Edit Wi‑Fi Configuration Files Using sed and C Wrapper Scripts

Using sed to modify Wi‑Fi configuration

Example wpa_supplicant.conf file:

r/run/wpa_supplicant
update_config=1
ap_scan=1

network={
        ssid="test"
        psk="12345678"
        scan_ssid=1
        key_mgmt=WPA-PSK
        priority=1
}

Replace the SSID and password with LinuxZn and 88888888 respectively:

sed -i 's/ssid="[^"]*"/ssid="LinuxZn"/g' ./wpa_supplicant.conf
sed -i 's/psk="[^"]*"/psk="88888888"/g' ./wpa_supplicant.conf

Calling the same substitutions from a C program

The program builds the sed commands with snprintf, resets the configuration file, and executes the commands via system():

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>

#define SHELL_CMD_MODIFY_WIFI_SSID   "sed -i 's/ssid=\"[^\"]*\"/ssid=\"%s\"/g' ./wpa_supplicant.conf"
#define SHELL_CMD_MODIFY_WIFI_PSW    "sed -i 's/psk=\"[^\"]*\"/psk=\"%s\"/g' ./wpa_supplicant.conf"
#define SHELL_CMD_RESET_WIFI_CONF    "cp default_wpa_supplicant.conf wpa_supplicant.conf"

int main(void) {
    char cmd[256] = {0};

    /* Reset configuration to a known state */
    system(SHELL_CMD_RESET_WIFI_CONF);

    /* Set SSID */
    snprintf(cmd, sizeof(cmd), SHELL_CMD_MODIFY_WIFI_SSID, "LinuxZn");
    system(cmd);
    memset(cmd, 0, sizeof(cmd));

    /* Set password */
    snprintf(cmd, sizeof(cmd), SHELL_CMD_MODIFY_WIFI_PSW, "88888888");
    system(cmd);
    return 0;
}

Running either the shell commands directly or the compiled program updates wpa_supplicant.conf with the new SSID and password. Resetting the file first ensures the original formatting is preserved.