Editing ad-hoc config files on Linux

I had a need to edit /etc/network/interfaces file on Ubuntu (from my program). At first I found some awk script on the web, which claimed to do the job, but when I tried, it wasn't addressing all the CRUD cases that I am interested in (also, I didn't want to have system() calls from my C code).
So, I searched for a better utility, and found Augeas - this is fantastic! you have to try it to believe! It has a neat command line utility (augtool) as well as an easy to use C API (+ bindings for many scripting languages, including my fav Lua too :-) ).

The following commands show how easy it is to add/rm an interface (XPATH expressions)

$ sudo augtool # Add an interface at the end (last)
augtool> set /files/etc/network/interfaces/iface[last()+1] eth1
augtool> set /files/etc/network/interfaces/iface[last()]/family inet
augtool> set /files/etc/network/interfaces/iface[last()]/method static
augtool> set /files/etc/network/interfaces/iface[last()]/address 10.1.1.1
augtool> save
Saved 1 file(s)
augtool> 


$ sudo augtool  # Edit the added interface (by name, not position)
augtool> set /files/etc/network/interfaces/iface[. = 'eth1']/netmask 255.255.255.0
augtool> save
Saved 1 file(s)
augtool> set /files/etc/network/interfaces/iface[. = 'eth1']/mtu 1500
augtool> save
Saved 1 file(s)
augtool> 

$ sudo cat /etc/network/interfaces 
auto lo
iface lo inet loopback
iface eth1 inet dhcp
   address 10.1.1.1
   netmask 255.255.255.0
   mtu 1500

$ sudo augtool # Lets just delete eth1 now
augtool> rm /files/etc/network/interfaces/iface[. = 'eth1']
rm : /files/etc/network/interfaces/iface[. = 'eth1'] 6  <-- 6="" augtool="" fields="" removed=""> save
Saved 1 file(s)
augtool> 

Now, the same/similar exercise programmatically:

	// Test CRUD of an iface in /etc/network/interfaces ( on Ubuntu)
	// using the Augeas lib
	// Aniruddha. A (aniruddha.a@gmail.com)
	// Nov 1, 2014
	#include <stdio.h>
	#include <assert.h>
	#include <stdarg.h>
	#include <stddef.h>
	#include <limits.h>
	#include <string.h>
	#include <augeas.h>
	// using the .so installed from libaugeas-dev :
	// gcc -I /usr/include/libxml2/ augparse.c -laugeas
	//
	// to run, just install libaugeas0 ( minus dev files )
	#define LENS_PATH "/usr/share/augeas/lenses/dist/"
	void if_create (augeas *aug, const char *ifname, int nfields, ...)
	{
	char key[128] = {0}, val[128] = {0};
	char *p = NULL;
	char *kv = NULL;
	va_list ap;
	int ret, i;
	char iffield_path[256]= {0};
	assert(aug && ifname && (nfields > 0));
	ret = aug_set(aug, "/files/etc/network/interfaces/iface[last()+1]", ifname);
	assert(ret == 0);
	va_start(ap, nfields);
	for (i = 0; i < nfields; i++) {
	kv = va_arg(ap,char*);
	p = strchr(kv, '=');
	if (!p) continue;
	strncpy(key, kv, (ptrdiff_t)(p - kv));
	key[p - kv] = '\0';
	strcpy(val, p + 1);
	sprintf(iffield_path, "/files/etc/network/interfaces/iface[last()]/%s",
	key);
	ret = aug_set(aug, iffield_path, val);
	assert(ret == 0);
	}
	va_end(ap);
	}
	void if_update (augeas *aug, const char *ifname, int nfields, ...)
	{
	char key[128] = {0}, val[128] = {0};
	char *p = NULL;
	char *kv = NULL;
	va_list ap;
	int ret, i;
	char iffield_path[256]= {0};
	assert(aug && ifname && (nfields > 0));
	va_start(ap, nfields);
	for (i = 0; i < nfields; i++) {
	kv = va_arg(ap,char*);
	p = strchr(kv, '=');
	if (!p) continue;
	strncpy(key, kv, (ptrdiff_t)(p - kv));
	key[p - kv] = '\0';
	strcpy(val, p + 1);
	sprintf(iffield_path, "/files/etc/network/interfaces/iface[. = '%s']/%s",
	ifname, key);
	ret = aug_set(aug, iffield_path, val);
	assert(ret == 0);
	}
	va_end(ap);
	}
	void if_rm (augeas *aug, const char *ifname)
	{
	int ret;
	char ifpath[256]= {0};
	assert(aug && ifname);
	sprintf(ifpath, "/files/etc/network/interfaces/iface[. = '%s']", ifname);
	ret = aug_rm(aug, ifpath);
	assert(ret != -1);
	}
	// TEST
	int main (int argc, char *argv[])
	{
	augeas *aug = NULL;
	aug = aug_init("/", LENS_PATH, AUG_SAVE_BACKUP|AUG_NO_ERR_CLOSE);
	if (!aug || (aug_error(aug) != AUG_NOERROR)) {
	fprintf(stderr, "Init failed: %s\n", aug_error_message(aug));
	exit(1);
	}
	if_create(aug, "eth2", 5,
	"family=inet", "method=static",
	"address=1.1.1.1", "netmask=255.255.0.0",
	"mtu=1500" );
	aug_save(aug);
	system("cat /etc/network/interfaces");
	if_update(aug, "eth2", 1, "address=2.2.2.2");
	aug_save(aug);
	system("cat /etc/network/interfaces");
	if_rm(aug, "eth2");
	aug_save(aug);
	system("cat /etc/network/interfaces");
	aug_close(aug);
	return 0;
	}

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