Week 6: Unix Command I/O and Redirection, Processes and Job Control

Question 1

When a Unix process starts, how many streams are opened?

Answer 1

Three streams:

• Standard Input (stdin): default place from which programs read
• Standard Output (stdout): default place to which programs write
• Standard Error (stderr): default place where errors are reported

Question 2

What is the default standard input/output/error?

Answer 2

Input: keyboard

Output: display

Error: display

Question 3

For file redirection, what is the symbol for redirecting standard input?

Answer 3

<

or

0

Question 4

For file redirection, what is the symbol for redirecting standard output?

Answer 4

>

or

1>

EX: cat file1 file2 > file3

Question 5

For file redirection, what is the symbol for redirecting standard error?

Answer 5

2>

Question 6

For file redirection, what do the following do?

cat file1 file2 > file3

cat file1 file2 >| file3

cat file1 file2&raquo_space; file3

cat > file3

Answer 6

cat file1 file2 > file3
– concatenates file1 and file2 into file3
– file3 is created if not there

cat file1 file2 >| file3
– file3 is clobbered if there

cat file1 file2&raquo_space; file3
– warning if file3 is not there
– file3 is appended to if it is there

cat > file3
– file3 is created from whatever user provides from
standard input

Question 7

What would the format be to redirect standard from a file named myfile:

Standard output to “yourfile” and standard error to “yourerrorfile”?

Answer 7

cat myfile > yourfile 2> yourerrorfile

Question 8

What does the following do?

cat myfile &> yourfile

Answer 8

If myfile exists, it is copied into yourfile

If myfile does not exist, an error message cat: myfile: No such file or directory is copied into yourfile

Question 9

What does the following do?

cat myfile > yourfile 2>&1

Answer 9

stdout goes to yourfile and stderr goes to where stdout goes. The same thing as cat myfile &> yourfile

Question 10

What does tr string1 string2 do?

Answer 10

character n of string1 translated to

character n of string2

Question 11

What do the following do?

compute[3] > tr aeoiu eoiua file2
compute[4] > tr eoiua aeoiu file2
compute[5] > tr a-z A-Z < file1 > file2

Answer 11

Replace every occurrence of the characters in string1 with those in string2

file1 as input, redirect it to the translation, and place the output in file2

Question 12

What is /dev/null

Answer 12

A virtual file that is always empty.

Copy a file to here and they disappear

Copy a file from here and get an empty file

Redirect error messages here and they disappear as well

Question 13

What are tr, grep, wc, and sort examples of?

Answer 13

Filters

Question 14

Grep, by default, writes the ____ containing the phrase to stdout

Answer 14

LINES

Question 15

What does the following do?

grep “unix is easy” < myfile1 > myfile2

Answer 15

Write all lines of myfile1 containing phrase unix is easy

to myfile2

Question 16

What does wc do?

Answer 16

Count the number of chars/words/lines on stdin and write the resulting statistics to stdout

Question 17

What does the sort filer do?

Answer 17

Sort all the input lines in alphabetical order and write to

the standard output.

Question 18

What does a “pipe” do in essence?

Answer 18

Connects stdout of one program with stdin of another

Question 19

What is an alternative way, using pipes, of writing the following?

compute[2] > grep unix < readme.txt > tmp
compute[3] > wc -l < tmp

Answer 19

cat readme.txt | grep unix | wc -l

Question 20

To include standard error in a pipe, what is the syntax?

Answer 20

command1 |& command2

Question 21

What will the shell do when given the following command?

readme.txt > grep unix | wc -l

Answer 21

Your shell will go looking for a program named

readme.txt

Question 22

What does the “tee” command do?

Answer 22

replicate the standard output:

cat readme.txt | tee myfile

Question 23

When you execute a task in Unix, it runs in the ______ of your shell

Answer 23

Foreground

Question 24

What is the syntax to run a program in the background?

Answer 24

a_heavy_task &

Question 25

When a task is put in the background, any output....

Answer 25

Appears on the screen

Question 26

When running a program in the background, what special command stops the job?

