Ch 4 Using Common Text File-Related Tools

Question 1

What command opens text file in a pager (to the console/terminal/stdout), allowing for easier reading?

Answer 1

less

(e.g.,

sudo dmesg | less
pipes the output of dmesg to a pager

Question 2

what command dumpts contents of a text file to screen

Answer 2

cat
(short for concatenate…chaining things together)

Question 3

because of the limitations of how much data you can see in a shell window, cat will only give you the last lines of a file. What command do you use if you want to see the first lines?

Answer 3

tac (cat spelled backwards)

Question 4

what command will dump the first ten lines of a file to stdout?

Answer 4

head

Question 5

what command will dump the last ten lines of a file to stdout?

Answer 5

tail

Question 6

What does this command do?

head -n 11 /etc/passwd | tail -n 1

Answer 6

The n flag tells it how many lines to give. So first it outputs the first 11 lines of /etc/passwd.

It pipes that to tail, which takes those 11 lines and only shows that last one

Question 7

What does the -f flag for the tail command do?

Answer 7

shows you the last ten lines of a file, but sends any additional lines to stdout as that process runs,

e.g. tail -f /var/log/messages

Question 8

what are the 11 main commands (well, one is a tech) used for working with text files?

Answer 8

less
cat
tac
tail
head
sort
cut
grep
regex
awk
sed

Question 9

what does the cut command do, broadly?

Answer 9

filters out specific fields.

e.g. list of all users in /etc/passwd/

Question 10

what is the -d flag for cut?

Answer 10

-d is for delimiter. pick something like a comma or a colon. must be followed by -f, for field (and this will be a number)

Question 11

What is this command doing?

cut -d : -f 1 /etc/passwd

Answer 11

looking at the /etc/passwd file, sort using the 1st field and use : as the delimiter

this filters OUT the first field of the file, which will give you just the usernames

Question 14

Translate this into plain english:

Cut -d : -f 1 /etc/passwd

Answer 14

Parse the etc/password file and, using a colon as a delimiter, take only the first field

Question 15

What would be the output of

Sort /etc/passwd?

Answer 15

It will print out the contents of the file, but in byte order – or how characters appear in an ASCII text table – looks like it’s alphabetic, but ISNT

Question 16

Is the output of sort alphabetical?

Answer 16

No

It’s based off the ASCII text table order. So since all capitals come first, Z would come before a, because it’s capitalized

Question 17

What command often goes with cut?

Answer 17

Sort

Question 18

What are different options with the sort command?

Answer 18

-n for numeric order
-rn for reverse numeric

Question 19

what does the command

sort -k3 -t : /etc/passwd do?

Answer 19

This sorts along a column of your choice – in this case, it’s filtering along the 3rd column in the /etc/passwd file, using : as the field delimiter

Question 20

what does this command do?

ps aux | sort -k 4 -n

Answer 20

sort the output of the ps aux alone the 4th column, and sort it numerically (don’t need a delimiter because it’s a command’s output, not a file’s?)

overall, this command tells us which processes are using the most memory

Question 21

What command will give you a word count?

Answer 21

wc

Question 22

what are the three different results you could get from the wc command?

Answer 22

1. number of lines
2. number of words
3. number of characters

Question 23

What would be the simplest/easiest/fastest command to figure out how many processes are running right now?

Answer 23

ps aux | wc

Question 24

what does this command do ?

grep anna /etc/passwd

Answer 24

looks for the word anna in the /etc/passwd file

Question 25

what is a regex line anchor?

Answer 25

^ as in ^anna as in "starts with anna"

Question 26

what would the following command do? grep ash$ /etc/passwd

Answer 26

grab any line in the /etc/passwd file that ends in ash

Question 27

what characters need to be escaped in regex?

Answer 27

* $ ?w

Question 28

which is better and why? grep ^anna /etc/passwd or grep '^anna' /etc/passwd

Answer 28

grep '^anna' /etc/passwd because '^anna" allows you to escape the bash command line interpreting the ^ in anna. We only want the grep command to interpret it.

