Four

Question 1

what are the file descriptor number of standard file input and output

Answer 1

stdin 0 cat output.txt

stderr 2 command file 2> errorfile.txt

Question 2

How to create an empty file

Answer 2

> newfile.name

Question 3

How to discard unwanted output

Answer 3

command > /dev/null

The output to /dev/null is ignored by the shell

Question 4

How to discard both the output and the error output

Answer 4

command &> /dev/null

Question 5

The cheat sheet for file checkings

Answer 5

Operator Description

• a file Returns true if file exists
• b file Returns true if file exists and is a block special file
• c file Returns true if file exists and is a character special file
• d file Returns true if file exists and is a directory
• e file Returns true if file exists
• r file Returns true if file exists and is readable
• s file Returns true if file exists and has a greater size that zero
• s file Returns true if file exists and has a greater size that zero
• w file Returns true if file exists and is writable
• x file Returns true if file exists and is executable
• N file Returns true if the file exists and has been modified since it was last read

Question 6

What is the normal exit value of a successful command

Answer 6

$? which is always 0 when command is implemented successfully

Question 7

what is the value of grep command when the string is not present

Answer 7

the exit value is non-zero then

Question 8

How to specify a exit value for a script

Answer 8

when exit is explicitly given, the exit value is the value of the last command

Question 9

What is command1 && command2

and command1 || command2

Answer 9

command1 && command2: command2 is executed iff command1 gives a 0 exit value
command1 || command2: command2 is executed iff command1 gives a non-zero value

Question 10

What is the difference between

Question 11

grep into string

Answer 11

grep “string”

Question 12

redirect error output

Answer 12

./scripts.sh 2>scripts.err

Question 13

Append a output

Answer 13

command_output&raquo_space; file

Question 14

strd output to strd output

Answer 14

2>&1

error output to standard output

Question 15

what does

Answer 15

-tells the I/O that in the file, the leading tab character is ignored

Question 16

what does quoting or escaping a limitstring in redirecting the input do?

Answer 16

this will tells the redirection that in the contxt, the $,’ and \ will be escaped.