Answer 26

Ctrl-z

Question 27

What command displays what is running and/or stopped?

Answer 27

"jobs" EX: compute[6] > jobs 1 Stopped make_noise 2 Stopped vi readme

Question 28

What command moves a job into the foreground?

Answer 28

"fg" Ex: fg %2

Question 29

What command moves a job into the background?

Answer 29

"bg" Ex: bg %1

Question 30

What is the unix command to terminate a job?

Answer 30

"kill" Ex: kill %1

Question 31

What options can be used with the "kill" command to terminate no matter what?

Answer 31

kill -9 %1 or kill -KILL %1

Question 32

What command lists Unix processes?

Answer 32

"ps"

Question 33

What are the options that can be used with ps command?

Answer 33

–l gives you a long listing of what is going on –u loginid tells you about loginid's processes

Question 34

What does the "ulimit" utility do?

Answer 34

Sets or reports the file-size writing limit imposed on files written by the shell and its child processes (files of any size may be read). User can decrease limit. Only a process with appropriate privileges can increase the limit.

Question 35

What are some "ulimit" options?

Answer 35

ulimit –a (prints all limits) ulimit -n (maximum number of open file descriptors) ulimit -u (maximum number of processes available)

Question 36

What does the following do: grep unix file

Answer 36

Matches all appearances of unix in file

Question 37

What does the following do: grep '[Uu]nix' file

Answer 37

Matches Unix and unix

Question 38

How are regular expressions different from file name expansion?

Answer 38

– Regular expressions are interpreted and matched by special utilities (such as grep). – File name expansions are interpreted and matched by shells

Question 39

What does a dot "." mean in wildcarding?

Answer 39

Match any single character

Question 40

What does an asterisk "*" mean in wildcarding?

Answer 40

Match zero or more of the previous single character pattern Ex: a*b matches b, ab, aab, aaab, aaaab, …

Question 41

What does a plus "+" mean in wildcarding?

Answer 41

+ matches one or more occurrences of the previous single character pattern a\+b matches ab, aab, aaab, aaaab, …

Question 42

What does "?" mean in wildcarding?

Answer 42

? matches zero or one occurrence of the previous single character pattern a\?b matches b and ab

Question 43

"+" and "?" have to be escaped with _ to have the special meaning

Answer 43

\ ex: \? or \+

Question 44

To match a set or range of characters, ___ is used

Answer 44

[...] is used: [wxyz] or [w-z]

Question 45

What does the following do for regular expressions? [aeiou]*

Answer 45

Matches any number of vowels

Question 46

Wildcards lose their specialness inside of...

Answer 46

[...]

Question 47

What can you use to match something at the beginning of a line in regular expressions?

Answer 47

^ ex: ^TITLE matches any line with TITLE at the beginning

Question 48

What can you use to match something at the end of a line in regular expressions?

Answer 48

$ ex: FINI$ matches any line ending with FINI

Question 49

A 'word' is...

Answer 49

a pattern containing only letters, | digits, and underscores (_)

Question 50

Match the beginning of a word with...

Answer 50

\< \

Question 51

Match the end of a word with...

Answer 51

\> ox\> matches ox if it appears at the end of a word

Question 52

Matching the complement of a set by using the...

Answer 52

^ – [^aeiou] - matches any non-vowel – ^[^a-z]*$ - matches any line containing no lower case letters

Question 53

To escape the 'specialness' of a wildcard, use...

Answer 53

the backslash \ symbol

Question 54

To remember portions of a regular expression, use

Answer 54

Surround them with \(...\): '^\([a-z]\)\1' matches lines beginning with a pair of duplicate (identical) letters '^.*\([a-z][a-z]*\).*\1.*\1' matches lines containing at least three copies of something which consists of lower case letters

Question 55

The -c option with grep does what?

Answer 55

Count the number of matches

Question 56

What does the | symbol do for regular expressions? Ex: r1|r2

Answer 56

r1|r2 matches regular expression r1 or r2 (| acts like a logical “or” operator). – red|blue will match either red or blue – Unix|UNIX will match either Unix or UNIX