Tue. Sep 27th, 2022

As a reasonably separated junior sysadmin, I remember seeing responses on Experts Exchange and later on Stack Exchange that baffled me. Authors and commenters might chain 10 commands together with pipes and angle brackets– something I never did in daily system administration.
Attempting to check out the male pages for the energies most often seen in these extended command chains didnt make them appear more approachable, either. The sed guy page weighs in at around 1,800 words alone without ever truly explaining how regular expressions work or the most common uses of sed itself.
If you discover yourself in the exact same boat, get a drink and buckle in. Rather of providing you encyclopedic listings of every possible argument and use case for each of these common commands, were going to teach you how to think about them– and how to easily, productively integrate them in your own daily command-line usage.
Redirection 101
Prior to we can talk about sed, awk, and grep, we need to speak about something a bit more basic– command-line redirection. Again, were going to keep this very easy:
Operator
Function
Example
;
Process the command on the right after youre done processing the command on the left.
echo one; echo 2
>>.
Place the output of the thing on the left in the empty file called on the right.
ls/ home/me > > myfilesonce.txt; ls/ home/me > > myfilesonce.txt.
>>> >.
Add the output of the thing on the left to the end of the existing file on the.
ls/ home/me > > myfilestwice.txt; ls/ home/me >> > > myfilestwice.txt.
< targetfile. | Pipe the standard output of the thing on the left into the standard input of the thing on the. echo "test123"|mail -s "subjectline" emailaddress. Understanding these redirection operators is essential to understanding the sort of wizardly command lines youre presumably here to find out. They make it possible to deal with specific, easy utilities as part of a greater whole. Which last principle-- breaking one complex task into numerous simpler tasks-- is similarly needed to finding out to believe in intricate command-line invocations in the very first place! Grep discovers strings. When first discovering tools like grep, I discover it helps to think about them as far easier than they truly are. In that vein, grep is the tool you use to discover lines that include a particular string of text. Ad. me@banshee:~$ sudo netstat -anp|head -n2; sudo netstat -anp|grep apache. Active Internet connections (servers and developed). Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name. tcp6 0 0:::80::: * LISTEN 4011/apache2. We introduced some brand-new commands above: head, which limits output to the very first n lines and after that truncates it. Theres likewise wc, which, with the argument -l, tells you the number of lines of text struck its basic input. So we can equate the 4 commands above into plain English:. sudo netstat -anp|head -n5: "Find all the open network sockets, however limit output to the first five lines.". sudo netstat -anp|wc -l: "Find all the open network sockets, then inform me how lots of total lines of text you d have used to tell me.". sudo netstat -anp|grep apache: "Find all the open network sockets, but only show me the results that consist of the word apache.". sudo netstat -anp|head -n2; sudo netstat -anp|grep apache: "Find all the open network sockets, but only reveal me the two header lines-- then do it again, however only show me the apache results.". By thinking about grep as something much easier than it really is, we can leap immediately to discovering efficient ways to use it-- and we can chain these easy uses together to easily describe more complex tasks! Lets say youre interested in discovering which ports the apache web internet browser has open on your system. Many energies can accomplish this goal; netstat is among the older and better-known options. Normally, we d conjure up netstat using the -anp arguments-- for all sockets, numeric display screen, and showing the owning pid of each socket. less. Or, you may rather redirect it to a file to be opened with a full-screen editor: netstat -anp > > netstat.txt. However theres a better option. Instead, we can utilize grep to return only the lines we actually want. In this case, what we would like to know about is the apache webserver. :.
me@banshee:~$ sudo netstat -anp|head -n5.
Active Internet connections (servers and developed).
Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name.
tcp 0 0 192.168.188.1:53 0.0.0.0: * LISTEN 5128/dnsmasq.
tcp 0 0 192.168.254.1:53 0.0.0.0: * LISTEN 5057/dnsmasq.
tcp 0 0 192.168.122.1:53 0.0.0.0: * LISTEN 4893/dnsmasq.

As a fairly isolated junior sysadmin, I remember seeing answers on Experts Exchange and later Stack Exchange that baffled me. Authors and commenters might chain 10 commands together with pipelines and angle brackets– something I never ever did in everyday system administration. Honestly, I doubted the real-world worth of that. Certainly, this was just an exercise in e-braggadocio?
Theres a much better alternative.

me@banshee:~$ sudo netstat -anp|grep apache.
tcp6 0 0:::80::: * LISTEN 4011/apache2.

me@banshee:~$ sudo netstat -anp|wc -l.
1694.

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