Exercise solutions

Introduction

1) Replace only the first occurrence of 5 with five for the given stdin source.

$ echo 'They ate 5 apples and 5 mangoes' | sed 's/5/five/'
They ate five apples and 5 mangoes

2) Replace all occurrences of 5 with five.

$ echo 'They ate 5 apples and 5 mangoes' | sed 's/5/five/g'
They ate five apples and five mangoes

3) Replace all occurrences of 0xA0 with 0x50 and 0xFF with 0x7F for the given input file.

$ cat hex.txt
start address: 0xA0, func1 address: 0xA0
end address: 0xFF, func2 address: 0xB0

# if you already know how to specify multiple commands,
# a better solution would be: sed 's/0xA0/0x50/g; s/0xFF/0x7F/g' hex.txt
$ sed 's/0xA0/0x50/g' hex.txt | sed 's/0xFF/0x7F/g'
start address: 0x50, func1 address: 0x50
end address: 0x7F, func2 address: 0xB0

4) The substitute command searches and replaces sequences of characters. When you need to map one or more characters with another set of corresponding characters, you can use the y command. Quoting from the manual:

y/src/dst/ Transliterate any characters in the pattern space which match any of the source-chars with the corresponding character in dest-chars.

Use the y command to transform the given input string to get the output string as shown below.

$ echo 'goal new user sit eat dinner' | sed 'y/aeiou/AEIOU/'
gOAl nEw UsEr sIt EAt dInnEr

5) Why does the following command produce an error? How'd you fix it?

Space is a shell metacharacter and thus you need to quote the sed script to avoid getting interpreted by the shell.

$ echo 'a sunny day' | sed s/sunny day/cloudy day/
sed: -e expression #1, char 7: unterminated `s' command

# expected output
$ echo 'a sunny day' | sed 's/sunny day/cloudy day/'
a cloudy day

In-place file editing

1) For the input file text.txt, replace all occurrences of in with an and write back the changes to text.txt itself. The original contents should get saved to text.txt.orig

$ cat text.txt
can ran want plant
tin fin fit mine line
$ sed -i.orig 's/in/an/g' text.txt

$ cat text.txt
can ran want plant
tan fan fit mane lane
$ cat text.txt.orig
can ran want plant
tin fin fit mine line

2) For the input file text.txt, replace all occurrences of an with in and write back the changes to text.txt itself. Do not create backups for this exercise. Note that you should have solved the previous exercise before starting this one.

$ cat text.txt
can ran want plant
tan fan fit mane lane
$ sed -i 's/an/in/g' text.txt

$ cat text.txt
cin rin wint plint
tin fin fit mine line
$ diff text.txt text.txt.orig
1c1
< cin rin wint plint
---
> can ran want plant

3) For the input file copyright.txt, replace copyright: 2018 with copyright: 2019 and write back the changes to copyright.txt itself. The original contents should get saved to 2018_copyright.txt.bkp

$ cat copyright.txt
bla bla 2015 bla
blah 2018 blah
bla bla bla
copyright: 2018
$ sed -i'2018_*.bkp' 's/copyright: 2018/copyright: 2019/' copyright.txt

$ cat copyright.txt
bla bla 2015 bla
blah 2018 blah
bla bla bla
copyright: 2019
$ cat 2018_copyright.txt.bkp
bla bla 2015 bla
blah 2018 blah
bla bla bla
copyright: 2018

4) In the code sample shown below, two files are created by redirecting the output of the echo command. Then a sed command is used to edit b1.txt in-place as well as create a backup named bkp.b1.txt. Will the sed command work as expected? If not, why?

$ echo '2 apples' > b1.txt
$ echo '5 bananas' > -ibkp.txt
$ sed -ibkp.* 's/2/two/' b1.txt

Unquoted strings on the command line are subjected to shell interpretation. So, -ibkp.* will get expanded as -ibkp.txt (as there exists a file whose name starts with -ibkp.). This results in back up filename as b1.txtbkp.txt (because bkp.txt will be treated as the suffix to be added to b1.txt). The correct usage is sed -i'bkp.*' 's/2/two/' b1.txt to get bkp.b1.txt as the back up filename.

5) For the input file pets.txt, remove the first occurrence of I like from each line and write back the changes to pets.txt itself. The original contents should get saved with the same filename inside the bkp directory. Assume that you do not know whether bkp exists or not in the current working directory.

