What's the best way to edit a file with a bash script?

Question

Using /bin/bash on RHEL 5.8 and I want to automate editing a file. I need the script to search the file and replace a line in the file. Line example:

Other line
Current date 01121990
Other line

Search for this line and replace the date string with a predetermined date. I can't echo to a new file and enter in my date since this file will be copied from a server-specific file. Thanks!

Answer 1

your_date='your desired date'
sed -i "s/^Current date.*/Current date ${your_date}/" /path/to/file

That's the easiest way. This assumes that all lines that contains a date to replace also are the only lines to start with 'Current date'.

Note that the user level that runs this command must also have permission to edit that file.

-i means inline edit, which means you are editing the file directly.

^Current date.*

Means all lines starting with: Current date and ending in anything. In other words, replace the entire line with what is in the second /.../ part of the sed thing.

Double " are used around the sed statement so that variables will be used as variables, not strings.

Answer 2

ed can do in-place editing of files as well! The inode number of the edited file remains unchanged.

file='/path/to/file'
your_date='your desired date'
ls -i "$file"
printf '%s\n' H ",g/^Current date.*/s//${your_date}/" wq | ed -s "$file"
ls -i "$file"