How to multiple a number by 2 (double)present in a particular line number of a file, in Linux?

Question

File_A

Name: John Smith
Grade: 8
Institute: Baldwin
Number of entries: 125
State: Texas

File_B

Name: David Buck
Grade: 9
Institute: High Prime
Number of entries: 123
State: California

There are many such similar files in which the Number of entries (present at line number 4 in all files) has to doubled.

For File_A it should be 250 and for File_B 246.

How to do this for all files in Linux?(using sed or awk or any other commands)

Tried commands:

sed -i '4s/$2/$2*2/g' *.txt               (nothing happening from this)
awk "FNR==4 {sub($2,$2*2)}" *.txt         (getting syntax error)

Answer 1

With your shown samples please try following awk code. Simple explanation would be look/search for string /Number of entries: and then multiply 2 into value of last field and save it within itself, then print lines by mentioning 1.

awk '/Number of entries:/{$NF = ($NF * 2)} 1' File_A File_B

Run above command it will print output on screen, once you are Happy with output and want to save output into Input_file itself then you can try awk's -inplace option(available in GNU awk 4.1+ version etc).

Also if your files extension is .txt then pass it to above awk program itself, awk can read multiple files itself.