Input and Output Redirects

Question 1

What is stdin?

Answer 1

Standard input and it has a file descriptor number as 0.

Question 2

What is stdout?

Answer 2

Standard Output and it has a file descriptor value as 0.

Question 3

What is stderr?

Answer 3

Standard error and it has a file descriptor value as 2.

Question 4

How can the output of a command be routed to a file?

Answer 4

All output of any given command is considered stdout. Using >.
If using the same file for additional output or to append to the same file use&raquo_space;. A single > will overwrite existing contents in file.
Example: ls -l > listings
ls -la&raquo_space; listings

Question 5

How can you feed file contents into a file or email.

Answer 5

Input is used when feeding file contents to a file or email. Using <.
Example: cat < listings
mail -s “office memo” …@gmail.com < ‘memo letter’

Question 6

How can you redirect error messages to a file?

Answer 6

Any error on the terminal is considered stderr. Using 2>.
Example: ls -l /root 2> ‘error file’
telnet localhost 2> ‘error file’

Question 7

What is this character and what is it used for | ?

Answer 7

The symbol is called a pipe and is used by the shell to connect the output of one command directly to the input of another command.
Example: command1 {argument} | command2 {argument}
cat /var/log/messages | grep error

Question 8

What command can you use to check the manual of a command?

Answer 8

man
Example: man ls
man grep

Question 9

What command can be used to find a command based on a keyword

Answer 9

Apropos
Example: apropos manual
apropos search

Question 10

What command looks up a given command?

Answer 10

whatis
Example: whatis ls
whatis systemctl

Question 11

What option can you universally use with any command for help?

Answer 11

–help
Example: ls –help