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CS2211 > Week 6: Unix Command I/O and Redirection,  Processes and Job Control > Flashcards

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

1
Q

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

A

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
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2
Q

What is the default standard input/output/error?

A

Input: keyboard

Output: display

Error: display

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3
Q

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

A

<

or

0

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4
Q

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

A

>

or

1>

EX: cat file1 file2 > file3

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5
Q

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

A

2>

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6
Q

For file redirection, what do the following do?

cat file1 file2 > file3

cat file1 file2 >| file3

cat file1 file2&raquo_space; file3

cat > file3

A

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

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7
Q

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

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

A

cat myfile > yourfile 2> yourerrorfile

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8
Q

What does the following do?

cat myfile &> yourfile

A

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

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9
Q

What does the following do?

cat myfile > yourfile 2>&1

A

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

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10
Q

What does tr string1 string2 do?

A

character n of string1 translated to

character n of string2

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11
Q

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

A

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

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12
Q

What is /dev/null

A

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

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13
Q

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

A

Filters

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14
Q

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

A

LINES

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15
Q

What does the following do?

grep “unix is easy” < myfile1 > myfile2

A

Write all lines of myfile1 containing phrase unix is easy

to myfile2

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16
Q

What does wc do?

A

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

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17
Q

What does the sort filer do?

A

Sort all the input lines in alphabetical order and write to

the standard output.

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18
Q

What does a “pipe” do in essence?

A

Connects stdout of one program with stdin of another

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19
Q

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

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

A

cat readme.txt | grep unix | wc -l

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20
Q

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

A

command1 |& command2

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21
Q

What will the shell do when given the following command?

readme.txt > grep unix | wc -l

A

Your shell will go looking for a program named

readme.txt

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22
Q

What does the “tee” command do?

A

replicate the standard output:

cat readme.txt | tee myfile

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23
Q

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

A

Foreground

24
Q

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

A

a_heavy_task &

25
Q

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

A

Appears on the screen

26
Q

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

A

Ctrl-z

27
Q

What command displays what is running and/or stopped?

A

“jobs”
EX:

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

28
Q

What command moves a job into the foreground?

A

“fg”

Ex: fg %2

29
Q

What command moves a job into the background?

A

“bg”

Ex: bg %1

30
Q

What is the unix command to terminate a job?

A

“kill”

Ex: kill %1

31
Q

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

A

kill -9 %1

or

kill -KILL %1

32
Q

What command lists Unix processes?

A

“ps”

33
Q

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

A

–l gives you a long listing of what is going on

–u loginid tells you about loginid’s processes

34
Q

What does the “ulimit” utility do?

A

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.

35
Q

What are some “ulimit” options?

A

ulimit –a (prints all limits)

ulimit -n (maximum number of open file descriptors)

ulimit -u (maximum number of processes available)

36
Q

What does the following do:

grep unix file

A

Matches all appearances of unix in file

37
Q

What does the following do:

grep ‘[Uu]nix’ file

A

Matches Unix and unix

38
Q

How are regular expressions different from file name expansion?

A

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

– File name expansions are interpreted and
matched by shells

39
Q

What does a dot “.” mean in wildcarding?

A

Match any single character

40
Q

What does an asterisk “*” mean in wildcarding?

A

Match zero or more of the previous single character pattern

Ex: a*b matches b, ab, aab, aaab, aaaab, …

41
Q

What does a plus “+” mean in wildcarding?

A

+ matches one or more occurrences of the
previous single character pattern

a+b matches ab, aab, aaab, aaaab, …

42
Q

What does “?” mean in wildcarding?

A

? matches zero or one occurrence of the
previous single character pattern

a\?b matches b and ab

43
Q

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

A

\

ex: \? or +

44
Q

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

A

[…] is used:

[wxyz] or [w-z]

45
Q

What does the following do for regular expressions?

[aeiou]*

A

Matches any number of vowels

46
Q

Wildcards lose their specialness inside of…

A

[…]

47
Q

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

A

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

48
Q

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

A

$

ex: FINI$ matches any line ending with FINI

49
Q

A ‘word’ is…

A

a pattern containing only letters,

digits, and underscores (_)

50
Q

Match the beginning of a word with…

A

<

\

51
Q

Match the end of a word with…

A

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

52
Q

Matching the complement of a set by using the…

A

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

53
Q

To escape the ‘specialness’ of a wildcard, use…

A

the backslash \ symbol

54
Q

To remember portions of a regular expression, use

A

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

55
Q

The -c option with grep does what?

A

Count the number of matches

56
Q

What does the | symbol do for regular expressions?

Ex: r1|r2

A

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

CS2211 (9 decks)

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