Conditional structure in Linux bash/shell script – if else

We use the if expression to check the pattern or command status and accordingly we can make certain decisions to execute scripts or commands.

The syntax of the if conditional is as follows:

if     command 
then 
	command 
	command 
fi

From the preceding syntax, we can clearly understand the working of the if conditional construct. Initially, if will execute the command. If the result of command execution is true or 0, then all the commands that are enclosed between then and fi will be executed. If the status of command execution after if is false or non-zero, then all the commands after then will be ignored and the control of execution will directly go to fi.

Let’s learn another variation of if constructs.

Syntax:

if command 
then 
	command 
	command 
else 
	command 
fi

In the preceding case, if the command after if is successfully executed or the status variable ? content is 0, then all the commands after then will be executed. If the result of the command is a failure or non-zero, then all the commands after else will be executed.

For numeric or string expression evaluations using if, the syntax is as follows:

if [ string/numeric expression ] 
then 
	command 
fi

Alternatively, use the following syntax:

if [[ string expression ]] 
then 
	command 
fi

Alternatively, use the following syntax:

if (( numeric expression )) 
then 
	command 
fi

A simple example of checking the status of the last command executed using the if construct is as follows:

#!/bin/bash 
if [ $? -eq 0 ] 
then 
	echo "Command was successful." 
else 
	echo "Command was successful." 
fi

Whenever we run any command, the exit status of the command will be stored in the ? variable. The preceding construct will be very useful in checking the status of the last command.

Numerical handling if constructs

Let’s learn about using the if construct for numerical decision-making.

We can use the test command for finding which variable contains the smaller value:

$ X=10 
$ y=20 
$ (( x < y )) 
$ echo $? 
0 
The result 0 shows that x is smaller than y.

In the shell script if_01.sh, we can use the test command along with the if construct for checking the equality of variables with numerical values as follows:

if_01.sh

#!/bin/bash 
a=100 
if [ $a -eq 100 ] 
then 
	echo "a is equal to $a" 
else 
	echo "a is not equal" 
fi

Let’s test the following program:

$ chmod +x if_01.sh
$ ./if_01.sh

The following will be the output after executing the preceding commands:

Output:

a is equal to 100

Use the script if_02.sh to check which product is costly. The script is as follows:

if_02.sh

#!/bin/bash 
echo "Enter the cost of product a" 
read a 
echo "Enter the cost of product b" 
read b 
 
if [ $a -gt $b ] 
then 
 echo " a is greater" 
else 
 echo " b is greater" 
fi

Let’s test the following program:

$ chmod +x if_02.sh
$ ./if_02.sh

The following will be the output after executing the preceding commands:

Output:

Enter the cost of product a100
Enter the cost of product b150 
b is greater
$

Using the exit command and the ? variable

If we need to terminate the shell script and come back to the command line, then we can use the exit command. The syntax is very simple:

exit 0

The given command will terminate the shell script and return to the command line. It will store the 0 value in the ? status variable. We can use any value between 0 and 255. Value 0 means success, and any other non-zero value means an error. We can use these values to indicate error information.

The script to check the value of a parameter that is passed along with the command (either less than 0 or greater than 30) is as follows. This will save us from using the nested if statement:

#!/bin/bash 
if (( $1 <  0 || $1 > 30 )) 
then 
	echo "mdays is out of range" 
	exit 2 
fi

The test command used in the preceding expression for OR can be written as follows:

[ $1 -lt 0 -o $1 -gt 30 ]

String handling with the if construct

Let’s learn about using string-related checking using the if expression.

