Linux Expert Guide

/proc Filesystem

Process Information

// /proc/<pid>/cmdline - Command line arguments (null-separated)
data, _ := os.ReadFile(fmt.Sprintf("/proc/%d/cmdline", pid))
args := strings.Split(string(data), "\x00")

// /proc/<pid>/exe - Symlink to executable
exe, _ := os.Readlink(fmt.Sprintf("/proc/%d/exe", pid))

// /proc/<pid>/cwd - Current working directory
cwd, _ := os.Readlink(fmt.Sprintf("/proc/%d/cwd", pid))

// /proc/<pid>/environ - Environment variables (null-separated)
data, _ := os.ReadFile(fmt.Sprintf("/proc/%d/environ", pid))
envs := strings.Split(string(data), "\x00")

// /proc/<pid>/status - Process status (parsed key-value)
// Contains: Name, State, Pid, PPid, Uid, Gid, etc.

// /proc/<pid>/fd/ - Open file descriptors (directory of symlinks)

System-wide Information

code

/proc/loadavg      - Load averages
/proc/meminfo      - Memory statistics
/proc/cpuinfo      - CPU information
/proc/mounts       - Mounted filesystems
/proc/net/tcp      - TCP connections

Cgroups

cgroup v1 (legacy)

code

/proc/<pid>/cgroup format:
hierarchy-ID:controller-list:cgroup-path

Example:
12:memory:/docker/abc123def456
11:cpu,cpuacct:/docker/abc123def456

cgroup v2 (unified)

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/proc/<pid>/cgroup format:
0::<cgroup-path>

Example:
0::/system.slice/docker-abc123def456.scope

Detecting Container from cgroup

func isContainerized(pid int) (bool, string) {
    data, err := os.ReadFile(fmt.Sprintf("/proc/%d/cgroup", pid))
    if err != nil {
        return false, ""
    }

    content := string(data)

    // Docker patterns
    if strings.Contains(content, "/docker/") {
        // Extract 64-char hex container ID
    }
    if strings.Contains(content, "docker-") && strings.Contains(content, ".scope") {
        // cgroup v2 docker pattern
    }

    // Other runtimes
    // containerd: /system.slice/containerd-<id>.scope
    // podman: /machine.slice/libpod-<id>.scope
    // cri-o: /crio-<id>.scope

    return false, ""
}

File Permissions

Permission Bits

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Owner  Group  Other
rwx    rwx    rwx
421    421    421

0644 = rw-r--r-- (files)
0755 = rwxr-xr-x (directories, executables)
0600 = rw------- (sensitive files)

Go Permission Operations

// Check if file is readable by all
info, _ := os.Stat(path)
mode := info.Mode()
worldReadable := mode&0004 != 0

// Set permissions
os.Chmod(path, 0644)

// Set ownership (requires root)
os.Chown(path, uid, gid)

// Check effective access
err := unix.Access(path, unix.R_OK)

Process Management

Process States

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R - Running
S - Sleeping (interruptible)
D - Disk sleep (uninterruptible)
Z - Zombie
T - Stopped

Finding Processes

// List all PIDs
entries, _ := os.ReadDir("/proc")
for _, e := range entries {
    if pid, err := strconv.Atoi(e.Name()); err == nil {
        // Valid PID directory
    }
}

// Using gopsutil (preferred)
import "github.com/shirou/gopsutil/v4/process"
procs, _ := process.Processes()

Process Relationships

// Get parent PID from /proc/<pid>/status
// PPid: 1234

// Get child PIDs from /proc/<pid>/task/<tid>/children

User/Group Resolution

import "os/user"

// Get user by UID
u, _ := user.LookupId("1000")
fmt.Println(u.Username) // "hardik"

// Get current user
current, _ := user.Current()

// Check if running as root
if os.Geteuid() == 0 {
    // Running as root
}

Common Permission Errors

EACCES (Permission denied)

•File/directory permissions insufficient
•Check with ls -la and verify user context

EPERM (Operation not permitted)

•Usually requires root/CAP_*
•Check if running with appropriate privileges

Debugging Permission Issues

bash

# Check file permissions
ls -la /path/to/file

# Check process user context
cat /proc/<pid>/status | grep -E "^(Uid|Gid):"

# Check capabilities
cat /proc/<pid>/status | grep Cap

# Trace syscalls for permission errors
strace -e trace=open,access,stat <command>

Namespaces (Container Context)

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/proc/<pid>/ns/ contains namespace references:
- mnt   (mount namespace)
- pid   (PID namespace)
- net   (network namespace)
- user  (user namespace)
- uts   (hostname namespace)
- ipc   (IPC namespace)
- cgroup (cgroup namespace)

Process in container has different namespace IDs than host processes.