Pre & Post Scripts Best Practices

Mr.OneProCloudSeptember 3, 2025About 6 min

Pre & Post Scripts Best Practices

Version Note

We recommend upgrading to v6.7.0 or later.

Why Use Pre-Scripts and Post-Scripts

In disaster recovery scenarios, different business systems often have varying operating environments and configurations. To maximize both generality and flexibility during recovery and quickly restore business operations, HyperMotion / HyperBDR provides advanced settings in the host startup functionality, allowing users to configure pre-scripts and post-scripts:

Pre-scripts: Execute before host startup by mounting disks for offline modifications. These are ideal for pre-configuration tasks that don't depend on system runtime status, such as batch replacement of configuration files, boot parameter adjustments, initialization file creation, etc. They ensure the system can complete bootstrapping and reach the login interface.
Post-scripts: Execute after system startup completion, leveraging the full operating system and network environment. These are suitable for runtime initialization tasks that require real-time interaction, such as starting critical services, registering business components, loading database initial data, etc.

This pre/post-script design approach enables disaster recovery processes to achieve both generalized rapid recovery and flexible configuration capabilities for business-specific differences, thereby reducing recovery time (RTO) and minimizing switching risks.

Functionality and Operating Principles

1. Pre-Scripts

Execution Timing Executed in the final stage of driver adaptation, before host startup.
Execution Environment
- Runs on a temporary transition host, with the operating system version depending on the product version (e.g., Ubuntu 20.04 / Ubuntu 24.04, with potential future upgrades).
- Scripts must be Linux Bash scripts.
- AWS Platform Specifics:
  - Linux target hosts → executed on Linux transition hosts.
  - Windows target hosts → executed on Windows Server 2019 transition hosts, with scripts requiring .bat file format.
Working Principle
- The transition host mounts user disks related to startup.
- Scripts can directly edit files on the mounted disk, primarily system/boot files.
Scope of Use
- Pre-scripts should only be used for boot-related configuration changes.
- For data disk modifications or runtime-dependent tasks, use post-scripts instead.
Limitations
- Maximum script length: 4096 characters.

1.1 Linux Path Mapping

In scripts, use $HOST_ROOT_PATH to reference the target system root partition.

1.2 Windows Path Mapping

In Windows pre-scripts, use %HOST_ROOT_PATH% to reference the target boot drive (typically C:).

On AWS, use a Windows transition host for driver fixes on Windows targets.

2. Post-Scripts

Execution Timing: Automatically runs after the operating system completes startup.
Execution Environment:
- Linux targets → Bash scripts.
- Windows targets → .bat scripts.
Working Principle:
- Executes after the system is operational and can perform system-level tasks.
- Typical applications:
  - Start or stop services
  - Update configuration files
  - Add Windows registry entries
  - Insert database records
- Execution behavior:
  - Linux: post-scripts run once and are removed after execution.
  - Windows: post-scripts run once and remain on disk for debugging; they will not run again automatically.
Script Locations:
- Linux: /root/.post_script
- Windows: C:\Program Files\Guestfs\Firstboot.Scripts
Limitations:
- Maximum script length: 4096 characters.

Best Practices and Debugging Tips

Pre-Script Guidelines

Only modify boot/system disk configuration. Avoid complex operations on data disks.
Keep scripts concise and reliable to ensure successful driver adaptation completion.
Use $HOST_ROOT_PATH (Linux) or %HOST_ROOT_PATH% (Windows) to reference the target root, improving portability.
Pre-script output is not directly displayed in the console UI. Retrieve logs from the transition host or download logs from the platform. Log file locations vary by cloud provider.

