Shell Scripting - True Command
Last Updated :
27 Jan, 2022
A shell provides an interface with the help of which users can interact with the system easily. To directly interact with a shell, we use an operating system. On a Unix-based operating system, every time we write a command using a terminal, we interact with the system. To interpret or analyze Unix commands, we use a shell. The main job of a shell is to take commands from the user and convert them into the kernel's understandable form. To summarize this, we can see it as a medium between a user and the kernel system of an OS. The kernel is a computer program that is considered the main part of a computer’s operating system.
This article focuses upon the shell scripting- True command.
True command:
This command is referred to as "Do nothing, successfully". This is because, on a UNIX-based operating system, the main purpose of this command is to return the successful exit status, which also means that it outputs nothing. It should be used if a part of a script always returns true. Its basic syntax is given below:
Syntax:
true [ arguments (optional) ]
It doesn't matter whether you provide arguments, the true command always returns successfully.
Exit status:
zero (0)
It signifies success
Example :
In this script, we have used true command without any argument.
#!/bin/sh
# true command without any argument
true
Output:
Example:
In this script, we are using the true command with an argument.
#!/bin/sh
# true command using an argument
true GeekforGeeks.txt
Output:
Verify exit status:
We can verify the exit statement of the true command but this command has to be used along with another command. For this purpose, a special shell variable (?) is used to store the status of the true command. This mechanism is illustrated in the below script.
Example:
#!/bin/sh
# Evaluates true then print the statement
true; echo "Status of the previous command is $?."
Output:
If statement:
We can use the true command even in the if statement. This mechanism is illustrated in the below script.
Example:
#!/bin/sh
if true; then echo "True Command"; else echo "Not A True Command"; fi
Output:
Example:
To execute the else part in the above script we can use Not (!) operator just before the true command.
#!/bin/sh
# Now it evaluate false
if ! true; then echo "True Command"; else echo "Not A True Command"; fi
Output:
While statement:
We can use the true command in a while loop also. It is used to create infinite loops not only in shell scripting but also in other programming languages.
Example:
#!/bin/sh
# while true: Print "Infinite loop"
while true; do
echo "Infinite loop";
done
Output:
