17

How might it be possible to alter some variable in the env of an already running process, for example through /proc/PID/environ? That "file" is read-only.

Need to change or unset the DISPLAY variable of a long-running batch job without killing it.

  • 3
    It's too late now, but for future reference, xpra could be interesting. – sr_ May 9 '12 at 9:56
  • xpra sounds useful. Normally I reroute to non-user displays hosted by Xvfb or Xephyr, but today I forgot and ran from cli rather than cron/at to troubleshoot output, so it's been annoying me at :0 – Marcos May 9 '12 at 11:12
17

You can't do this without a nasty hacks - there's no API for this, no way to notify the process that its environment has changed (since that's not really possible anyway).
Even if you do manage to do that, there is no way to be sure that it will have any effect - the process could very well have cached the environment variable you're trying to poke (since nothing is supposed to be able to change it).

If you really do want to do this, and are prepared to pick up the pieces if things go wrong, you can use a debugger. See for instance this Stack Overflow question:
Is there a way to change another process's environment variables?

Essentially:

(gdb) attach process_id
(gdb) call putenv ("DISPLAY=your.new:value")
(gdb) detach

Other possible functions you could try to call are setenv or unsetenv.

Please do really keep in mind that this may not work, or have dire consequences if the process you target does "interesting" things with its environment block. Do test it out on non-critical processes first, but make sure these test processes mirror as close as possible the one you're trying to poke.

  • 3
    Yes I realize it's somewhat of a hack, risky and not guaranteed for reasons you mentioned. (Part of the reason I visit this group is for such non-conventional needs I can't seem to find ordinarily.) In this case setting DISPLAY to junk or empty merely resolves an annoyance and delay(unnecessary frequent screenshots over network,fine if they fail). Since child copies parent, I need only mod the parent env. Many new child &subchild processes are getting spawned and exit quickly in my batch job; those matter. I figured a debugger could do this, thanks--I could wrap that into a shell function. – Marcos May 9 '12 at 11:02
0

There is no need to do so if a batch job can read from a file system to retrieve a change. Just run a job with path to a temporary unique directory and pass the same path to the child shell script. Script will lock a file in that directory and write a file with new values near the lock file. A job script time to time will lock the same file, parse and read changes back from the values file. To find out how to make a lock in the unix shell just search for unix shell lock file or bash lock file, there is already exist a plenty solutions for that.

Benefits from this solution:

  • portable between almost any OS like Windows or Unix
  • no need to write and duplicate complex parsers for each interpreter (unix/windows/etc) to read back values from the file as long as the values file stays simple

Issues in implementation below:

  • Implementation relies on a file lock in a shell redirection phase (flock in Linux to achieve exclusion effect, in Windows has a builtin exclusion)
  • Each value for a variable is a single line value (not a multiline)

Implementation is stored here: https://sourceforge.net/p/contools/contools/HEAD/tree/trunk/Scripts/Tools

The bash implementation:

set_vars_from_locked_file_pair.sh

#!/bin/bash

# Another variant of a configuration file variables read and set script.
# The script must stay as simple as possible, so for this task it uses these parameters:
# 1. path where to lock a lock file
# 2. path where to read a file with variable names (each per line)
# 3. path where to read a file with variable values (each per line, must be the same quantity of lines with the variable names file)

# Script can be ONLY included by "source" command.
if [[ -n "$BASH" && (-z "$BASH_LINENO" || ${BASH_LINENO[0]} -gt 0) ]]; then 

function set_vars_from_locked_file_pair()
{
  # the lock file directory must already exist
  if [[ ! -d "${1%[/\\]*}" ]]; then
    echo "$0: error: lock file directory does not exist: \`${1%[/\\]*}\`" >&2
    return 1
  fi

  if [[ ! -f "${2//\\//}" ]]; then
    echo "$0: error: variable names file does not exist: \`$2\`" >&2
    return 2
  fi

  if [[ ! -f "${3//\\//}" ]]; then
    echo "$0: error: variable values file does not exist: \`$3\`" >&2
    return 3
  fi

  function LocalMain()
  {
    # open file for direct reading by the `read` in the same shell process
    exec 7< "$2"
    exec 8< "$3"