Checking Results (Generation-2 Engine)

Search the log /opt/installer/production/logs/mistral/mistral-server.log for:
---Execute user pre scripts---

Below that line, the pre-script content provided by the user will appear, for example:
2025-08-14 17:54:53,496 21096 INFO [-] ---Execute user pre scripts---
+ echo before_pre

Platform Name	Chinese Name (Translated)	Engine Generation	Supported Storage Types
Alibaba Cloud Apsara Stack(v3.16.x)	Alibaba Cloud Private Cloud (v3.16.x)	v2	Block Storage
Alibaba Cloud Apsara Stack(v3.18.x)	Alibaba Cloud Private Cloud (v3.18.x)	v2	Block Storage
AWS China(SDK v1.34.93)	AWS China	v2	Block Storage; Object Storage
AWS(SDK v1.34.93)	AWS	v2	Block Storage; Object Storage
ctyun JC	China Telecom Joint Cloud	v2	Object Storage
eSurfingCloud4.0	China Telecom Cloud 4.0	v2	Block Storage
FiXo Cloud BS	FiXo Cloud BS	v2	Block Storage
FusionCompute(v8.6.x)	FusionCompute	v2	Object Storage
Google Cloud(SDK v1.19.0)	Google Cloud	v2	Block Storage
GridCloud	Green Energy Cloud	v2	Block Storage
Huawei Cloud Stack Online(v23.3)	Huawei Cloud HCS Online	v2	Block Storage; Object Storage
Huawei Cloud(Recommended, SDK v3.1.86)	Huawei Cloud (Recommended, SDK v3.1.86)	v2	Block Storage; Object Storage
Microsoft Azure(SDK v30.3)	Microsoft Azure	v2	Block Storage
Open Telekom Cloud(SDK v3.1.86)	Open Telekom Cloud	v2	Block Storage; Object Storage
Oracle Cloud(SDK v2.126.3)	Oracle Cloud	v2	Block Storage
SMTX OS(v6.x.x)	SMTX OS	v2	Block Storage
Tencent Cloud	Tencent Cloud	v2	Object Storage
Tencent Cloud Enterprise	Tencent Cloud Enterprise Edition	v2	Block Storage; Object Storage
UCloudStack	UCloudStack	v2	Block Storage
Volcengine	Volcengine	v2	Object Storage
XHERE(NeutonOS_3.x)	XHERE (NeutonOS_3.x)	v2	Block Storage; Object Storage

Checking Results (Generation-1 Engine)

Search the log /opt/installer/production/logs/unicloud/unicloud-engine.log for:
---Execute user pre scripts---

Below that line, the pre-script content provided by the user will appear, for example:
2025-08-14 17:54:53,496 21096 INFO [-] ---Execute user pre scripts---
+ echo before_pre

Platform Name	Chinese Name (Translated)	Engine Generation	Supported Storage Types
Alibaba Cloud	Alibaba Cloud	v1	Block Storage; Object Storage
Alibaba Cloud Apsara Stack(v3.16.x)	Alibaba Cloud Private Cloud (v3.16.x)	v1	Object Storage
Alibaba Cloud Apsara Stack(v3.18.x)	Alibaba Cloud Private Cloud (v3.18.x)	v1	Object Storage
ecloud	China Mobile Cloud	v1	Block Storage; Object Storage
ecloud JC	China Mobile Joint Cloud	v1	Block Storage
GDS	GDS Local Data Center Cloud	v1	Object Storage
Huawei Cloud Stack(HCS)(v8.2.x / v8.3.x)	Huawei Cloud Stack (HCS)(v8.2.x / v8.3.x)	v1	Block Storage
Jinshan Cloud	Kingsoft Cloud	v1	Block Storage
OpenStackCommunity(Juno+)	OpenStack Community Version (Juno+)	v1	Block Storage; Object Storage
QingCloud	QingCloud	v1	Block Storage
Tencent Cloud	Tencent Cloud	v1	Block Storage
Tencent Cloud TStack Enterprise	Tencent Cloud TStack Enterprise Edition	v1	Block Storage
Tencent Cloud TStack Ultimate	Tencent Cloud TStack Ultimate Edition	v1	Block Storage
TM CAE	TM CAE	v1	Block Storage; Object Storage
VMware	VMware	v1	Object Storage
ZStack(v4.x.x)	ZStack(v4.x.x)	v1	Block Storage

Post-Script Best Practices

Use post-scripts for tasks that must run after startup, such as:
- Start/stop services
- Initialize databases
- Update running system configuration
Post-scripts can depend on system runtime. Ensure scripts are idempotent (safe to run multiple times) to avoid issues.
For changes that depend on NICs, services, or boot order, add waiting logic (check service/NIC availability) to reduce errors. Validate scripts on a target platform before use.