$ cat pets.txt
I like cats
I like parrots
I like dogs

$ mkdir -p bkp
$ sed -i'bkp/*' 's/I like //' pets.txt

$ cat pets.txt
cats
parrots
dogs
$ cat bkp/pets.txt
I like cats
I like parrots
I like dogs

Selective editing

1) For the given input, display except the third line.

$ seq 34 37 | sed '3d'
34
35
37

2) Display only the fourth, fifth, sixth and seventh lines for the given input.

$ seq 65 78 | sed -n '4,7p'
68
69
70
71

3) For the input file addr.txt, replace all occurrences of are with are not and is with is not only from line number 4 till the end of file. Also, only the lines that were changed should be displayed in the output.

$ cat addr.txt
Hello World
How are you
This game is good
Today is sunny
12345
You are funny

$ sed -n '4,$ {s/are/are not/gp; s/is/is not/gp}' addr.txt
Today is not sunny
You are not funny

4) Use sed to get the output shown below for the given input. You'll have to first understand the input to output transformation logic and then use commands introduced in this chapter to construct a solution.

# can also use: sed -n '2~5{p; n; n; p}'
$ seq 15 | sed -n 'n; p; n; n; p; n'
2
4
7
9
12
14

5) For the input file addr.txt, display all lines from the start of the file till the first occurrence of is.

$ sed '/is/q' addr.txt
Hello World
How are you
This game is good

6) For the input file addr.txt, display all lines that contain is but not good.

$ sed -n '/is/{/good/!p}' addr.txt
Today is sunny

7) n and N commands will not execute further commands if there are no more input lines to fetch. Correct the command shown below to get the expected output.

# wrong output
$ seq 11 | sed 'N; N; s/\n/-/g'
1-2-3
4-5-6
7-8-9
10
11

# expected output
# can also use: sed 'N; $!N; s/\n/-/g'
$ seq 11 | sed 'N; $!N; y/\n/-/'
1-2-3
4-5-6
7-8-9
10-11

8) For the input file addr.txt, add line numbers in the format as shown below.

$ sed '=' addr.txt
1
Hello World
2
How are you
3
This game is good
4
Today is sunny
5
12345
6
You are funny

9) For the input file addr.txt, print all lines that contain are and the line that comes after, if any.

$ sed -n '/are/,+1 p' addr.txt
How are you
This game is good
You are funny

Bonus: For the above input file, will sed -n '/is/,+1 p' addr.txt produce identical results as grep -A1 'is' addr.txt? If not, why?

No. sed will not try to match is in the context lines whereas grep will match the search term in the context lines as well. Also, in general, you'll need to use the --no-group-separator option to prevent grep from adding a separator between the different matching groups.

10) Print all lines if their line numbers follow the sequence 1, 15, 29, 43, etc but not if the line contains 4 in it.

$ seq 32 100 | sed -n '1~14 {/4/!p}'
32
60
88

11) For the input file sample.txt, display from the start of the file till the first occurrence of are, excluding the matching line.

$ cat sample.txt
Hello World

Hi there
How are you

Just do-it
Believe it

banana
papaya
mango

Much ado about nothing
He he he
Adios amigo

$ sed '/are/Q' sample.txt
Hello World

Hi there

12) For the input file sample.txt, display from the last occurrence of do till the end of the file.

$ tac sample.txt | sed '/do/q' | tac
Much ado about nothing
He he he
Adios amigo

13) For the input file sample.txt, display from the 9th line till a line containing go.

$ sed -n '9,/go/p' sample.txt
banana
papaya
mango

14) For the input file sample.txt, display from a line containing it till the next line number that is divisible by 3.

$ sed -n '/it/,~3p' sample.txt
Just do-it
Believe it

banana

15) Display only the odd numbered lines from addr.txt.

# can also use: sed -n '1~2p' addr.txt
$ sed -n 'p; n' addr.txt
Hello World
This game is good
12345

BRE/ERE Regular Expressions

1) For the input file patterns.txt, display all lines that start with den or end with ly.

$ sed -nE '/^den|ly$/p' patterns.txt
2 lonely
dent
lovely

2) For the input file patterns.txt, replace all occurrences of 42 with [42] unless it is at the edge of a word. Display only the modified lines.

