How to use if-else in shell script
It is easy to see the syntax of a function and believe you know how to use it. But it is always a better choice to understand a function through examples because they help you understand the role that different aspects of a function play.
Here are some useful examples of if-else in shell scripts to give you a better idea of how to use this tool.
| Command | Description |
|---|---|
| && | Logical AND |
| $0 | Argument 0 i.e. the command that’s used to run the script |
| $1 | First argument (change number to access further arguments) |
| -eq | Equality check |
| -ne | Inequality check |
| -lt | Less Than |
| -le | Less Than or Equal |
| -gt | Greater Than |
| -ge | Greater Than or Equal |
1. Using if-else to check whether two numbers are equal
When trying to understand the working of a function like if-else in a shell script, it is good to start things simple. Here, we initialize two variables a and b, then use the if-else function to check if the two variables are equal. The bash script should look as follows for this task.
#!/bin/bash
m=1
n=2
if [ $n -eq $m ]
then
echo "Both variables are the same"
else
echo "Both variables are different"
fi
Output:
Both variables are different
2. Using if-else to compare two values
The more common use of if-else in shell scripts is for comparing two values. Comparing a variable against another variable or a fixed value helps is used in a variety of cases by all sorts of programmers.
For the sake of this example, we will be initializing two variables and using the if-else function to find the variable which is greater than the other.
#!/bin/bash a=2 b=7 if [ $a -ge $b ] then echo "The variable 'a' is greater than the variable 'b'." else echo "The variable 'b' is greater than the variable 'a'." fi
Output:
The variable 'b' is greater than the variable 'a'.
3. Using if-else to check whether a number is even
Sometimes we come across situations where we need to deal with and differentiate between even and odd numbers. This can be done with if-else in shell scripts if we take the help of the modulus operator.
The modulus operator divides a number with a divisor and returns the remainder.
As we know all even numbers are a multiple of 2, we can use the following shell script to check for us whether a number is even or odd.
#!/bin/bash n=10 if [ $((n%2))==0 ] then echo "The number is even." else echo "The number is odd." fi
Output:
The number is even
As you can see, we’ve enclosed a part of the condition within double brackets. That’s because we need the modulus operation to be performed before the condition is checked.
Also, enclosing in double brackets runs statements in C-style allowing you to process some C-style commands within bash scripts.
4. Using if-else as a simple password prompt
The if-else function is known for its versatility and range of application. In this example, we will use if-else in shell script to make the interface for a password prompt.
To do this, we will ask the user to enter the password and store it in the variable pass.
If it matches the pre-defined password, which is ‘password’ in this example, the user will get the output as -“The password is correct”.
Else, the shell script will tell the user that the password was incorrect and ask them to try again.
#!/bin/bash echo "Enter password" read pass if [ $pass="password" ] then echo "The password is correct." else echo "The password is incorrect, try again." fi
5. นับจำนวนไฟล์ว่าเท่ากับจำนวนที่กำหนดหรือไม่
#!/bin/bash a=NUM_FILE=`find . -type f -print | wc -l` b=4 if [ $a -eq $b ] then echo "Exactly the same." else echo "Wrong number" fi
6. start, stop process
ถ้า process ไม่ได้รันอยู่ ให้ start process
#!/bin/bash a=`ps -ef | grep PROCESS_NAME | wc -l` b=1 if [ $a -gt $b ] then echo "Found existing process" else echo "Not found existing, START PROCESS_NAME" start.sh fi
ถ้า process รันอยู่ ให้ stop process
#!/bin/bash a=`ps -ef | grep PROCESS_NAME | wc -l` b=1 if [ $a -gt $b ] then echo "Found existing, STOP PROCESS_NAME " stop.sh else echo "Process not found" fi