How to check readiness/execution:

Check post-script execution (generation-2 engine)

Search the log /opt/installer/production/logs/mistral/mistral-server.log for:
----Install post scripts service---

Supported cloud platforms and storage types are the same as listed for pre-scripts.

Check post-script execution (generation-1 engine)

Search the log /opt/installer/production/logs/unicloud/unicloud-engine.log for:
----Install post scripts service---

Supported cloud platforms and storage types are the same as listed for pre-scripts.

Viewing Execution Output

We recommend redirecting stdout and stderr to files within your post-script for easier inspection.

Common Usage Examples

1. Pre-Script Examples

# Modify SSH configuration
sed -i 's/^#PermitRootLogin.*/PermitRootLogin yes/' $HOST_ROOT_PATH/etc/ssh/sshd_config
# Disable SELinux
sed -i 's/^SELINUX=.*/SELINUX=disabled/' $HOST_ROOT_PATH/etc/selinux/config

Example: Remove crashkernel=auto for Oracle 6 in KVM and Rebuild grub.cfg

set -e
# 1. Edit grub default and remove the field
GRUB_FILE="$HOST_ROOT_PATH/etc/default/grub"
if [ -f "$GRUB_FILE" ]; then
    echo "[INFO] Modifying $GRUB_FILE ..."
    sed -i 's/\s*crashkernel=auto//g' "$GRUB_FILE"
else
    echo "[ERROR] $GRUB_FILE not found"
    exit 1
fi

# 2. Bind necessary system directories for chroot
for dir in dev proc sys run; do
    mount --bind /$dir $HOST_ROOT_PATH/$dir
done

# 3. Detect BIOS or UEFI
if [ -d "$HOST_ROOT_PATH/boot/efi/EFI" ]; then
    echo "[INFO] UEFI detected..."
    chroot "$HOST_ROOT_PATH" /usr/sbin/grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg
else
    echo "[INFO] BIOS detected..."
    chroot "$HOST_ROOT_PATH" /usr/sbin/grub2-mkconfig -o /boot/grub2/grub.cfg
fi

# 4. Unmount bound directories
for dir in sys proc dev run; do
    umount $HOST_ROOT_PATH/$dir
done

echo "[INFO] Modification complete."

Example: Add a File on Windows

:: Create readme.txt in system drive root
echo This system has been pre-configured. >> %HOST_ROOT_PATH%\readme.txt

Example: Install a Kernel and Set it as Default on Linux

Scenario: A VM migrated from Azure to AWS needs a generic kernel because the Azure kernel lacks AWS ENA driver.

ROOT=$HOST_ROOT_PATH

mount --bind /dev $ROOT/dev
mount --bind /sys $ROOT/sys
mount --bind /proc $ROOT/proc
mount --bind /run $ROOT/run

cat <<EOF > /tmp/change-kernel.sh
DEFGRUB=/etc/default/grub
KERNEL=5.15.0-139-generic

apt update
apt install -y linux-image-\$KERNEL

sed -i 's/GRUB_TIMEOUT_STYLE=hidden/GRUB_TIMEOUT_STYLE=menu/g' \$DEFGRUB
sed -i 's/GRUB_TIMEOUT=0/GRUB_TIMEOUT=10/g' \$DEFGRUB
sed -i "s/GRUB_DEFAULT=0/GRUB_DEFAULT=\\\"Advanced options for Ubuntu>Ubuntu, with Linux \\\$KERNEL\\\"/g" \$DEFGRUB

update-grub

exit
EOF

cp /tmp/change-kernel.sh $ROOT/tmp/change-kernel.sh
chroot $ROOT /bin/bash -x /tmp/change-kernel.sh

umount $ROOT/dev $ROOT/sys $ROOT/proc $ROOT/run

Example: Remove Problematic Drivers Before Windows Failover

Scenario: A VMware Windows 2016 source contains problematic drivers that cause a blue screen on the target—remove them in a pre-script.

