1.6 Redirection and Piping

Question 2

What is the difference between redirection and piping?

Answer 2

• Redirection occurs to and from files - sending command output to a file or getting command input from a file.
• Piping occurs between commands - connecting the output of one command to the input of another.

Question 3

When might you choose to redirect the input of a command?

Answer 3

When you wish to take command input from a file rather than the keyboard.

Question 4

What are the three default file descriptors that Linux uses to classify information for a command?

Answer 4

1. Standard input or STDIN, file descriptor 0
2. Standard output or STDOUT, file descriptor 1
3. Standard error or STDERR, file descriptor 2

Question 5

How can you overcome the 128 KB shell command size restriction?

Answer 5

You can overcome the 128 KB shell command size restriction by using the xargs command.

Question 6

What is the default file descriptor if none is specified?

Answer 6

File descriptor 1, standard output.

Question 7

What are the symbols for redirecting standard intput, standard output, and standard error?

Answer 7

• Standard input - command < filename
• Standard output - command 1> filename
• Standard error - command 2> filename

Question 8

How do you redirect both standard output and standard error to a file?

Answer 8

command 1> filename 2> &1

Question 9

How do you connect a file to the standard input of the sort command?

Answer 9

sort < filename

Question 10

What is redirection?

Answer 10

Redirection directs standard input, output, and error streams from and to locations other than the default.

Question 11

What is piping?

Answer 11

Piping connects the output stream of one command to the input stream of another command.

Question 12

What is standard input and what is its file descriptor?

Answer 12

Standard input (stdin) comes from the keyboard. In redirection, 0 represents stdin.

Question 13

What is standard output and what is its file descriptor?

Answer 13

Standard output (stdout) displays on the monitor. In redirection, 1 represents stdout.

Question 14

What is standard error and what is its file descriptor?

Answer 14

Standard errors (stderr) display on the monitor. In redirection, 2 represents stderr.

Question 15

What are the redirection symbols and how do they work?

Answer 15

• Greater-than symbol (>) - redirects output
• Less-than symbol ( - redirects input
• Double greater-than symbol (>>) - appends output

Question 16

What does the tee command do?

Answer 16

The tee command reads from standard input and writes to standard output and to files.

Question 17

What does the command ls /usr > /tmp/deleteme do?

Answer 17

ls /usr > /tmp/deleteme places the list of files in the /usr directory into a file named /tmp/deleteme.

Question 18

What does the command ls /nonesuch > /tmp/deleteme do?

Answer 18

ls /nonesuch > /tmp/deleteme does not write anything to a file and sends an error message to the monitor ‘/nonesuch not found’.

Question 19

What does the command ls /nonesuch 2 > /tmp/deleteme do?

Answer 19

ls /nonesuch 2> /tmp/deleteme writes the standard error message to a file named /tmp/deleteme.

Question 20

What does the command ls /bin /nonesuch > /tmp/deleteme do?

Answer 20

ls /bin /nonesuch > /tmp/deleteme writes the contents of the /bin directory to the /tmp/deleteme file, but sends the error message ‘/nonesuch not found’ to the screen.

Question 21

What does the command ls /bin /nonesuch > /tmp/deleteme 2>&1 do?

Answer 21

ls /bin /nonesuch > /tmp/deleteme 2> &1 directs the standard output to the /tmp/deleteme file, then directs that the standard error messages be sent to the same place as the standard output. Both the list of files in the /bin directory and the error message are written to the file.

Question 22

What does the command ls /bin /nonesuch 2>&1 > /tmp/deleteme do?

Answer 22

ls /bin /nonesuch 2> &1 > /tmp/deleteme writes the contents of the /bin directory to the /tmp/deleteme file, but sends the error message ‘/nonesuch not found’ to the screen. This is because standard error messages are directed to the same place that standard output goes, but this is before standard output has been directed to the file.

Question 23

What does the command ls /bin >> /tmp/deleteme do?

Answer 23

ls /bin >> /tmp/deleteme appends the list of files from the /usr directory on to the end of the /tmp/deleteme file.

Question 24

What does the command sort < unordered_file.txt > ordered_file.txt do?

Answer 24

sort < unordered_file.txt > ordered_file.txt takes input from the unordered_file.txt file sends it to the sort command, and then writes a new file named ordered_file.txt.

Question 25

What does the command cat /usr/wordlist1 /usr/wordlist2 | sort do?

Answer 25

cat /usr/wordlist1 /usr/wordlist2 | sort sends the output of the cat command, the contents of wordlist1 and wordlist2, to the input of the sort command. The result is a sorted list of the combined contents of wordlist1 and wordlist2.

Question 26

What does the command cat /usr/wordlist1 /usr/wordlist2 | mail jdoe do?

Answer 26

cat /usr/wordlist1 /usr/wordlist2 | mail jdoe mails the combined list of words in wordlist1 and wordlist2 to the user jdoe.

Question 27

What does the command ls /bin | sort | mail jdoe do?

Answer 27

ls /bin | sort | mail jdoe lists the contents of /bin then sorts the combined contents and mails them to jdoe.

Question 28

What does the command cat /usr/wordlist1 /usr/wordlist2 | sort | tee sortedwordlist do?

Answer 28

cat /usr/wordlist1 /usr/wordlist2 | sort | tee sortedwordlist lists the contents of wordlist1 and wordlist2 then sorts the combined contents, then sends the results to the monitor and a file named sortedwordlist.

Question 29

What does the command cat /usr/wordlist1 | tee log.txt do?

Answer 29

cat /usr/wordlist1 | tee log.txt writes to the standard output and the log.txt file.

Question 30

What does the xargs command do?

Answer 30

The xargs command reads items from the standard input and breaks up long lists of arguments into smaller, usable lists.

Question 31

What are the main features of the xargs command?

Answer 31

• Makes it easier to pipe input into commands that take arguments.
• Overcomes a 128 KB shell command size restriction in older Linux kernels.

Question 32

Which commands does xargs commonly take input from?

Answer 32

• find
• ls
• locate
• grep -l

Question 33

Which xargs option ignores spaces in file names

Answer 33

xargs -0 ignores spaces in file names.

Question 34

What does the command find / -print0 -name *.odt | xargs -0 rm do?

Answer 34

find / -print0 -name *.odt | xargs -0 rm deletes all .odt files in the file system, even those with spaces in the file names.

Question 35

Which xargs option replace the initial argument of a command with the argument from the standard input?

Answer 35

xargs -I variable replaces the initial argument of a command with the argument from the standard input.

Question 36

What does the command find / -name ‘*.jpg’ | xargs -I var1 cp var1 /home/user/Pictures do?

Answer 36

find / -name ‘*.jpg’ | xargs -I var1 cp var1 /home/user/Pictures finds all the .jpg files on the computer and copies them into the /home/user/Pictures directory.