$ sed -n 's/\B42\B/[&]/gp' patterns.txt
Hi[42]Bye nice1[42]3 bad42
eqn2 = pressure*3+42/5-1[42]56
cool_[42]a 42fake
_[42]_

3) For the input file patterns.txt, add [] around words starting with s and containing e and t in any order. Display only the modified lines.

$ sed -nE 's/\bs\w*(e\w*t|t\w*e)\w*/[&]/gp' patterns.txt
[sets] tests Sauerkraut
[site] cite kite bite [store_2]
[subtle] sequoia
a [set]

4) From the input file patterns.txt, display lines having the same first and last word character.

# can also use: sed -nE '/^(\w|(\w).*\2)$/p' patterns.txt
$ sed -nE '/^(\w)(.*\1)?$/p' patterns.txt
Not a pip DOWN
y
1 dentist 1
_42_

5) For the input file patterns.txt, display lines containing *[5] literally.

$ sed -n '/\*\[5]/p' patterns.txt
(9-2)*[5]

6) sed -nE '/\b[a-z](on|no)[a-z]\b/p' is same as sed -nE '/\b[a-z][on]{2}[a-z]\b/p'. True or False? Sample input shown below might help to understand the differences, if any.

False. [on]{2} will also match oo and nn.

$ printf 'known\nmood\nknow\npony\ninns\n'
known
mood
know
pony
inns

7) For the input file patterns.txt, display all lines starting with hand and ending immediately with s or y or le or no further characters.

$ sed -nE '/^hand([sy]|le)?$/p' patterns.txt
handle
handy
hands
hand

8) For the input file patterns.txt, replace 42//5 or 42/5 with 8. Display only the modified lines.

$ sed -nE 's#42//?5#8#gp' patterns.txt
eqn3 = r*42-5/3+42///5-83+a
eqn1 = a+8-c
eqn2 = pressure*3+8-14256

9) For the given quantifiers, what would be the equivalent form using the {m,n} representation?

? is same as {,1}
* is same as {0,}
+ is same as {1,}

10) True or False? In ERE, (a*|b*) is same as (a|b)*.

False. Because (a*|b*) will match only sequences like a, aaa, bb, bbbbbbbb. But (a|b)* can match a mixed sequence like ababbba too.

11) For the input file patterns.txt, construct two different REGEXPs to get the outputs as shown below. Display only the modified lines.

# delete from '(' till next ')'
$ sed -n 's/([^)]*)//gp' patterns.txt
a/b + c%d
*[5]
def factorial
12- *4)
Hi there. Nice day

# delete from '(' till next ')' but not if there is '(' in between
$ sed -n 's/([^()]*)//gp' patterns.txt
a/b + c%d
*[5]
def factorial
12- (e+*4)
Hi there. Nice day(a

12) For the input file anchors.txt, convert markdown anchors to corresponding hyperlinks as shown below.

$ cat anchors.txt
# <a name="regular-expressions"></a>Regular Expressions
## <a name="subexpression-calls"></a>Subexpression calls
## <a name="the-dot-meta-character"></a>The dot meta character

$ sed -E 's|[^"]+"([^"]+)"></a>(.+)|[\2](#\1)|' anchors.txt
[Regular Expressions](#regular-expressions)
[Subexpression calls](#subexpression-calls)
[The dot meta character](#the-dot-meta-character)

13) For the input file patterns.txt, replace the space character that occurs after a word ending with a or r with a newline character, only if the line also contains an uppercase letter. Display only the modified lines.

$ sed -nE '/[A-Z]/ s/([ar])\b /\1\n/gp' patterns.txt
par
car
tar
far
Cart
Not a
pip DOWN

14) Surround all whole words with (). Additionally, if the whole word is imp or ant, delete them. Can you do it with a single substitution?

$ words='tiger imp goat eagle ant important'
$ echo "$words" | sed -E 's/\b(imp|ant|(\w+))\b/(\2)/g'
(tiger) () (goat) (eagle) () (important)

15) For the input file patterns.txt, display lines containing car but not as a whole word.

$ sed -nE '/\Bcar|car\B/p' patterns.txt
scar
care
a huge discarded pile of books
scare
part cart mart

16) Will the ERE pattern ^a\w+([0-9]+:fig)? match the same characters for the input apple42:banana314 and apple42:fig100? If not, why not?

