![]() ![]() $ ps -ef | grep systemd | awk '' | grep -v grep To list the process IDs (PIDs) for all systemd-related processes: You can also perform complex tasks using pipes. For example, you can list the number of files in the /etc directory with this:Īgain, your results might look different, but you know something is wrong if the command returns a small number of files. This listing is from a fresh "RHEL 8 server no GUI" install. The -l switch displays the number of lines. An easy option is to pipe the results of your ls command to the wc (word count) command: You still need to find out how many there are, and a visual count would take a long time. The results are too long to list here, but as you can see from your displayed list, there are a lot of them. ![]() ![]() Maybe you want to find out how many directories under /etc are writeable by root: You could also grep for "RUNNING" or "RUN" to display the same information. Note: Don't confuse pipe ( |) redirection with file redirection ( >) and ( file or cmd The pipe redirects that output as input to cmd2. When you pipe one command's output to another, however, the information from cmd1 doesn't produce output to the screen. Let's look at a theoretical example as an illustration of how this process works:īoth cmd1 and cmd2 are command line utilities that output their results to the screen ( stdout). You can pipe the output to any command that accepts stream input. This purpose is why the most popular use for pipes involves the commands grep and sort. You use piping to filter the contents of a large file-to find a particular string or word, for example. One of the main purposes of piping is filtering. And, you're not limited to a single piped command-you can stack them as many times as you like, or until you run out of output or file descriptors. The pipe takes output from one command and uses it as input for another. One of the most powerful shell operators is the pipe ( |). It is an efficient way of quickly gathering and processing information, creating new scripts, and configuring systems. The Linux command line is alive and well, and still going strong. I'm sorry to inform you, but the command line didn't die off with the dinosaurs, nor did it disappear with the dodo or the carrier pigeon. ![]()
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