Editing a Java properties file with Bash

Question

I have the following Java properties file that is maintained by humans.

# file.input = /very/old/name
# file.input = /old/name
file.input = /current/name
file.input.default = /default/name
other.file.input = /current/other/name
# Other comments
other.properties = must stay the same

I must create a script that will edit this file without touching at all at the rest of the files. Therefore I cannot dump new properties in the config file.

Humans will often copy/paste a line and comment it for further use. My application must edit exclusively config file without touching any of the comments.

In this particular case, I want to edit the line file.input to set the value to /new/name without any commented line being touched and without any other properties being touched as well (file.input.default, other.file.input). My script will not keep the current line for history purpose like humans usually do. Also, I don't know the current value set to the key file.input.

Where am I stuck? At the very beginning. I'm not a Bash expert, I rarely use it except to call commands in sequence and I use some ifs sometimes. But that's all.

I could use perl to use regular expressions, but perl is apparently not installed on this system. So I turned to sed, in turn, but I don't get any of this: it seems I can only replace a specific value to another. I could wrap that in the reading of the file, and rewrite it except for the expected line that I could change. Also, I would barely be able to handle human-added spaces in front of the property I want to change.

So if you have an alternative please share it, because I start to become alienated.

Answer 1

Using the -i option of sed (which edits inplace). The caveat is the character used to delimiter the substitution command must not appear in the NEW string. The example uses ':' rather than '/' to delimit (s:old_string:new_string:)

NEW=/new/name    
sed -i 's:^[ \t]*file.input[ \t]*=\([ \t]*.*\)$:file.input = '${NEW}':' f

Answer 2

An solution using awk:

$ awk -F'=' '$1 ~ /^[ \t]*file.input[ \t]*$/{$2="= /new/name"}{print}' config
# file.input = /very/old/name
# file.input = /old/name
file.input = /new/name
file.input.default = /default/name
other.file.input = /current/other/name
# Other comments
other.properties = must stay the same

Then you can save it to new config file:

awk -F'=' '$1 ~ /^\s*file.input\s*$/{$2="= /new/name"}{print > "new_config"}' config

Answer 3

Note if you need this in a script to work cross platform (to work on MacOSX too) then you need to use the complete -i flag, supplying a backup file extension:

NEW=/new/thing
sed -i.backup 's|\s*file.input\s*=.*$|file.input = '${NEW}'|' filename

## Then you can check exit code and remove the backup file if all is well
## rm filename.backup

Otherwise on MacOSX you'll get an error something like "extra characters at end of command".