My main workstation is a Linux laptop, which has a few applications installed that I need to use, such as Libre Office, Lyx, Jabref and g++. I invariably have a terminal window open somewhere on my workspace, which I use for various tasks. There are a few commands that I find myself using most days. This post is a list of four of these commands.
Netstat
The first command on my list is netstat. There are a few options with netstat; the options I tend to use are -r and -a. The -r option prints out the routing table. This is useful when you want to check the default gateway and subnet mask. But the option I generally use is the -a option, which shows all TCP and UDP connections. It also gives the name of the application (if known) that is using a connection. Finally, it shows the state of TCP connections. See for instance the below output, which is a sample of output from running netstat -a.
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address Foreign Address State
tcp 0 0 *:50450 *:* LISTEN
tcp 0 0 localhost:ipp *:* LISTEN
tcp 0 0 *:17500 *:* LISTEN
tcp 38 0 gudaring.local:33581 v-d-2a.sjc.dropbo:https CLOSE_WAIT
tcp 0 0 gudaring.local:46072 tf-in-f125.1e100.:https ESTABLISHED
tcp 0 0 gudaring.local:49496 r-199-59-148-20.t:https ESTABLISHED
tcp 0 0 gudaring.local:42965 sjc-not15.sjc.dropb:www ESTABLISHED
tcp 0 0 gudaring.local:50959 kwaimuk.canonical:https ESTABLISHED
tcp 0 0 gudaring.local:58176 111.221.77.149:40005 ESTABLISHED
tcp 38 0 gudaring.local:51187 v-client-1a.sjc.d:https CLOSE_WAIT
tcp 0 0 gudaring.local:54352 rproxy2.msg.vip.sp2:www ESTABLISHED
tcp 38 0 gudaring.local:35819 75.126.110.108-st:https CLOSE_WAIT
Awk
Another command that I use is awk. I like being able to run a command and pipe the output to awk to view just the fields I want to look at, and use a regular expression to extract the lines I want to view. Going back to netstat, for example, I want to look at only the connections that are established, and of those, I want to look only at the fifth field. I can do this by simply typing in the command string netstat -a | awk ‘/ESTABLISHED/ {print $5}’. This will print out the data I want.
tf-in-f125.1e100.:https
sjc-not15.sjc.dropb:www
kwaimuk.canonical:https
111.221.77.149:40005
r-199-59-149-232.:https
rproxy2.msg.vip.sp2:www
Regular expressions are very handy to know, especially when using awk on the CLI. Going back to the netstat output, you could extract the lines beginning with “tcp” and again print the fifth field of the output.
root@gudaring:/home# netstat -a | awk '/^tcp / {print $5}'
*:*
*:*
*:*
v-d-2a.sjc.dropbo:https
r2.ycpi.vip.aue.yah:www
tf-in-f125.1e100.:https
sjc-not15.sjc.dropb:www
kwaimuk.canonical:https
111.221.77.149:40005
v-client-1a.sjc.d:https
rproxy2.msg.vip.sp2:www
75.126.110.108-st:https
root@gudaring:/home#
Time
Another command I use is time. What this does in its most basic form is tell you the time it takes to execute a command. As I am at present trying to optimise some code, I use time regularly in order to obtain a measurement of the execution time of a program. The output of time is shown below; it also tells you the time the process spends in system and in user mode.
root@gudaring:/home# time ls
scott
real 0m0.132s
user 0m0.000s
sys 0m0.004s
root@gudaring:/home#
Man
The final command would likely upset one of my oldest Linux/Unix colleagues if he saw it was last on my list. The command is of course man. My colleague would invariably start any Linux introductory shell programming course stating that man was possibly the most useful command to know on Linux, and that you should, as a start, type in man man.
These are commands that I invariably end up using from day to day. Some, such as netstat, I use daily, whereas others such as time, man and awk are every other day.
What are the commands you can’t do without?