Question 29

what are the regex wildcards and multipliers

Answer 29

. -- one specific character (r.t matches rat and rut) * -- matches zero or more of a character \{2\} -- or whatever number. match that number of characters (regardless of what they are) ? -- character is optional - match zero or one of character

Question 30

What does this mean in regex? re\{2\}d

Answer 30

match a word that begins with re, ends with d, and has any 2 other characters in between

Question 30

what are some characters in extended regex that base regex doesn't have?

Answer 30

+ -- a character should occur one or more times ? -- a character should occur zero or more times

Question 30

what flag does grep need to use to have the extra features of the extended regex?

Answer 30

grep -E OR just use egrep

Question 31

^text

Answer 31

match a line that starts with 'text'

Question 32

text$

Answer 32

match a line that ends with 'text'

Question 33

.

Answer 33

wildcard -- match any single character (match one, but no more than one, unlike +, which is 1 or more)

Question 34

[abc]

Answer 34

match a, b, or c

Question 35

?

Answer 35

extended regex that matches 0 or more of character preceding

Question 36

+

Answer 36

extended regex that matches one or more of character preceding

Question 37

*

Answer 37

matches zero to an infinite number of the preceding character

Question 38

\{2\}

Answer 38

match exactly 2 of the previous character

Question 39

\{1,3\}

Answer 39

match a min of one and a max of three of the previous character

Question 40

*

Answer 40

match zero to an infinite number of the previous character

Question 41

what does this mean in regex? colou?r

Answer 41

match the word color or colour, because the ? means the u is optional

Question 42

(...) in regex

Answer 42

the () are used to group multiple characters so that the regex can be applied to the group

Question 43

what selinux manpage has an example of regex being used for selinux?

Answer 43

semanage-fcontext

Question 44

what does this mean: "/web(/.*)?"

Answer 44

this regex is referring to the text /web this is followed by the regex (/.*)? It's basically saying, in the /web directory, look for subdirectory files (/web/.* but the .* are optional)?

Question 45

what does grep stand for?

Answer 45

general regular expression parser

Question 46

list 7 of grep's most useful flags

Answer 46

-i -- ignore case -v -- show lines that DON'T have the regular expression -r -- search files recursively -e -- search for lines matching more than one regex, with an addnl -e flag before each regex to be used -E -- interpret using extended regex -A -- show of lines AFTER the matching regex -B -- show of lines BEFORE the matching regex

Question 47

grep -v '^#' /etc/services -B 5

Answer 47

show lines that DON'T start with #, and then the 5 preceding lines to that

Question 48

grep -v -e '^#' -e '^$' /etc/services

Answer 48

excludes all blank lines and lines that start with # (because blank lines start with #$ or, for lines that start with # and also start with a $ after that, exclude them

Question 49

vim option to show hidden characters

Answer 49

:set list, or :set nolist to hide

Question 50

What has made sed and awk historically so powerful and useful?

Answer 50

they were developed in a time when computers didn't have screens, so it's good at creating and parsing text files in a scriptlike way

Question 51

what does this do? awk -F : '{ print $4 }' /etc/passwd

Answer 51

print the fourth field (column) in the /etc/passwd file?

Question 52

what sets awk apart from cut?

Answer 52

awk is better at knowing which fields to use. So if cut fails, use awk it can also do what grep does

Question 53

what does awk -F : ' /user/ { print $4 }' /etc/passwd do?

Answer 53

search the /etc/passwd file for the text 'user' -- if any lines match, print only the fourth field

Question 54

what does sed stand for?

Answer 54

stream editor

Question 55

what does sed -n 5p /etc/passwd do?

Question 56

what does sed do?

Answer 56

filters text from text files (like grep) but also allows you to make modifications as well

Question 57

what does sed -i s/old-text/new-text/g ~/myfile

Answer 57

search for old-text in myfile, and replace with new-text (ALL occurrences...that's what the g is for)

Question 58

what does the -i flag in sed do?

Answer 58

write the output to the output file instead of STDOUT (the default)

Question 59

what does sed -i -e '2d' ~/myfile do?

Answer 59

delete the second line in myfile

Question 60

what does sed -i -e '2d;20,25d' ~/myfile do?

Answer 60

delete lines 2, 20, and 25 from myfile