Line 6 - The backslash ( \ ) in front of the single quote ( ' ) is needed as the single quote has a special meaning for bash and we don't want that special meaning. Enter a number: 45 Number is odd. We can precede the command to be tested with ! Output of the above program. grep -q regex options; then printf '%s\n' 'myscript: Pattern not found!' >&2 exit 1 fi Conclusion # Checking if a string contains a substring is one of the most basic and frequently used operations in Bash scripting. After reading this tutorial, you should have a good understanding of how to test … [is itself a command, very nearly equivalent to test. If the first test (4 = 4) fails, the test command terminates with a non-zero exit code. Enter a number: 88 Number is even. Let's break it down: Line 4 - Let's see if the first command line argument is greater than 100; Line 6 and 7 - Will only get run if the test on line 4 returns true. Here, in this tutorial, we have discussed several command line option this tool offers. If your Linux work involves accessing information related to block devices, then lsblk is a must know command for you. Use /dev/null to send any unwanted output from program/command and syntax is: command >/dev/null This syntax redirects the command standard output messages to /dev/null where it is ignored by the shell. The branching of the test command is significant. Explanation of the above code-We have asked a user to enter a number and stored the user response in a number variable. In this example, the variable count specifies a condition that is used as part of the if statement.Before the if statement is executed, the variable count is assigned the value 5.The if statement then checks whether the value of count is 5.If that is the case, the statement between the keywords then and fi are executed.Otherwise, any statements following the if statement are executed. Bash check if process is running or not. To build a condition in if statement, we have used $(()) and [].$(()) is used to check whether a number is divisible by 2 or not. Unix & Linux Stack Exchange is a question and answer site for users of Linux, FreeBSD and other Un*x-like operating systems. For example, I used the nm command with the 'apl' binary file: nm apl. The basic usage of this command is very straight forward - all you have to do is to run the 'nm' command and pass the name of the object file as input to it. The -q option tells grep to be quiet, to omit the output. test.sh #!/usr/bin/env bash if [ $# -ge 3 ] then echo script has at least 3 arguments fi produces the following output Write a Bash script so that it receives arguments that are specified when the script is called from the command line. To know more about lsblk, head to its man page. About Himanshu Arora The aforementioned command produced the following output: Conclusion. But if you want to test whether a command succeeded or not, use the command itself directly with if, as shown above. to negate the test as well, to prevent us having to use else as well: # Best if ! Bash commands to check running process: pgrep command – Looks through the currently running bash processes on Linux and lists the process IDs (PID) on screen. Thus, we jump to the double-pipe symbol and "it is not a cow" prints to standard output. It's probably the most common command to use in an if, which can lead to the assumption that it's part of the shell's syntax. The following script demonstrates how this works. You can have as many commands here as you like. The following screenshot shows the kind of output the above command produced: Greetings. It only takes a minute to sign up. I have a nice bash shell script that runs a multi-step analysis well. Command with the 'apl ' binary file: nm apl command, very nearly equivalent to …... 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