The following script, if_03.sh, will check the equality of two strings:

if_03.sh

echo "Enter the first string to compare" 
read name1 
echo "Enter the Second string to compare" 
read name2 
 
if [ "$name1" == "$name2" ] 
then 
    echo "First string is equal to Second string" 
else 
    echo "Strings are not same" 
fi

Let’s test the following program:

$ chmod +x if_03.sh
$ ./if_03.sh

The following will be the output after executing the preceding commands:

Output:

$ ./ if_03.sh 
Enter the first string to compare 
LEVANA 
Enter the Second string to compare 
TECHNOLOGIES 
Strings are not same 
$ ./ if_03.sh

The following will be the output after executing the preceding commands:

Output:

Enter the first string to compare 
LEVANA 
Enter the Second string to compare 
LEVANA 
First string is equal to Second string 
$

We will write the script for performing various other string operations using a test. Let’s write the script if_04.sh to compare two strings for various attributes:

if_04.sh

#!/bin/bash 
 
str1="Ganesh" 
str2="Naik" 
 
if [ $str1 = $str2 ] 
then 
    echo "Two Strings Are Equal" 
fi 
 
if [ $str1 != $str2 ] 
then 
    echo "Two Strings are not equal" 
fi 
 
if [ $str1 ] 
then 
    echo "String One Has Size Greater Than Zero" 
fi 
 
if [ $str2 ] 
then 
    echo "String Two Has Size Greater Than Zero" 
fi

Let’s test the following program:

$ chmod +x if_04.sh
$ ./if_04.sh

The following will be the output after executing the preceding commands:

Output:

Two Strings are not equal
String One Has Size Greater Than Zero
String Two Has Size Greater Than Zero

If we want to verify whether the entered password is valid, then script if_05.sh will be as follows:

if_05.sh

#!/bin/bash 
stty -echo        # password will not be printed on screen 
read -p "Please enter a password  :" password 
if test "$password" == "Abrakadabra" 
then 
	echo "Password is matching" 
fi 
stty echo

Let’s test the following program:

$ chmod +x if_05.sh
$ ./if_05.sh

The following will be the output after executing the preceding commands:

Output:

$ ./if_05.sh
Please enter a password  :  levana
$ ./if_05.sh
Please enter a password  : Abrakadabra
Password is matching
$

Checking for null values

Many a time we need to check the value of variable, such as whether it is null. The null value means zero value. If we want to create the string with the null value, then we should use double quotes ("") while declaring it:

if [ "$string" = "" ] 
then 
	echo "The string is null" 
fi

We can even use [ ! "$string" ] or [ -z "$string" ] for null checking of strings.

Let’s write the script if_08.sh, which will search for the entered person’s name and tell us whether the user is on the computer system:

if_08.sh

#!/bin/bash 
read -p "Enter a user name : " user_name 
 
# try to locate username in in /etc/passwd 
# 
grep "^$user_name" /etc/passwd > /dev/null 
 
status=$? 
 
if test $status -eq 0 
then 
    echo "User '$user_name' is found in /etc/passwd." 
else 
    echo "User '$user_name' is not found in /etc/passwd." 
fi

Let’s test the following program:

$ chmod +x if_08.sh
$ ./if_08.sh

The following will be the output after executing the preceding commands:

Output:

Enter a user name : ganesh
User 'ganesh' is not found in /etc/passwd.

In the preceding script, we are searching for the username in the /etc/passwd file. If a person’s name is not found in the /etc/passwd file, then we can conclude that the username has not been created in the system.

Let’s write a script to check the disk space being used. The script will print a warning if 90 percent or more of the disk space is used on one of the mounted partitions.

The shell script if_09.sh for solving the disk filesystem usage warning will be as follows:

if_09.sh

#!/bin/bash 
df -h | grep /dev/sda1 | cut -c 35-36 > log.txt 
read usage < log.txt 
if [ $usage -gt 80 ] 
then 
    echo "Warning - Disk file system has exceeded 80% !" 
    echo "Please move extra data to backup device." 
else 
    echo "Good - You have enough disk space to continue working !" 
fi 

Let’s test the following program:

$ chmod +x if_09.sh
$ ./if_09.sh

If the preceding program does not work, due to some hardware differences,  then make the following changes to the script:

  • Check to see whether your partition for storage is sda1, sda2, or any other by entering the $df -h command.
  • Check whether the % disk utilization value is at character count 35 and 36. If not, then make changes in the code accordingly.

