I have sent Gedit command in the background this is why it shows three processes otherwise you will normally see just ps and bash. If you use the ps command without any options in Linux, it will show the running processes in the current shell: ps This is why I will show you some of the most common and useful examples of the ps command in Linux. You can understand its strength and complexity. The ps command is an extensive tool and has over 80 command options. You can get information like process ID (PID) for the processes you or any other user is running on the same Linux system. We may easily combine the two forms of redirection we have seen so far into a single command as seen in the example below.The ps command in Linux displays running processes on the system. As a result, this mechanism is often used in order to get ancillary data (which may not be required) to not be printed. So in the above example, wc recieved some content to process, but it has no knowledge of where it came from so it may not print this information. This is because whenever we use redirection or piping, the data is sent anonymously. When we ran it redirecting the contents of the file into wc the file name was not printed. You'll notice that when we ran wc supplying the file to process as a command line argument, the output from the program included the name of the file that was processed. The above example illustrates a subtle but useful difference. Given this, you may be asking why we would need to use this operator. This also allows us to easier manipulate that data later on as we'll see further down the page.Ī lot of programs (as we've seen in previous sections) allow us to supply a file as a command line argument and it will read and process the contents of that file. When we are redirecting, it may be to a file, or it could be somewhere else, so the safest option is to format it as one entry per line. The reason for this is that the screen is a known width and the program can format its output to suit that. You'll notice that in the above example, the output saved in the file was one file per line instead of all across one line when printed to the screen. Line 6 Let's have a look at what was saved in there.Line 4 As you can see, our new file has been created.The terminal will create it automatically if it does not exist. You'll notice that we don't need to create the file before saving to it. Line 3 Now we'll run the same command but this time we use the > to tell the terminal to save the output into the file myoutput.Line 1 Let's start off by seeing what's in our current directory.barry.txt bob example.png firstfile foo1 myoutput video.mpeg.barry.txt bob example.png firstfile foo1 video.mpeg.The greater than operator ( > ) indicates to the command line that we wish the programs output (or whatever it sends to STDOUT) to be saved in a file instead of printed to the screen. Normally, we will get our output on the screen, which is convenient most of the time, but sometimes we may wish to save it into a file to keep as a record, feed into another system, or send to someone else. We'll demonstrate piping and redirection below with several examples but these mechanisms will work with every program on the command line, not just the ones we have used in the examples. Piping and redirection is the means by which we may connect these streams between programs and files to direct data in interesting and useful ways. STDERR (2) - Standard error (for error messages, also defaults to the terminal).STDOUT (1) - Standard output (data printed by the program, defaults to the terminal).STDIN (0) - Standard input (data fed into the program).So what are they?Įvery program we run on the command line automatically has three data streams connected to it. Even though the mechanisms and their use are quite simple, it is important to understand various characteristics about their behaviour if you wish to use them effectively. There is a bit of reading involved in this section. In this section we will see how we may join them together to do more powerful data manipulation. In the previous two sections we looked at a collection of filters that would manipulate data for us. Learn how easy it is to use piping and redirection to create powerful workflows that will automate your work, saving you time and effort.
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