    # cleanup on return
    trap "rm -f \"$1\" 2> /dev/null; exec 8>&-; exec 7>&-; trap - RETURN" RETURN

    local __VarName
    local __VarValue

    # shared acquire of the lock file
    while :; do
      # lock via redirection to file
      {
        flock -s 9

        # simultaneous iteration over 2 lists in the same time
        while read -r -u 7 __VarName; do
          read -r -u 8 __VarValue
          # drop line returns
          __VarName="${__VarName//[$'\r\n']}"
          __VarValue="${__VarValue//[$'\r\n']}"
          # instead of `declare -gx` because `-g` is introduced only in `bash-4.2-alpha`
          export $__VarName="$__VarValue"
          (( ${4:-0} )) && echo "$__VarName=\`$__VarValue\`"
        done

        break

        # return with previous code
      } 9> "$1" 2> /dev/null # has exclusive lock been acquired?

      # busy wait
      sleep 0.02
    done
  }

  LocalMain "${1//\\//}" "${2//\\//}" "${3//\\//}" "${4:-0}"
}

fi

testlock.sh

#!/bin/bash

{
  flock -x 9 2> /dev/null
  read -n1 -r -p "Press any key to continue..."
  echo >&2
} 9> "lock"

The same on Windows (as an example of portability):

set_vars_from_locked_file_pair.bat

@echo off

rem Another variant of a configuration file variables read and set script.
rem The script must stay as simple as possible, so for this task it uses these parameters:
rem 1. path where to lock a lock file
rem 2. path where to read a file with variable names (each per line)
rem 3. path where to read a file with variable values (each per line, must be the same quantity of lines with the variable names file)

rem disable alternative variables expansion to avoid `!` character consumption
setlocal DISABLEDELAYEDEXPANSION

set "FILE_LOCK_PATH=%~1"
set "FILE_VAR_NAMES_PATH=%~2"
set "FILE_VAR_VALUES_PATH=%~3"
set "PRINT_VARS_SET=%~4"

set "FILE_LOCK_DIR=%~d1"

rem the lock file directory must already exist
if not exist "%FILE_LOCK_DIR%" (
  echo.%~nx0: error: FILE_LOCK_DIR does not exist: "%FILE_LOCK_DIR%"
  exit /b 1
) >&2

if not exist "%FILE_VAR_NAMES_PATH%" (
  echo.%~nx0: error: FILE_VAR_NAMES_PATH does not exist: "%FILE_VAR_NAMES_PATH%"
  exit /b 2
) >&2

if not exist "%FILE_VAR_VALUES_PATH%" (
  echo.%~nx0: error: FILE_VAR_VALUES_PATH does not exist: "%FILE_VAR_VALUES_PATH%"
  exit /b 3
) >&2

rem The endlocal works only in the same call context
endlocal

rem exclusive acquire of the lock file
:REPEAT_LOCK_LOOP

(
  (
    rem if lock is acquired, then we are in...
    call :MAIN "%%~2" "%%~3" "%%~4"
    call set "LASTERROR=%%ERRORLEVEL%%"

    rem exit with return code from the MAIN
  ) 9> "%~1" && (del /F /Q /A:-D "%~1" & goto EXIT)
) 2>nul

rem Busy wait: with external call significantly reduces CPU consumption while in a waiting state
pathping localhost -n -q 1 -p 20 >nul 2>&1
goto REPEAT_LOCK_LOOP

:EXIT
exit /b %LASTERROR%

:MAIN
rem drop last error
type nul>nul

if %~30 NEQ 0 goto SET_WITH_PRINT

rem trick with simultaneous iteration over 2 lists in the same time
(
  for /f "usebackq eol=# tokens=* delims=" %%i in ("%~1") do (
    set /p "%%i="
  )
) < "%~2"

exit /b

:SET_WITH_PRINT
rem trick with simultaneous iteration over 2 lists in the same time
(
  for /f "usebackq eol=# tokens=* delims=" %%i in ("%~1") do (
    set /p "%%i="
    rem to filter out wrong matches of a variable from the `set "%%i"`
    for /f "usebackq eol=# tokens=1,* delims==" %%j in (`set "%%i"`) do if "%%j" == "%%i" echo.%%i=%%k
  )
) < "%~2"

testlock.bat

@echo off

(
  pause
) 9> ./lock

To write the files in just make the same way lock in your code.

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