If you are new to Linux or any Unix operating system here is a list of must know commands that can help you in every day task
cat
Shows the content of a file. Run cat /etc/hosts
127.0.0.1 localhost
255.255.255.255 broadcasthost
::1 localhost
is the output on my computer. /etc/hosts is the path of the file.
mkdir
Make/Create a directory. Run mkdir a mkdir a/b mkdir a/b/c
this will create the directory a and in it will be directory b and then c in b directory.
pwd
print working directory. This commands shows the current directory you are in, output from my machine.
expanse@Ankurs-Mac-mini ~ % pwd
/Users/expanse
I am in expanse directory which is in the Users directory.
cd
cd allows you to navigate from one directory to another. You can give an absolute path or
relative path. Relative path is the location of the directory from your current folder.
Let us try to navigate the directory created above aka a b and c
cd a - Enters directory a and this is relative path from current directory.
cd b - Enters directory b from current directory
cd /Users/expanse/a/b/c - Enter directory c using absolute path aka the complete path on the filesystem.
you can use
cd .. to go to parent directory from current directory
cd - to go back to previous directory before you entered this one.
cd without any argument goes to the home directory of the user.
free
free tells you how much of free memory is there on the system. e.g.
deploy@linux:~$ free
total used free shared buff/cache available
Mem: 8167616 3140156 795416 1156 4232044 4723404
Swap: 0 0 0
this show you how much of free Mem and swap space is there. To know the same in MB and GB do this
free -mh
deploy@linux:~$ free
total used free shared buff/cache available
Mem: 7.8G 3.0G 776M 1.1M 4.0G 4.5G
Swap: 0B 0B 0B
df
df tell you how much disk space is free in every partition. e.g.
df -h
deploy@deploy:~$ df -h
Filesystem Size Used Avail Use% Mounted on
udev 3.9G 0 3.9G 0% /dev
tmpfs 798M 960K 797M 1% /run
/dev/vda1 78G 61G 17G 79% /
tmpfs 3.9G 0 3.9G 0% /dev/shm
tmpfs 5.0M 0 5.0M 0% /run/lock
tmpfs 3.9G 0 3.9G 0% /sys/fs/cgroup
/dev/loop0 100M 100M 0 100% /snap/core/11316
/dev/loop2 100M 100M 0 100% /snap/core/11187
/dev/loop1 43M 43M 0 100% /snap/certbot/1201
/dev/loop4 62M 62M 0 100% /snap/core20/1026
/dev/vda15 105M 8.8M 96M 9% /boot/efi
tmpfs 798M 0 798M 0% /run/user/1000
/dev/loop5 43M 43M 0 100% /snap/certbot/1280
head
head allows you to display the first few lines of a file. I have a file with all rss feeds of tennis websites.
deploy@linux:~$ head tennisnews.txt
http://news.bbc.co.uk/rss/sportonline_world_edition/tennis/rss091.xml
http://sports.espn.go.com/espn/rss/tennis/news?null
http://www.tennis-x.com/tennisxnews.xml
http://www.atpworldtour.com/en/media/rss-feed/xml-feed
http://straightsets.blogs.nytimes.com/feed/
http://feeds.tennis.com/concrete-elbow-tignor
http://www.guardian.co.uk/sport/tennis/rss
http://feeds.feedburner.com/tennisx
http://bleacherreport.com/articles/feed?tag_id=12
http://rss.cnn.com/rss/edition_tennis.rss
how about if I want to display only the top 10 lines?.
deploy@linux:~$ head -n 5 tennisnews.txt
http://news.bbc.co.uk/rss/sportonline_world_edition/tennis/rss091.xml
http://sports.espn.go.com/espn/rss/tennis/news?null
http://www.tennis-x.com/tennisxnews.xml
http://www.atpworldtour.com/en/media/rss-feed/xml-feed
http://straightsets.blogs.nytimes.com/feed/
watch
watch allows you to run a command again and again periodically. Say that your ram's is getting utilised completely and you want to monitor the change or growth every 3-4 seconds.
deploy@deploy:~$ watch -n 3 free -m
Every 3.0s: free -m total used free shared buff/cache available
Mem: 7976 3068 722 1 4185 4610
Swap: 0 0 0
screen
say you want to run a command and let it run even after you have existed the terminal aka run the program in background without the terminal running. Use the command screen
Run screen and you will be greeted by a copyright message, [Press Space or Return to end.] and you will see a normal shell prompt. Here you can simply run a script and exit screen by entering the command ctrl + ad and to enter the screen again simply type screen -r -d.
| and grep
say you have a file with thousands of lines and want to find amongst that a word. We have seen that we can print the content of a file with cat. grep allows you to find the existence of a word in a line. | will allow you to take the output of one and make it the input of another. What the hell am I talking?. e.g.
deploy@deploy:~$ cat tennisnews.txt | grep swordsmaster
http://blog.xuite.net/swordsmaster/tennis/rss.xml
so | made the output of tennisnews.txt as the input of grep.
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