME/testing

$ ./test19

The file is not owned by your group

$

​

A script to compare the creation date of two files.

It is often used as a way to compare the installation date of a file and prevent installation when the file to be installed is an outdated version.

​

file1 -nt file2 : run if file1 is not older than file2 file1 -ot file2 : run if file1 is older than file2

file1 -nt file2 : run if file1 is not older than file2 file1

​

#!/bin/bash

# testing file dates

if [ ./test19 -nt ./test18 ]

then

echo "The test19 file is newer than test18"

else

echo "The test18 file is newer than test19"

fi

​

if [ ./test17 -ot ./test19 ]

then

echo "The test17 file is older than the test19 file"

fi

​

let's run

$ ./test20

The test19 file is newer than test18

The test17 file is older than the test19 file

​

$ ls -l test17 test18 test19

-rwxrw-r-- 1 rich rich 167 2007-09-30 16:31 test17

-rwxrw-r-- 1 rich rich 185 2007-09-30 17:46 test18

-rwxrw-r-- 1 rich rich 167 2007-09-30 17:50 test19

$

​

3. Compare multiple conditional clauses

It can be used by attaching multiple conditional clauses. That way, you can do an and operation or an or operation.

​

■ [ condition1 ] && [ condition2 ] ==> condition 1 and condition 2 must be true then statement is executed

​

■ [ condition1 ] || [ condition2 ] ==> If either condition 1 or condition 2 is true, then statement is executed

​

Example

$ cat test22

#!/bin/bash

# testing compound comparisons

if [ -d $HOME ] && [ -w $HOME/testing ]

then

echo "The file exists and you can write to it"

else

echo "I can’t write to the file"

fi

$ ./test22

I can’t write to the file

​

$ touch $HOME/testing

$ ./test22

The file exists and you can write to it

$

​

4. Advanced If-Then function

Among the if-then statements, there are two recently added functions

.

■ Use of parentheses for mathematical expressions

■ Using double brackets for string handling

​

Script example using double brackets

#!/bin/bash

# using double parenthesis

val1=10

if (( $val1 ** 2 > 90 ))

then

(( val2 = $val1 ** 2 ))

echo "The square of $val1 is $val2"

fi

$ ./test23

The square of 10 is 100

$

The types of operators using double parentheses are as follows.

​

​Scripts with double brackets

#!/bin/bash

# using pattern matching

if [[ $USER == r* ]]

then

echo "Hello $USER"

else

echo "Sorry, I don’t know you"

fi

$ ./test24

Hello rich

$

​

5. Case command

In case, the command to be executed in each case can be designated separately.

#!/bin/bash

# looking for a possible value

if [ $USER = "rich" ]

then

echo "Welcome $USER"

echo "Please enjoy your visit"

elif [ $USER = barbara ]

then

echo "Welcome $USER"

echo "Please enjoy your visit"

elif [ $USER = testing ]

then

echo "Special testing account"

elif [ $USER = jessica ]

then

echo "Don’t forget to logout when you’re done"

else

echo "Sorry, you’re not allowed here"

fi

$ ./test25

Welcome rich

Please enjoy your visit

$