$ echo 'apple42:banana314' | sed -E 's/^a\w+([0-9]+:fig)?/[&]/'
[apple42]:banana314

$ echo 'apple42:fig100' | sed -E 's/^a\w+([0-9]+:fig)?/[&]/'
[apple42:fig]100

For patterns matching from the same starting location, longest match wins in case of BRE/ERE flavors. So, \w+ will give up characters to allow ([0-9]+:fig)? to also match in the second case. In other flavors like PCRE, apple42 will be matched both the cases.

17) For the input file patterns.txt, display lines starting with 4 or - or u or sub or care.

$ sed -nE '/^([4u-]|sub|care)/p' patterns.txt
care
4*5]
-handy
subtle sequoia
unhand

18) For the given input string, replace all occurrences of digit sequences with only the unique non-repeating sequence. For example, 232323 should be changed to 23 and 897897 should be changed to 897. If there are no repeats (for example 1234) or if the repeats end prematurely (for example 12121), it should not be changed.

$ s='1234 2323 453545354535 9339 11 60260260'
$ echo "$s" | sed -E 's/\b([0-9]+)\1+\b/\1/g'
1234 23 4535 9339 1 60260260

19) Replace sequences made up of words separated by : or . by the first word of the sequence. Such sequences will end when : or . is not followed by a word character.

$ ip='wow:Good:2_two.five: hi-2 bye kite.777:water.'
$ echo "$ip" | sed -E 's/([:.]\w*)+//g'
wow hi-2 bye kite

20) Replace sequences made up of words separated by : or . by the last word of the sequence. Such sequences will end when : or . is not followed by a word character.

$ ip='wow:Good:2_two.five: hi-2 bye kite.777:water.'
$ echo "$ip" | sed -E 's/((\w+)[:.])+/\2/g'
five hi-2 bye water

21) Replace all whole words with X unless it is preceded by a ( character.

$ s='guava (apple) berry) apple (mango) (grape'
$ echo "$s" | sed -E 's/(^|[^(])\b\w+/\1X/g'
X (apple) X) X (mango) (grape

22) Surround whole words with [] only if they are followed by : or , or -.

$ ip='Poke,on=-=so_good:ink.to/is(vast)ever2-sit'
$ echo "$ip" | sed -E 's/(\w+)([:,-])/[\1]\2/g'
[Poke],on=-=[so_good]:ink.to/is(vast)[ever2]-sit

23) The fields.txt file has fields separated by the : character. Delete : and the last field if there is a digit character anywhere before the last field.

$ cat fields.txt
42:cat
twelve:a2b
we:be:he:0:a:b:bother
apple:banana-42:cherry:
dragon:unicorn:centaur

# can also use: sed -E '/[0-9].*:/ s/:[^:]*$//' fields.txt
$ sed -E 's/([0-9].*):.*/\1/' fields.txt
42
twelve:a2b
we:be:he:0:a:b
apple:banana-42:cherry
dragon:unicorn:centaur

24) Are the commands sed -n '/a^b/p' and sed -nE '/a^b/p' equivalent?

They are not equivalent. In BRE, ^ is not special when not used in the customary position. In ERE, ^ has to be escaped to match it literally. The same holds true for the $ metacharacter as well.

$ echo 'a^b+c' | sed -n '/a^b/p'
a^b+c

$ echo 'a^b+c' | sed -nE '/a^b/p'
$ echo 'a^b+c' | sed -nE '/a\^b/p'
a^b+c

25) What characters can be used as REGEXP delimiters?

Any character other than \ and the newline character can be used as delimiters.

Flags

1) For the input file para.txt, remove all groups of lines marked with a line beginning with start and a line ending with end. Match both these markers case insensitively.

$ cat para.txt
good start
Start working on that
project you always wanted
to, do not let it end
hi there
start and try to
finish the End
bye

$ sed '/^start/I,/end$/I d' para.txt
good start
hi there
bye

2) The headers.txt file contains one header per line, starting with one or more # characters followed by one or more whitespace characters and then some words. Convert such lines to the corresponding output as shown below.

$ cat headers.txt
# Regular Expressions
## Subexpression calls
## The dot meta character

$ sed -E 's/^#+\s+(.+)/\L\1/; y/ /-/' headers.txt
regular-expressions
subexpression-calls
the-dot-meta-character

3) Using para.txt, create a file named five.txt with lines that contain a whole word of length 5 and a file named seven.txt with lines that contain a whole word of length 7.

