Extract integers from file and output to another file?

This extracts lines that contain only digits:

$ grep -E '^[[:digit:]]+$' foo.txt
1
8192
8291

This sends the output to bar.txt:

grep -E '^[[:digit:]]+$' foo.txt >bar.txt

How it works

When you want to select lines from a file, grep is first utility to try.

^[[:digit:]]+$ is a regular expression. In it, ^ matches the beginning of a line, [[:digit:]]+ matches one or more digits, and $ matches the end of a line. Because this regex starts with ^ and ends with $, it only matches whole lines. An alternative way of matching only whole lines is to use the -x option:

grep -xE '[[:digit:]]+' foo.txt >bar.txt

The option -E tells grep to use extended regular expressions. This reduces the need to escape things in the regex.

The > signifies redirection. It causes the output that would have appeared on the screen to go to a file named bar.txt.

AWK solution:

$> cat input.txt
1
AbcdJ
8192
Pak78
8291
3 blind mice
$> awk '/^[[:digit:]]+$/' input.txt
1
8192
8291

Use > to redirect output to file

awk '/^[[:digit:]]+$/' input.txt > output.txt

Some other tools:

• sed:

  $ sed -n '/^[0-9]\+$/ p' foo.txt
  1
  8192
  8291
  

• bash, slower than other approaches:

  $ while IFS= read -r line; do [[ $line =~ ^[0-9]+$ ]] && echo "$line"; done <foo.txt
  1
  8192
  8291
  

To save the output on another file, use output redirection, >:

sed -n '/^[0-9]\+$/ p' foo.txt >output.txt
while IFS= read -r line; do [[ $line =~ ^[0-9]+$ ]] && echo "$line"; done <foo.txt >output.txt