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Accepted
deepblue_86
Asked: 2015-10-21 21:37:07 +0800 CST

Adding a number to a text file with a loop

  • 772

I have a day-and-month text file like this:

day=

8
9
10
15
1

month=

3
6
7
10
1

I need to add "0" to lines, including only one-digit numbers. I need to write a loop to find a one-digit number in all the lines and add "0" to the left of the number like;

day=

03
06
07
10
01

month=

03
06
07
10
01
bash

5 Answers

  1. xargs printf "%.2d\n" <file.txt
    • 5

  2. Perl one-liner approach, suitable as probably every shell have perl preinstalled:

    perl -ne 'printf("%02d\n",$_)'<numbers.txt > numbers_out.txt
    • 4

  3. Using sed - tool suited for tasks as batch or stream editing of text files or streams.

    • as a filter to be used in a pipe: sed 's/^[0-9]$/0&/'

    • as a command to edit a file: sed -i 's/^[0-9]$/0&/' numfile.txt

    The code will prepend 0 to lines containing just a single digit 0 - 9. Instead of [0-9] you can also use [[:digit:]].

    • 3
  4. Best Answer

    You can use printf to pad the numbers with 0 accordingly:

    while read num; do printf '%0.2d\n' "$num"; done <file.txt

    To save the output in the same file:

    while read num; do printf '%0.2d\n' "$num"; done <file.txt | tee file.txt
    • 2

  5. awk approach. Redirecting output back to file with creating tmp.txt and replacing original file with tmp.txt once the command is done.

    Command

    awk '/^[[:digit:]]$/ {$1="0"$1}1'  numFile.txt  > tmp.txt && mv tmp.txt numFile.txt

    Sample output

    xieerqi:$ cat numFile.txt                                                      
1
2
3
4
5
6
7
8
9
10
11
12

xieerqi:$ awk '/^[[:digit:]]$/ {$1="0"$1}1'  numFile.txt  > tmp.txt && mv tmp.txt numFile.txt 

xieerqi:$ cat numFile.txt                                                      
01
02
03
04
05
06
07
08
09
10
11
12
    • 0