$ sed -nE -e '/\b\w{5}\b/w five.txt' -e '/\b\w{7}\b/w seven.txt' para.txt

$ cat five.txt
good start
Start working on that
hi there
start and try to

$ cat seven.txt
Start working on that
project you always wanted

4) Given sample strings have fields separated by , where field values can be empty as well. Use sed to replace the third field with 42.

$ echo 'lion,,ant,road,neon' | sed 's/[^,]*/42/3'
lion,,42,road,neon

$ echo ',,,' | sed 's/[^,]*/42/3'
,,42,

5) Replace all occurrences of e with 3 except the first two matches.

$ echo 'asset sets tests site' | sed 's/e/3/3g'
asset sets t3sts sit3

$ echo 'sample item teem eel' | sed 's/e/3/3g'
sample item t33m 33l

6) For the input file addr.txt, replace all input lines with the number of characters in those lines. wc -L is one of the ways to get the length of a line as shown below. Assume that the input file doesn't have single or double quote characters.

# note that newline character isn't counted, which is preferable here
$ echo "Hello World" | wc -L
11

$ sed 's/.*/echo "&" | wc -L/e' addr.txt
11
11
17
14
5
13

7) For the input file para.txt, assume that it'll always have lines in multiples of 4. Use sed commands such that there are 4 lines at a time in the pattern space. Then, delete from start till end provided start is matched only at the start of a line. Also, match these two keywords case insensitively.

$ sed -E 'N;N;N; s/^start(\s|\S)*end//Im' para.txt
good start

hi there

bye

8) For the input file patterns.txt, replace the last but second ar with X. Display only the modified lines.

$ sed -nE 's/(.*)ar((.*ar){2})/\1X\2/p' patterns.txt
par car tX far Cart
pXt cart mart

9) Display lines from sample.txt that satisfy both of these conditions:

he matched irrespective of case
either World or Hi matched case sensitively

$ sed -nE '/he/I{/World|Hi/p}' sample.txt
Hello World
Hi there

10) For the input file patterns.txt, surround all hexadecimal sequences with a minimum of four characters with []. Match 0x as an optional prefix, but shouldn't be counted for determining the length. Match the characters case insensitively, and the sequences shouldn't be surrounded by other word characters. Display only the modified lines.

# can also use: sed -nE 's/\b(0x)?[[:xdigit:]]{4,}\b/[&]/Igp' patterns.txt
$ sed -nE 's/\b(0x)?[0-9a-f]{4,}\b/[&]/Igp' patterns.txt
"should not match [0XdeadBEEF]"
Hi42Bye nice1423 [bad42]
took 0xbad 22 [0x0ff1ce]
eqn2 = pressure*3+42/5-[14256]

Shell substitutions

1) Replace #expr# with the value of the usr_ip shell variable. Assume that this variable can only contain the metacharacters as shown in the sample below.

$ usr_ip='c = (a/b) && (x-5)'
$ mod_ip=$(echo "$usr_ip" | sed 's|[/&]|\\&|g')
$ echo 'Expression: #expr#' | sed 's/#expr#/'"$mod_ip"'/'
Expression: c = (a/b) && (x-5)

2) Repeat the previous exercise, but this time with command substitution instead of using a temporary variable.

$ usr_ip='c = (a/b/y) && (x-5)'
$ echo 'Expression: #expr#' | sed 's/#expr#/'"$(echo "$usr_ip" | sed 's|[/&]|\\&|g')"'/'
Expression: c = (a/b/y) && (x-5)

z, s and f command line options

1) Replace any character other than word and . characters with the _ character for the sample filenames shown below.

$ mkdir test_dir && cd $_
$ touch 'file with spaces.txt' $'weird$ch\nars.txt' '!f@oo.txt'
# > at start of line indicates continuation of multiline shell command
$ for file in *; do
>   new_name=$(printf '%s' "$file" | sed -z 's/[^a-z0-9_.]/_/Ig')
>   mv "$file" "$new_name"
> done

$ ls
file_with_spaces.txt  _f_oo.txt  weird_ch_ars.txt
$ cd .. && rm -r test_dir

2) Print only the third line, if any, from these input files: addr.txt, para.txt and copyright.txt

$ sed -sn '3p' addr.txt para.txt copyright.txt
This game is good
project you always wanted
bla bla bla

3) For the input file hex.txt, use content from replace.txt to perform search and replace operations. Each line in replace.txt starts with the search term, followed by a space and then followed by the replace term. Assume that these terms do not contain any sed metacharacters.

$ cat hex.txt
start address: 0xA0, func1 address: 0xA0
end address: 0xFF, func2 address: 0xB0
$ cat replace.txt
0xA0 0x5000
0xB0 0x6000
0xFF 0x7000

$ sed -f <(sed -E 's|(.+) (.+)|s/\1/\2/g|' replace.txt) hex.txt
start address: 0x5000, func1 address: 0x5000
end address: 0x7000, func2 address: 0x6000

4) For the input file nul_separated, use the ASCII NUL character as the line separator and display lines containing fig. Also, change NUL characters in the output to the newline character.

$ sed -zn '/fig/p' nul_separated | tr '\0' '\n'
apple
fig
mango
icecream

5) For the input file addr.txt, change a newline character that comes before an uppercase letter to . followed by a space character. Assume that the input file doesn't have ASCII NUL characters.

$ sed -zE 's/\n([A-Z])/. \1/g' addr.txt
Hello World. How are you. This game is good. Today is sunny
12345. You are funny

append, change, insert

1) For the input file addr.txt, display the third line surrounded by -----.

$ sed -n -e '3 {i-----' -e 'p; a-----' -e '}' addr.txt
-----
This game is good
-----

2) For the input file addr.txt, replace all lines starting from a line containing you till the end of file with a blank line followed by Have a nice day.

$ sed '/you/,$ c\\nHave a nice day' addr.txt
Hello World

Have a nice day

3) For the input file addr.txt, replace even numbered lines with ---.

$ sed 'n; c---' addr.txt
Hello World
---
This game is good
---
12345
---

Adding content from file

1) For the input file addr.txt, replace from the third to fifth lines with the second to fourth lines from para.txt.

$ sed -n '2,4p' para.txt | sed -e '5r /dev/stdin' -e '3,5d' addr.txt
Hello World
How are you
Start working on that
project you always wanted
to, do not let it end
You are funny

2) Add one line from hex.txt after every two lines of copyright.txt.

$ sed 'n; R hex.txt' copyright.txt
bla bla 2015 bla
blah 2018 blah
start address: 0xA0, func1 address: 0xA0
bla bla bla
copyright: 2019
end address: 0xFF, func2 address: 0xB0

3) For every line of the input file hex.txt, insert --- before the line and append one line from replace.txt as shown below.

$ sed -e 'R replace.txt' -e 'i---' hex.txt
---
start address: 0xA0, func1 address: 0xA0
0xA0 0x5000
---
end address: 0xFF, func2 address: 0xB0
0xB0 0x6000

4) Insert the contents of hex.txt before a line matching 0x6000 of the input file replace.txt.

$ sed '/0x6000/e cat hex.txt' replace.txt
0xA0 0x5000
start address: 0xA0, func1 address: 0xA0
end address: 0xFF, func2 address: 0xB0
0xB0 0x6000
0xFF 0x7000

5) For the input file addr.txt, replace lines containing are with contents of hex.txt.

$ sed -e '/are/r hex.txt' -e '//d' addr.txt
Hello World
start address: 0xA0, func1 address: 0xA0
end address: 0xFF, func2 address: 0xB0
This game is good
Today is sunny
12345
start address: 0xA0, func1 address: 0xA0
end address: 0xFF, func2 address: 0xB0

6) The two commands shown below are equivalent. True or False?

sed '/are/r pets.txt' addr.txt

cat pets.txt | sed '/are/r /dev/stdin' addr.txt

False. The first solution will append the contents of pets.txt for every line matching are. The second solution will append only once as the stdin data will not be available for subsequent matches.

7) What do the commands sed '0r file1' file2 and sed '0R file1' file2 do?

sed '0r file1' file2 will insert the contents of file1 before the first line of file2. The second command will produce an error since 0 as an address isn't supported for the R command.

Control structures

1) Using the input file para.txt, create a file named markers.txt with all lines that contain start or end (matched case insensitively) and a file named rest.txt with the remaining lines.

