Command Line Invocation

The simplest way SONiC communicates with the kernel is through command-line calls, which are implemented in common/exec.h. The interface is straight-forward:

// File: common/exec.h
// Namespace: swss
int exec(const std::string &cmd, std::string &stdout);

Here, cmd is the command to execute, and stdout captures the command output. The exec function is a synchronous call that blocks until the command finishes. Internally, it creates a child process via popen and retrieves output via fgets. However, although this function returns output, it is rarely used in practice. Most code only checks the return value for success, and sometimes even error logs won't be logged in the output.

Despite its simplicity, this function is widely used, especially in various *mgrd services. For instance, portmgrd calls it to set each port's status:

// File: sonic-swss - cfgmgr/portmgr.cpp
bool PortMgr::setPortAdminStatus(const string &alias, const bool up)
{
    stringstream cmd;
    string res, cmd_str;

    // ip link set dev <port_name> [up|down]
    cmd << IP_CMD << " link set dev " << shellquote(alias) << (up ? " up" : " down");
    cmd_str = cmd.str();
    int ret = swss::exec(cmd_str, res);

    // ...

References

1. Github repo: sonic-swss-common