Find Specific Lines in Bash

grep phone numbers

Given a text file file.txt that contains a 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.

Example:

Assume that file.txt has the following content:

987-123-4567
123 456 7890
(123) 456-7890
0(001) 345-0000

Your script should output the following valid phone numbers:

987-123-4567
(123) 456-7890

• grep -E '^\([0-9]{3}\) [0-9]{3}-[0-9]{4}$|^[0-9]{3}-[0-9]{3}-[0-9]{4}$' file.txt

This uses the grep command with the -E option (or egrep) to enable extended regular expressions. The regular expression ^\([0-9]{3}\) [0-9]{3}-[0-9]{4}$|^[0-9]{3}-[0-9]{3}-[0-9]{4}$ matches lines that match either of the two valid phone number formats.

Print a specific line

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.

sed

command	description
sed '10!d' file.txt	10!d means delete all of the lines except line ten.
sed -n '10p' file.txt	-n '10p' means supress all other and print only the tenth line

Both of these commands will read through the entire file so the sed command come with another command q that will stop reading the file after the specified command.

command	description
sed '10!d;q' file.txt	10!d;q means delete all of the lines except line ten. And stop reading after line ten.

awk

We can also use awk to output the line of a file. We can also use a command to stop processing after { print; exit }

command	description
awk 'NR==10' file.txt	Prints the tenth line of file.txt.
awk 'NR==10{ print; exit } file.txt	Prints the tenth line, then stops the process.

These are just a few of the commands I have come accross so far that have come in useful.