$ sed -nE -e '/start|end/I{w markers.txt' -e 'b}' -e 'w rest.txt' para.txt

$ cat markers.txt
good start
Start working on that
to, do not let it end
start and try to
finish the End

$ cat rest.txt
project you always wanted
hi there
bye

2) For the input file addr.txt:

if a line contains e, surround all consecutive repeated characters with {} as well as uppercase those characters
else, if a line contains u, surround all uppercase letters in that line with []

# note that H in the second line and Y in the last line aren't modified
$ sed -E '/e/ {s/(.)\1/{\U&}/g; b}; /u/ s/[A-Z]/[&]/g' addr.txt
He{LL}o World
How are you
This game is g{OO}d
[T]oday is sunny
12345
You are fu{NN}y

3) The nums.txt file uses a space character as the field separator. The first field of some lines has one or more numbers separated by the - character. Surround such numbers in the first field with [] as shown below.

$ cat nums.txt
123-87-593 42-3 fig
apple 42-42-42 1000 banana 4-3
53783-0913 hi 3 4-2
1000 guava mango

$ sed -E ':a s/^((\[[0-9]+\]-)*)?([0-9]+)/\1[\3]/; ta' nums.txt
[123]-[87]-[593] 42-3 fig
apple 42-42-42 1000 banana 4-3
[53783]-[0913] hi 3 4-2
[1000] guava mango

4) Convert the contents of headers.txt such that it matches the content of anchors.txt. The input file headers.txt contains one header per line, starting with one or more # characters followed by a space character and then followed by the heading. You have to convert such headings into anchor tags as shown by the contents of anchors.txt. Save the output in out.txt.

$ cat headers.txt
# Regular Expressions
## Subexpression calls
## The dot meta character
$ cat anchors.txt
# <a name="regular-expressions"></a>Regular Expressions
## <a name="subexpression-calls"></a>Subexpression calls
## <a name="the-dot-meta-character"></a>The dot meta character

$ sed -E 's|\w.*|<a name="\L&\E"></a>&|; :a s/("[^"]+) ([^"]+")/\1-\2/; ta' headers.txt > out.txt
$ diff -s out.txt anchors.txt
Files out.txt and anchors.txt are identical

5) What is the difference between t and T commands?

Any successful substitution triggers the t command. T is triggered only if all the prior substitutions have failed.

6) The blocks.txt file uses %=%= to separate group of lines. Display the first such group.

$ cat blocks.txt
%=%=
apple
banana
%=%=
3.14
42
1000
%=%=
brown
green
%=%=
hi
hello there
bye

$ sed ':a n; /%=%=/Q; ba' blocks.txt
%=%=
apple
banana

Processing lines bounded by distinct markers

1) The blocks.txt file uses %=%= to separate group of lines. Display the last such group.

$ tac blocks.txt | sed '/%=%=/q' | tac
%=%=
hi
hello there
bye

2) The code.txt file has code snippets that are surrounded by whole lines containing %%Code: python%% and %%end%%. The end of such snippets is always followed by an empty line. Assume that there will always be at least one line of code between the markers. Delete all such code snippets as well as the empty line that follows.

$ sed '/^%%Code: python%%$/,/^%%end%%$/{//N; d}' code.txt
H1: Introduction

REPL is a good way to learn Python for beginners.

H2: String methods

Python comes loaded with awesome methods.
Enjoy learning Python.

3) The code.txt file has code snippets that are surrounded by whole lines containing %%Code: python%% and %%end%%. Display the lines between such markers only for the first block.

$ sed -n '/^%%Code: python%%$/,/^%%end%%$/{//!p; /^%%end%%$/q}' code.txt
>>> 3 + 7
10
>>> 22 / 7
3.142857142857143
>>> 22 // 7
3

4) For the input file broken.txt, display lines between the markers top and bottom. The first sed command shown below doesn't work because lines till the end of file are printed when the second address isn't found.

$ cat broken.txt
--top--
3.14
[bottom]
--top--
1234567890
[bottom]
--top--
Hi there
Have a nice day
Good bye

# wrong output
$ sed -n '/top/,/bottom/ {//!p}' broken.txt
3.14
1234567890
Hi there
Have a nice day
Good bye

# expected output
$ tac broken.txt | sed -n '/bottom/,/top/ {//!p}' | tac
3.14
1234567890

5) For the input file addr.txt, replace the lines occurring between the markers How and 12345 with the contents of the file hex.txt.

$ sed -e '/How/r hex.txt' -e '//,/12345/{//!d}' addr.txt
Hello World
How are you
start address: 0xA0, func1 address: 0xA0
end address: 0xFF, func2 address: 0xB0
12345
You are funny

CLI text processing with GNU sed