Using the df command, we get the disk filesystem usage information. The grep command is filtering the hard disk partition, which contains our data. Then, we filter the disc % utilization number and store that value in the log.txt file. Using the read command, we read the % utilization and store it in the usage variable. Later on, using the if command, we check and warn the user if the % utilization is greater than 80.

File handling with the if command

You have already learned about how to use the test command for checking various file operations such as checking the file’s permissions and similar other attributes. A command’s task in any script is to check whether the file or folder is present or not. Then, accordingly, we need to proceed. We will see how to use the if command along with the test command.

Use the simple script if_10.sh to check whether the file exists or not in the current directory as follows:

if_10.sh

#!/bin/bash 
read filename 
if test -e $filename 
then 
    echo "file exists" 
else 
    echo " file does not exist" 
fi

Let’s test the program as follows:

$ chmod +x if_10.sh
$ ./if_10.sh

The following will be the output after executing the preceding commands:

Output:

sample.txt
file does not exist
$ touch sample.txt
$ ./if_10.sh
sample.txt
file exists

First, we checked without the file. Then, we created a file with the touch command. We can very easily check for the presence of the file.

Let’s learn how to use the if command to check various file attributes, such as whether it exists, whether it has file permissions to read, write, execute, and similar by writing script if_11.sh as follows:

if_11.sh

#!/bin/bash 
echo "$1 is: " 
if ! [ -e $1 ] 
then 
  echo "..Do not exists" 
  exit 
else 
  echo "file is present" 
fi 
 
if [ -x $1 ] 
then 
  echo "..Executable" 
fi 
 
if [ -r $1 ] 
then 
  echo "..Readable" 
fi 
 
if [ -w $1 ] 
then 
  echo "..Writable" 
fi

Let’s test the following program:

$ chmod +x if_11.sh
$ ./if_11.sh sample.txt

This should be the output:

Output:

sample.txt is:"file is present"..Readable..Writable

The shell script if_12.sh for performing the file copy operation and then checking whether the copy operation was successful will be as follows:

if_12.sh

#!/bin/bash 
file1="File1" 
file2="File2" 
if cp $file1 $file2 
then 
  echo "Copy Command Executed Successfully" 
  echo "Content of file named Fil1 copied in another file named File2" 
else 
  echo "Some problem in command execution" 
fi

Let’s test the program:

$ chmod +x if_12.sh
$ ./if_12.sh

The following will be the output after executing the preceding commands:

Output:

$ touch File1
$ ./if_12.sh
Copy Command Executed Successfully
Content of file named Fil1 copied in another file named File2

Multiple test commands and if constructs

These type of constructs enable us to execute the second command depending on the success or failure of the first command:

command1    &&    command2 
command1    ||    command2

Let’s write script if_13.sh. In this script, we will ask the user to input two numbers. Then, the if statement will evaluate two expressions. If both are true, then the command after then will be executed; otherwise, commands after else will be called:

if_13.sh

#!/bin/bash 
echo "Enter the first number" 
read val_a 
echo "Enter the Second number" 
read val_b 
 
if [ $val_a == 1 ] && [ $val_b == 10 ] 
then 
  echo "testing is successful" 
else 
  echo "testing is not successful" 
fi

Let’s test the program:

$ chmod +x if_13.sh
$ ./if_13.sh

The following will be the output after executing the preceding commands:

Output:

Enter the first number10
Enter the Second number20
testing is not successful
$ ./if_13.sh
Enter the first number1
Enter the Second number10
testing is successful

Sometimes, we may need to enter a command to check whether the file has the execute permission ? If it is executable, then the file should be executed. The script for such a requirement will be as follows:

test -e file &&  .  file.

