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Linux is an operating system that many developers will use.

Therefore, it’s a good idea to learn some Linux commands.

In this article, we’ll look at some useful Linux commands we should know.

kill

The kill command lets us send signals to a process.

The general format is:

kill <PID>

where PID is the process ID.

We can send signals like:

kill -HUP <PID>
kill -INT <PID>
kill -KILL <PID>
kill -TERM <PID>
kill -CONT <PID>
kill -STOP <PID>

HUP means hang up. It’s sent when a terminal window that started a process is closed before terminating it.

INT means interrupt. It sends the signal when we press ctrl+c.

KILL is sent to the OS kernel to stop and terminate the process.

TERM means terminate. The process that receives it will terminate.

CONT means continue. It lets us resume a stopped process.

STOP is sent to the OS kernel instead of the process, which stops but doesn’t terminate the process.

We can also use a number to replace these signals. 1 is HUP , 2 is INT , 9 is KILL , 15 is TERM , 18 is CONT .

top

The top command lets us list processes running in real time.

We can quit top with ctrl+c.

And we can sort processes by the amount of memory used by running:

top -o mem

echo

echo lets us print arguments passed to it.

For instance, we run:

echo "hello" >> output.txt

to put hello into the output.txt file.

Also, we can interpolate environment variables in the string:

echo "path=$PATH"

We have to escape special characters like $ in the command.

We can echo the files in the current folder with:”

echo *

And we can echo the files that start with a with:

echo a*

We can print the home folder path with:

echo ~

We can also print the results of a command with $() like:

echo $(ls)

ps

The ps command lets us list the processes currently running in the system.

We can list all processes with the ps ax command.

a is used to list other users' processes.

x shows processes not linked to any terminal.

ps axww continues the command listing on a new line instead of truncating it.

The output has the PID which is the process ID, TT tells us the terminal ID used.

STAT tells us the state of the process.

I means the process is idle. R is a runnable process. S is a process that’s sleeping for less than 20 seconds.

T is a stopped process, U is a process in uninterruptable wait and Z is a dead process.

+ means the process is in the foreground. s means the process is a session leader.

ln

ln lets us create links in the filesystem.

The syntax of the command is:

ln <original> <link>

For instance, we run

ln foo.txt newfoo.txt

to create the newfoo.txt link to the foo.txt file.

Links are indistinguishable from a regular file from the user’s perspective.

Conclusion

We can manage processes and create symbolic links with Linux commands.