reged -L -e ${HOST_ROOT_PATH}/Windows/System32/config/SYSTEM << EOF 
 cd \DriverDatabase\DeviceIds\PCI 
 rdel VEN_1AF4&DEV_1001 
 rdel VEN_1AF4&DEV_1042 
 rdel VEN_1AF4&DEV_1001&SUBSYS_00021AF4&REV_00 
 rdel VEN_1AF4&DEV_1042&SUBSYS_11001AF4&REV_01 
 rdel VEN_1AF4&DEV_1004 
 rdel VEN_1AF4&DEV_1048 
 rdel VEN_1AF4&DEV_1004&SUBSYS_00081AF4&REV_00 
 rdel VEN_1AF4&DEV_1048&SUBSYS_11001AF4&REV_01 
 q 
 EOF

2. Post-Script Examples

Example: Linux — Wait for Network, Start Services and Initialize Database

#!/bin/bash
# Wait for network connectivity
until ping -c 1 8.8.8.8 &> /dev/null; do
    echo "Waiting for network..."
    sleep 3
done

# Start nginx service
systemctl start nginx

# Initialize MySQL (assumes root has no password)
mysql -uroot -e "CREATE DATABASE IF NOT EXISTS myapp; \
    CREATE USER 'myuser'@'%' IDENTIFIED BY 'mypassword'; \
    GRANT ALL PRIVILEGES ON myapp.* TO 'myuser'@'%'; \
    FLUSH PRIVILEGES;"

Example: Windows — Delayed Start and Add Registry Key

@echo off
:: Wait 30 seconds for system services to start
timeout /t 30 /nobreak

:: Start IIS service
net start W3SVC

:: Add registry key example
reg add "HKLM\SOFTWARE\MyApp" /v InstallDate /t REG_SZ /d "%date%" /f

Example: Windows — Set Custom DNS After Boot

Scenario: After migrating to Huawei Cloud, configure DNS on Windows 2016.

@echo off
setlocal enabledelayedexpansion
for /f "tokens=2 delims==" %%a in ('wmic nic where "NetEnabled=True" get NetConnectionID /value^|findstr .') do (
    set "ADAPTER_NAME=%%a"
    goto :configdns
)
timeout /t 600 /nobreak>nul
echo No active network adapter detected
exit /b
:configdns
netsh interface ip set dns name="%ADAPTER_NAME%" static 100.120.0.15 primary validate=no
netsh interface ip add dns name="%ADAPTER_NAME%" addr=172.24.206.130 index=2 validate=no
netsh interface ip add dns name="%ADAPTER_NAME%" addr=172.24.206.131 index=3 validate=no

Example: Windows — Set Static IP and DNS on VMware→HCS Windows 2022

@echo off
setlocal enabledelayedexpansion

:retry
for /f "tokens=2 delims==" %%a in ('wmic nic where "NetEnabled=True" get InterfaceIndex /value^|findstr .') do (
    set "INTERFACE_INDEX=%%a"
    set "INTERFACE_INDEX=!INTERFACE_INDEX:~0,-1!"
    netsh interface set interface name=!INTERFACE_INDEX! admin=enable
    goto :configdns
)
timeout /t 5 /nobreak >nul
goto retry

:configdns
netsh interface ip set address name=!INTERFACE_INDEX! source=static addr=10.1.178.113 mask=255.255.255.0 gateway=10.1.178.254
netsh interface ip set dns name=!INTERFACE_INDEX! static 10.1.206.37 primary validate=no
netsh interface ip add dns name=!INTERFACE_INDEX! addr=10.1.224.66 index=2 validate=no
exit /b