Readwriteconfig - Click here to download

readwriteconfig is a little utility to manipulate flat files that contain text in the VARIABLE=value form. Its written in plain C and is completely free. If you write embedded systems, or just want to configure parts of an operating system with a very simple tool, then this is for you. It doesnt do anything that sed/grep cant do, but it does provide a consistent interface for embedded code, CGI scripts, BASH etc.

License: "Free code, you may use it in whole or part. This code is supplied "As is" without warranty."
Why not GNU ? Completely free makes it simpler to mix with BSD and MIT style code, if you wish to fork it to a GNU version then you are free to do so, a credit and a pointer back here would be polite though !

The code is tested only on gcc on GNU/Linix, its not tested on any other platform but should compile on systems that impliment the truncate system call.

The following is a large pile of examples of using readwriteconfig, they mostly use network settings but any plain text file of the same form can be manipulated this way :-)

Example: readwriteconfig run with no arguments

# readwriteconfig

readwriteconfig: read or write a text file of type VARIABLE=value, variable must be uppercase value may be mixed case.
Read:        readwriteconfig -r filename variable
Read silent: readwriteconfig -rs filename variable
Write:       readwriteconfig -w filename variable=value

  Update value                readwriteconfig -w /etc/sysconfig/networking/profiles/default/ifcfg-eth0 IPADDR=
  Read value                  readwriteconfig -r /etc/sysconfig/networking/profiles/default/ifcfg-eth0 IPADDR
  Read value,fail silently    readwriteconfig -rs /etc/sysconfig/networking/profiles/default/ifcfg-eth0 IPADDR

Writing will return a single character A=appended C=Changed N=File to large.
Reading will return the variables value only or nothing if the string is not found

Example a Fedora/Redhat configuration file for networking may look something like this :

[root]# cat /etc/sysconfig/networking/devices/ifcfg-eth0 

Readwriteconfig can be used to read a single value from the file, much like grep. -r is read :

[root]# readwriteconfig -r /etc/sysconfig/networking/devices/ifcfg-eth0 IPADDR

Or you can change the value with the -w option, readwriteconfig replied with 'C' for changed :

[root]# readwriteconfig -w /etc/sysconfig/networking/devices/ifcfg-eth0 IPADDR=

Now add an extra field to the file, readwriteconfig replied with 'A' for Added :

[root]# readwriteconfig -w /etc/sysconfig/networking/devices/ifcfg-eth0 FORWARD=1

The useful part of readwriteconfig is within scripts or CGI code, lets write a script to reconfigure the IP details for a network interface and restart it Redhat/Fedora :

# Example for setting an IP Address and Subnet mask and restarting the network interface

# This just saves some typing :-)
# Write new setting for eth0
readwriteconfig -w $FILENAME IPADDR=
readwriteconfig -w $FILENAME NETMASK=
readwriteconfig -w $FILENAME GATEWAY=
readwriteconfig -w $FILENAME ONBOOT=yes
# Restart the interface with new values
ifdown eth0
ifup eth0

Why the replies ? Whats the 'C' and 'A' for ...... here is a bash script fragment to illustrate

RESULT=`readwriteconfig -w $FILENAME IPADDR=`
if [ "$RESULT" == "A" ]; then
	echo "Warning: That interface did not have an IP Address assigned, i've added one"
	echo "IP Address has been changed"

I'm writing a shell script, but I want readwriteconfig to shut up and stop telling me its 'A' and 'C's.

readwriteconfig -w $FILENAME IPADDR= &>/dev/null

You are writing a web front end for an embedded product, it needs to be lite on code. You can write CGI forms in bash, Here is a simple example: For a web server try mini_httpd or apache.

(Question) I need to write C code, I just want the functions - can I lift them?
(Aanswer) Yes, The code is broken down into 3 main files, the readwriteconfig utility itself and the two functions for reading and writing text files. As the code is free you can simply drop it into your source and include the chunks of C.

//These two files are self contained suitable for inclusion in other source
#include "readconfigfile_inc.c"
#include "writeconfigfile_inc.c"

main ()
	// Example write
	char c;
	printf("write_configfile returned %c\n");		// 'C' 'A' or 'E'

	// Example read with errors reported on stdout
	char st[1024];
	printf ("THISVARIABLE=%s\n",st);

	// Example read silent, no errors reported
	char st[1024];
	if (st[0]=='\0')
		printf ("THISVARIABLE was not found in this file\n");
	else	printf ("THISVARIABLE=%s\n",st);

Linux, gcc, GNU, Free, Free Software, Embedded, Code, Flatfile C, Flat file database, CGI

All content on this site is (c) 2018 Jonathan Andrews, This article is marked as free and open. Feel free to copy and or republish it