Man
Every Linux system has some well documented Manuals to help users get information about commands and services. These manuals can be accessed using the command
# man command/file | syntax |
# man ls | shows the manual of the command ‘ls’ |
# man resolv.conf | shows the manual of the file ‘/etc/resolv.conf’ |
Some manuals contain further references at the bottom. These references usually contain page numbers that can be accessed using –
# man 5 resolv.conf | shows page no. 5 of resolv.conf |
whereis, whatis & which
whatis displays an overview of the command. whereis & whatis have similar output that contains the location where the command is stored, as well as files related to the command.
# whatis pwd
pwd (1p) - return working directory name
pwd (1) - print name of current/working directory
pwd [builtins] (1) - bash built-in commands, see bash(1)
pwd.h [pwd] (0p) - password structure
# whatis cp
cp (1) - copy files and directories
cp (1p) - copy files
# whereis pwd
pwd: /bin/pwd /usr/share/man/man1/pwd.1.gz /usr/share/man/man1p/pwd.1p.gz
# which pwd
/bin/pwd
Output Redirections
Output can be redirected from one place to another by using the”>” sign. For example:
# echo hello | Shows hello in the terminal |
# echo hello > /root/greetings | Instead of showing hello in the terminal, creates a new file /root/greetings and dumps ‘hello’ there. |
# cat /etc/passwd | Shows the content of the file in the terminal |
# cat /etc/passwd> /root/user_info | Instead of showing the file in the terminal, creates a new file /root/user_info and dumps the output there. |
Redirecting output this way always overwrites the output file. To append in the output file, the sign “ >> ” is used.
# echo hello >> /root/user_info | Appends hello at the bottom the file /root/user_info |
Pipelining
Pipelining ( | ) is applied to use the output of one command as input of the next command. For example-
# cat /etc/passwd | grep root | The first command’s output is the entire content of the file /etc/passwd. The second command filters ‘root’ from the output of the first command. |
# tailf /var/log/maillog | grep palash | The output of the first command shows entries of the mail log in real time, and the second command filters the output and shows ‘palash’ only |
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