Let’s learn one more example of && and multiple expressions using the test command. In the following script, if_14.sh, we will check whether file_one is present, then we will print Hello and then immediately we will check whether file_two is present. Then we will print there on the screen:

if_14.sh

#!/bin/bash 
 
touch file_one 
touch file_two 
 
if [ -f "file_one" ] && echo "Hello" && [ -f file_two ] && echo  "there" 
then 
  echo  "in if" 
else 
  echo    "in else" 
fiexit 0

Let’s test the program:

$ chmod +x if_14.sh
$ ./if_14.sh

The following will be the output after executing the preceding commands:

Output:

Hellotherein if

The following script, if_15.sh, will check file permissions such as read, write, and execute in the same if command using multiple && with the test command:

if_15.sh

#!/bin/bash 
echo "Please enter file name for checking file permissions" 
read file 
if [[ -r $file && -w $file && -x $file ]] 
then 
     echo "The file has read, write,and execute permission" 
fi

Let’s test the program:

$ chmod +x if_15.sh
$ touch sample.txt
$ chmod +rwx sample.txt
$ ./if_15.sh sample.txt

The following will be the output after executing the preceding commands:

Output:

The file has read, write, and execute permissions.

Till now, we have seen multiple expressions using the && logical operator. Now we will see one example with the OR (||) logical operator. In the following script, if_16.sh, we will check the existence of file_one and then we will print Hello on the screen. If the first expression of file checking fails, then the second expression of echo will be executed:

if_16.sh

#!/bin/sh 
if [ -f file_one ] || echo "Hello" 
then 
    echo "In if" 
else 
    echo "In else" 
fi

Let’s test the program:

$ chmod +x if_16.sh
$ ./if_16.sh

The following will be the output after executing the preceding commands:

Output:

helloIn if
$ touch file_one
$ ./if_16.sh

This is the output:

Output:

In if

We checked in the preceding script whether file_one is absent or present.

The if/elif/else command

Sometimes, we need to make a decision on multiple situations or options, such as whether a city is the capital of a country, the state capital, a major city, or a small town. In such situations where, depending on various options, we need to execute different commands, if/else or if/elif/else decision-making commands are useful.

Using the if/elif/else commands, we can have multiple decision-making processes. If the if command succeeds, the command after then will be executed. If it fails, the command after the elif statement will be tested. If that statement succeeds, then statements under the elif are executed. However, suppose none of the elif conditions are true, then statements after the else command are executed. Here, the else block is executed by default. The fi statement will close the if/elif/else command.

The syntax of decision-making using the if elif construct is as follows:

If    expression_1 
then 
    Command 
 
elif 
     expression_2 
then 
    Command 
 
elif 
    expression_3 
then 
    Command 
 
else 
  Command 
 
fi

Let’s write script if_18.sh as follows. In this script, we are checking whether the directory with a given name exists or not. If this fails, then we are checking whether the file with the given name exists. Even if this fails, then we will inform the user that neither the file nor the directory exists with the given name:

if_18.sh

#!/bin/bash 
echo "Kindly enter name of directory : " 
read file 
 
if [[ -d $file ]] 
then 
        echo "$file is a directory" 
elif [[ -f $file ]] 
    then 
    echo "$file is a file." 
 else 
       echo "$file is neither a file nor a directory. " 
fi

Let’s test the program:

$ chmod +x if_18.sh
$ ./if_18.sh

The following will be the output after executing the preceding commands:

Output:

$ ./if_18.sh
Kindly enter name of directory :File1
File1 is a file.
$ mkdir dir1
$ ./if_18.sh
Kindly enter name of directory :dir1
dir1 is a directory
$ ./if_18.sh
Kindly enter name of directory :File007
File007 is neither a file nor a directory.

The null command

In many situations, we may need a command that does nothing and returns a success status such as 0. In such cases, we can use the null command. It is represented by a colon (:). For example, in the if loop, we do not want to add any command if it is successful, but we have certain commands to execute if it fails. In such situations, we can use the null command. This is illustrated in the following if_19.sh script. If we want to loop for ever, then the null command can be used in the for loop:

if_19.sh

#!/bin/bash 
city=London 
if grep "$city" city_database_file >& /dev/null 
then 
      : 
else 
      echo "City is not found in city_database_file " 
      exit 1 
fi

Let’s test the program:

$ chmod +x if_19.sh
$ ./if_19.sh

The following will be the output after executing the preceding commands

Output:

City is not found in city_database_file

We can observe from the preceding script that the colon is a null command and it does nothing.

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