We have recently explored command-line list, we also dived into the test shell builtin command, which can not only be used to check file types but can also be used to compare values. The issue with using those conditions is that aren't easy to read, and it would become difficult to maintain if the complexity of the tasks increases.
In this guide, we would learn how to use if statement to make our script more readable and logical, well, since that is how we think, it can make things a bit easier for us.
The caveat is that it can make the script longer, but since we can achieve more, it doesn't hurt to use if statement. Consider the following, before and after:
#!/bin/bash test -z $1 || echo "Hello $1" exit 0
#!/bin/bash if [ $# -gt 0 ] ; then echo "Hello $1" exit 0 fi echo "Instruction: $0 <firstname>" exit 1
if [ $# -gt 0 ] - '$#' Can be used to get the total number of arguments passed to script,, we then check if the value is gt (greater than) zero, we are basically checking if the argument is set, or the user passed a first positional parameter, we then echo it:
echo "Hello $1", this is the output if I supplied the expected input:
user@server:~/bin$ isset2.sh Devsrealmer Hello Devsrealmer
The end of the if block is denoted with fi - if backwards, If you just run the script as is, without passing the argument, you would get the instructional statement, which assist you on how to use the script, see example:
user@server:~/bin$ isset2.sh Instruction: /home/user/bin/isset2.sh <firstname>
This ensure that we only display the Hello message if we have supplied a name to be welcomed. You might wonder what the exit 1 and exit 0 does:
The exit is used to reflect the true status of the script, 0 stands for success, and anything other than zero stands for failure, so when used, it ensures you are stoping the execution of our script when a condition is satisfied. Imagine, I didn't use the exit 0 when the condition is satisfied, say, I have something like so:
#!/bin/bash if [ $# -gt 0 ] ; then echo "Hello $1" # I didn't add a successful exit here fi echo "Instruction: $0 <firstname>" # I didn't add a failed exit here
Now, even if I supplied the right input, I'll still get the instructional statement:
user@server:~/bin$ isset2.sh Devsrealmer Hello Devsrealmer Instruction: /home/user/bin/isset2.sh <firstname>
This is because I am not telling the script when to exit successfully, it just keeps running! So, by employing exit status,it ensures you are stoping the execution of the script at the right time.
Another way of writing the script is as follows:
#!/bin/bash if [ $# -lt 1 ] ; then echo "Instruction: $0 <firstname>" exit 1 fi echo "Hello $1" exit 0
The only difference is the way we are checking if the variable is set, and the exit status.
Extending if with else
If condition ; then statement else statement fi
There are cases where a script would be required to continue regardless of the result of the if condition. What to do when
it is true, as well as, false. This is where the else keyword comes to play, it allows the execution of one block of code when the condition is true and another when the condition is evaluated as false.
Let's build on how earlier script, using if and else:
#!/bin/bash if [ $# -lt 1 ] ; then read -p "Enter a Name: " name=$REPLY else name=$1 fi echo "Hello $name" exit 0
The following script extend the options for passing name to the arguments, you can either use the script name and the value, e.g:
user@server:~/bin$ isset3.sh Devsrealmer Hello Devsrealmer
or just run the script, and follow the prompt, so, we have following output:
user@server:~/bin$ isset3.sh Enter a Name: Devsrealmer Hello Devsrealmer