Retrieving System and Process Information
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The /proc file system
Unix provides the proc virtual file system to investigate processes and metadata about them.
Obtaining Information About a Process: /proc/PID
The kernel provides a directory named /proc/PID for each process on the system. In the directory are files and subdirectories containing information about the process. For example, if you cat /proc/1/status, you can find information about the init process.
$ cat /proc/1/status
Name: init Name of command run by this process
State: S (sleeping) State of this process
Tgid: 1 Thread group ID (traditional PID, getpid())
Pid: 1 Actually, thread ID (gettid())
PPid: 0 Parent process ID
TracerPid: 0 PID of tracing process (0 if not traced)
Uid: 0 0 0 0 Real, effective, saved set, and FS UIDs
Gid: 0 0 0 0 Real, effective, saved set, and FS GIDs
FDSize: 256 # of file descriptor slots currently allocated
Groups: Supplementary group IDs
VmPeak: 852 kB Peak virtual memory size
VmSize: 724 kB Current virtual memory size
VmLck: 0 kB Locked memory
VmHWM: 288 kB Peak resident set size
VmRSS: 288 kB Current resident set size
VmData: 148 kB Data segment size
VmStk: 88 kB Stack size
VmExe: 484 kB Text (executable code) size
VmLib: 0 kB Shared library code size
VmPTE: 12 kB Size of page table (since 2.6.10)
Threads: 1 # of threads in this thread’s thread group
SigQ: 0/3067 Current/max. queued signals (since 2.6.12)
SigPnd: 0000000000000000 Signals pending for thread
ShdPnd: 0000000000000000 Signals pending for process (since 2.6)
SigBlk: 0000000000000000 Blocked signals
SigIgn: fffffffe5770d8fc Ignored signals
SigCgt: 00000000280b2603 Caught signals
CapInh: 0000000000000000 Inheritable capabilities
CapPrm: 00000000ffffffff Permitted capabilities
CapEff: 00000000fffffeff Effective capabilities
CapBnd: 00000000ffffffff Capability bounding set (since 2.6.26)
Cpus_allowed: 1 CPUs allowed, mask (since 2.6.24)
Cpus_allowed_list: 0 Same as above, list format (since 2.6.26)
Mems_allowed: 1 Memory nodes allowed, mask (since 2.6.24)
Mems_allowed_list: 0 Same as above, list format (since 2.6.26)
voluntary_ctxt_switches: 6998 Voluntary context switches (since 2.6.23)
nonvoluntary_ctxt_switches: 107 Involuntary context switches (since 2.6.23)
Stack usage: 8 kB Stack usage high-water mark (since 2.6.32)
Some interesting information in the /proc directory includes:
| File | Description (process attribute) |
|---|---|
| cmdline | Command-line arguments delimited by \0 |
| cwd | Symbolic link to current working directory |
| environ | Environment list NAME=value pairs, delimited by \0 |
| exe | Symbolic link to file being executed |
| fd | Directory containing symbolic links to files opened by this process |
| maps | Memory mappings |
| mem | Process virtual memory (must lseek() to valid offset before I/O) |
| mounts | Mount points for this process |
| root | Symbolic link to root directory |
| status | Various information (e.g., process IDs, credentials, memory usage, signals) |
| task | Contains one subdirectory for each thread in process (Linux 2.6) |
The /proc/PID/fd directory
This directory contains one symbolic link for each file descriptor that the process has open. Any process can access its own /proc/PID directory using /proc/self.
Threads: the /proc/PID/task directory
Each thread
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