Let us learn a few commands, which are required very often, such as man
, echo
, cat
and similar:
Enter the following command. It will show the various types of manual pages displayed by the
man
command:$ man man
From the following table, you can get an idea about various types of
man
pages for the same command:Section number
Subject area
1
User commands
2
System calls
3
Library calls
4
Special files
5
File formats
6
Games
7
Miscellaneous
8
System admin
9
Kernel routines
We can enter the
man
command to display corresponding manual pages as follows:$ man 1 command $ man 5 command
Suppose we need to know more about the
passwd
command, which is used for changing the current password of a user, you can type the command as follows:$ man command man -k passwd // show all pages with keyword man –K passwd // will search all manual pages for pattern $ man passwd
This will show information about the
passwd
command:$ man 5 passwd
The preceding command will give information about the file
passwd
, which is stored in/etc /passwd
.We can get brief information about the command as follows:
$ whatis passwd
Output:
passwd (1ssl) - compute password hashes passwd (1) - change user password passwd (5) - the password file
Every command we type in the terminal has an executable binary program file associated with it. We can check the location of a binary file as follows:
$ which passwd /usr/bin/passwd
The preceding line tells us that the binary file of the
passwd
command is located in the/usr/bin/passwd
folder.We can get complete information about the binary file location as well as manual page location of any command by following:
$ whereis passwd
The output will be as follows:
passwd: /usr/bin/passwd /etc/passwd /usr/bin/X11/passwd /usr/share/man/man1/passwd.1.gz /usr/share/man/man1/passwd.1ssl.gz /usr/share/man/man5/passwd.5.gz
Change the user login and effective user name:
$ whoami
This command displays the user name of the logged in user:
$ su
The
su
command (switch user) will make the user as the administrator; but, you should know the administrators, password. Thesudo
command (superuser do) will run the command with administrator's privileges. It is necessary that the user should have been added in thesudoers
list.# who am i
This command will show the effective user who is working at that moment.
# exit
Many a times, you might need to create new commands from existing commands. Sometimes, existing commands have complex options to remember. In such cases, we can create new commands as follows:
$ alias ll='ls –l' $ alias copy='cp –rf'
To list all declared aliases, use the following command:
$ alias
To remove an alias, use the following command:
$ unalias copy
We can check about the operating system details such as UNIX/Linux or the distribution that is installed by the following command:
$ uname
Output:
Linux
This will display the basic OS information (UNIX name)
Linux kernel version information will be displayed by the following:
$ uname –r
Output:
3.13.0-32-generic
To get all the information about a Linux machine, use the following command:
$ uname –a
Output:
Linux ubuntu 3.13.0-32-generic #57~precise1-Ubuntu SMP Tue Jul 15 03:50:54 UTC 2014 i686 i686 i386 GNU/Linux
The following commands will give you more information about the distribution of Linux:
$ cat /proc/version // detailed info about distribution $ cat /etc/*release # lsb_release -a // will tell distribution info for Ubuntu
The command
cat
is used for reading files and displayed on the standard output.Sometimes, we need to copy a file or directory in many places. In such situations, instead of copying the original file or directory again and again, we can create soft links. In Windows, a similar feature is called as creating a shortcut.
$ ln -s file file_link
To learn about the type of file, you can use the command file. In Linux, various types of files exist. Some examples are as follows:
Regular file (-)
Directory (d)
Soft link (l)
Character device driver (c)
Block device driver (b)
Pipe file (p)
Socket file (s)
We can get information about a file using the following command:
$ file fil_name // show type of file
Printing some text on the screen for showing results to the user or to ask details is an essential activity.
But this is very rarely used, as many powerful editors are already existing, such as vi or gedit.
The following command will print
Hello World
on the console. Theecho
command is very useful for Shell script writers:$ echo "Hello World"
The following command will copy the string
Hello World
to thehello.c
file:$ echo "Hello World" > hello.c
The command
echo
with>
overwrites the content of the file. If content already exists in the file, it will be deleted and new content will be added in the file. In a situation, when we need to append the text to the file, then we can use theecho
command as follows:$ echo "Hello World" >> hello.c will append the text
The following command will display the content of the file on screen:
$ cat hello.c