Ash Shell PID Tracking Issue¶

This document explains a known limitation when using SVC_BACKGROUND=y with SVC_WRITE_PID=y on DSM 5 and SRM systems.

Overview¶

On DSM < 6 and SRM, /bin/sh is ash (BusyBox), not bash. This causes issues with PID tracking for background services.

The Problem¶

When the framework starts a service with SERVICE_USER set on DSM < 6:

su ${EFF_USER} -s /bin/sh -c "${service}" >> ${LOG} 2>&1 &

The $! variable captures the PID of /bin/sh, not the actual command. The shell may exit while the command continues running, leaving an incorrect PID in the file.

Bash handles this differently, but ash has this limitation.

Affected Configurations¶

This issue affects packages with all of the following:

Condition	Why It Matters
`SERVICE_USER` set in Makefile	Triggers `su` wrapper
`SERVICE_COMMAND` with parameters	Triggers `/bin/sh -c`
`SVC_BACKGROUND=y`	Runs command in background
`SVC_WRITE_PID=y`	Attempts to capture PID with `$!`
Target is DSM < 6 or SRM	Uses ash shell

Workarounds¶

Option 1: Wrapper Script (Recommended)¶

Create a wrapper script that handles backgrounding and PID capture:

#!/bin/sh
# start.sh - wrapper for DSM 5/SRM compatibility

if [ -z "${SYNOPKG_PKGDEST}" ] || [ -z "${PID_FILE}" ]; then
    echo "ERROR: Required variables not set"
    exit 1
fi

${SYNOPKG_PKGDEST}/bin/myapp --some-arg &
echo "$!" > "${PID_FILE}"

Important

When using a wrapper that handles backgrounding:

Do NOT set SVC_BACKGROUND=y in service-setup.sh
Export required variables (SYNOPKG_PKGDEST, PID_FILE) before calling
The wrapper handles both backgrounding and PID capture

Option 2: Application Daemon Mode¶

If the application supports native daemon mode with PID file:

SERVICE_COMMAND="${SYNOPKG_PKGDEST}/bin/myapp --daemon --pid-file ${PID_FILE}"

Do not set SVC_BACKGROUND or SVC_WRITE_PID - let the application handle it.

Option 3: DSM 6+ Only¶

If DSM 5/SRM support is not required, this issue doesn't apply:

DSM 6+ uses conf/privilege for user context (no su wrapper)
The framework runs services directly: ${service} >> ${OUT} 2>&1 &
$! correctly captures the PID

Framework Code Reference¶

The relevant code is in mk/spksrc.service.start-stop-status:

# DSM < 6 code path (problematic with ash)
if [ -n "${USER}" -a "$SYNOPKG_DSM_VERSION_MAJOR" -lt 6 ]; then
    $SU /bin/sh -c "${service}" >> ${OUT} 2>&1 &
fi

# DSM 6+ code path (works correctly)
else
    ${service} >> ${OUT} 2>&1 &
fi

# PID capture (uses $!)
if [ -n "${SVC_WRITE_PID}" -a -n "${SVC_BACKGROUND}" ]; then
    echo -ne "$!" > ${PID_FILE}
fi

Current Status¶

DSM 5 reached end-of-life in 2019
Default toolchains are DSM 7.1 and 7.2
SRM (Synology Router Manager) still uses ash
Most packages no longer explicitly support DSM 5 or SRM

This issue primarily affects SRM deployments or legacy DSM 5 systems.

See GitHub issues and PRs related to SVC_BACKGROUND, SVC_WRITE_PID, and ash for historical context.