replacing strings in a configuration file with shell scripting - shell

Question

I have a configuration file with fields separated by semicolons ;. Something like:
user#raspberrypi /home/pi $ cat file
string11;string12;string13;
string21;string22;string23;
string31;string32;string33;
I can get the strings I need with awk:
user#raspberrypi /home/pi $ cat file | grep 21 | awk -F ";" '{print $2}'
string22
And I'd like to change string22 to hello_world via a script.
Any idea how to do it? I think it should be with sed but I have no idea how.

Answer 1

I prefer perl better than sed. Here a one-liner that modifies the file in-place.
perl -i -F';' -lane '
BEGIN { $" = q|;| }
if ( m/21/ ) { $F[1] = q|hello_world| };
print qq|#F|
' infile
Use -i.bak instead of -i to create a backup file with .bak as suffix.
It yields:
string11;string12;string13
string21;hello_world;string23
string31;string32;string33

Answer 2

First drop the useless use of cat and grep so:
$ cat file | grep 21 | awk -F';' '{print $2}'
Becomes:
$ awk -F';' '/21/{print $2}' file
To change this value you would do:
$ awk '/21/{$2="hello_world"}1' FS=';' OFS=';' file
To store the changes back to the file:
$ awk '/21/{$2="hello_world"}1' FS=';' OFS=';' file > tmp && mv tmp file
However if all you want to do is replace string22 with hello_world I would suggest using sed instead:
$ sed 's/string22;/hello_world;/g' file
With sed you can use the -i option to store the changes back to the file:
$ sed -i 's/string22;/hello_world;/g' file

Answer 3

Even though we can do this in awkeasily as Sudo suggested i prefer perl since it does inline replacement.
perl -pe 's/(^[^\;]*;)[^\;]*(;.*)/$1hello_world$2/g if(/21/)' your_file
for in line just add an i
perl -pi -e 's/(^[^\;]*;)[^\;]*(;.*)/$1hello_world$2/g if(/21/)' your_file
Tested below:
> perl -pe 's/(^[^\;]*;)[^\;]*(;.*)/$1"hello_world"$2/g if(/21/)' temp
string11;string12;string13;
string21;"hello_world";string23;
string31;string32;string33;
> perl -pe 's/(^[^\;]*;)[^\;]*(;.*)/$1hello_world$2/g if(/21/)' temp
string11;string12;string13;
string21;hello_world;string23;
string31;string32;string33;
>