how do I use the grep command to match a particular pattern ? if pattern specified in grep is “4” it should not return lines containing 44

Question

When i use the grep command as follows :

rollno=4
grep $rollno=4 new.txt

it returns the records that contain 44 or 4444 or 24,etc as roll no.How do i eliminate this flaw?

Answer 1

If you have GNU grep, then grep -w 4 will look for 'words' that consist of the digit 4 alone.

grep -w 4 <<eof
This line contains 4 all alone.
This line doesn't because of this 44.
This line contains '4' all alone too.
This line does not because 4x4 looks like a word.
eof

Answer 2

If your file contains one record per line, you can use

grep -E "^4$"

The -E option tells grep to accept regular expressions, and in the expression ^ stands for "beginning of line" and $ stands for "end of line".

Answer 3

check this link: http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_04_02.html use "^" if it is not starting with 'roll number'

Answer 4

Check that 4 is alone:

$ echo "4 24 4aa 44 74 4 9 kk4 09 4 yu4 4 " | tr ' ' '\n' | egrep '\b4\b'
4
4
4
4