192. Word Frequency

Write a bash script to calculate the frequency of each word in a text file words.txt.
For simplicity sake, you may assume:

• words.txt contains only lowercase characters and space ' ' characters.
• Each word must consist of lowercase characters only.
• Words are separated by one or more whitespace characters.
  Example:
  Assume that words.txt has the following content:

the day is sunny the the
the sunny is is

Your script should output the following, sorted by descending frequency:

the 4
is 3
sunny 2
day 1

Note:

• Don't worry about handling ties, it is guaranteed that each word's frequency count is unique.
• Could you write it in one-line using Unix pipes?

Ans:

cat words.txt | tr -s ' ' '\n' | sort  | uniq -c | sort -r | awk '{print $2,$1}'

193. valid-phone-numbers ref

Given a text file file.txt that contains list of phone numbers (one per line), write a one liner bash script to print all valid phone numbers.
You may assume that a valid phone number must appear in one of the following two formats: (xxx) xxx-xxxx or xxx-xxx-xxxx. (x means a digit)
You may also assume each line in the text file must not contain leading or trailing white spaces.
For example, assume that file.txt has the following content:

987-123-4567
123 456 7890
(123) 456-7890

Your script should output the following valid phone numbers:

987-123-4567
(123) 456-7890

Ans:

awk '/^(\([0-9]{3}\) |[0-9]{3}-)[0-9]{3}-[0-9]{4}$/' file.txt
# Mac sed, -E 正则  GNU/Linux也OK
sed -n -E '/^(\([0-9]{3}\) |[0-9]{3}-)[0-9]{3}-[0-9]{4}$/p' file.txt
# GNU/Linux sed  -r 正则
sed -n -r '/^(\([0-9]{3}\) |[0-9]{3}-)[0-9]{3}-[0-9]{4}$/p' file.txt
# both GNU/Linux and Mac, --extended-regexp (或者直接 egrep '^(\([0-9]{3}\) |[0-9]{3}-)[0-9]{3}-[0-9]{4}$' file.txt)
grep -E '^(\([0-9]{3}\) |[0-9]{3}-)[0-9]{3}-[0-9]{4}$' file.txt 
# only GNU/Linux --perl-regexp
grep -P '^(\([0-9]{3}\) |[0-9]{3}-)[0-9]{3}-[0-9]{4}$' file.txt

194. Transpose File

Given a text file file.txt, transpose its content.
You may assume that each row has the same number of columns and each field is separated by the ' ' character.
For example, if file.txt has the following content:

name age
alice 21
ryan 30

Output the following:
name alice ryan
age 21 30

ncol=`head -n1 file.txt | wc -w`
for i in `seq 1 $ncol`
do
    echo `cut -d' ' -f$i file.txt`
done

195. Tenth Line

DescriptionHintsSubmissionsDiscussSolution
Given a text file file.txt, print just the 10th line of the file.
Example:
Assume that file.txt has the following content:

Line 1
Line 2
Line 3
Line 4
Line 5
Line 6
Line 7
Line 8
Line 9
Line 10

Your script should output the tenth line, which is:

Line 10
Note:

1. If the file contains less than 10 lines, what should you output?
2. There's at least three different solutions. Try to explore all possibilities.
   Ans: