# System issue

***Copyright © Quectel Wireless Solutions Co., Ltd. 2026. All rights reserved.***

---

# Does the system have a default username and password?

No default credentials are provided. When Debian starts for the first time, you must create a user account. Keep a record of the username and password, as these credentials are required for future logins.

# How do I enable a Chinese input method?

Follow these steps to install and configure a Chinese input method:

```python
# 1. Install
sudo apt install ibus ibus-libpinyin fonts-noto-cjk -y

# 2. Check the current configuration
gsettings get org.gnome.desktop.input-sources sources

# 3. Configure the input sources
gsettings set org.gnome.desktop.input-sources sources "[('xkb', 'us'), ('ibus', 'libpinyin')]"

# 4. Restart the system
reboot

# 5. Switch input methods. Super is the Windows logo key
Super + Space
```

# How do I check the current system status?

```bash
# Monitor CPU activity in real time
top

# View currently running and waiting processes
ps aux | grep <comm>

# View overall memory and swap usage
free -h

# View disk partition usage
df -h
```

# How do I check the system version?

```bash
# Check the build version
cat /etc/quectel-release

# Check the kernel version
uname -a
```

# How do I upgrade the system?

```bash
sudo apt update
sudo apt upgrade
```

```{note}
1. If the system fails to start properly, reflash the system mirror.
2. Visit the [Firmware download center](<https://developer.quectel.com/en/resource-download>) to obtain the latest firmware release.
3. For instructions on flashing the system mirror, see [System installation](<../../Operating system/Debian/System installation/System installation.md>).
```

# What should I do if packages cannot be downloaded?

## Check the network connection and proxy settings

- Network connectivity: First, make sure the device is connected to the network. Run **ping -c 4 8.8.8.8** or **ping -c 4** archive.ubuntu.com to test network connectivity. If neither command succeeds, check the Ethernet cable, Wi-Fi connection, or IP address configuration.

- Proxy server: If a proxy is configured, verify its settings or temporarily disable it. Check */etc/apt/apt.conf.d/* for proxy configuration files such as *proxy.conf*, and temporarily rename the relevant file if necessary.

## Check and update the package repositories

Incorrect repository configuration is a common cause of download failures.

- View the current repositories: Run **cat /etc/apt/sources.list** and **ls -l /etc/apt/sources.list.d/**, and check for invalid or outdated repository URLs.

- Change the mirror: If the default repositories are unstable or slow, switch to a reliable mirror in China, such as one provided by Tsinghua University, Alibaba Cloud, or the University of Science and Technology of China. After changing the mirror, run **sudo apt update** to apply the change. For details, see [Software update](<../../Operating system/Debian/Software update/Software update.md>).

- Back up the repository configuration: Before making changes, run **sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak** to create a backup.

## Update package lists and repair dependencies

Outdated package lists or broken dependencies can cause download failures.

- Update the package list: This is an essential step. Run **sudo apt update** to ensure that the system can retrieve the latest package information.

- Repair dependencies: If the issue persists after the update, try running **sudo apt --fix-broken install** to repair broken dependencies.

- Use the *--fix-missing* option: If you encounter an *Unable to fetch some archives* error, try running **sudo apt update --fix-missing**.

## Check disk space and package manager processes

Insufficient disk space or another running package-management process can also cause download failures.

- Check disk space: Run **df -h** to check the available space on the *root (/)* and */var* partitions. If space is low, run **sudo apt clean** to clear the APT cache, and run **sudo apt autoremove** to remove unnecessary packages.

- Check package manager processes: If another APT or dpkg process is running, run **ps aux | grep -E "apt|dpkg"** to view it. Wait for the process to finish, or run **sudo kill -9 `<process_ID>`** to forcefully terminate it. If it cannot be terminated, run **sudo rm /var/lib/apt/lists/lock** to try to remove the lock file.

## Check other possible causes

- Verify the package name: APT cannot locate a package if its name is misspelled. Run **apt-cache search `<keyword>`** to identify the correct package name.

- Import the GPG key: If a GPG error appears during an update, a key may be missing. Run **sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys `<missing_key_ID>`** to import it.

- Check the system time: An incorrect system time can cause HTTPS certificate verification to fail. Run **date** to check the time. If the time is incorrect, run **sudo timedatectl set-ntp true** to synchronize it.

- If the package is unavailable from the configured repositories, download the .deb package and run **sudo dpkg -i xxx.deb** to install it locally.

# How do I troubleshoot high memory usage?

## Confirm that the system is actually low on memory

```plaintext
free -h
cat /proc/meminfo | head -30
```

Pay particular attention to these fields:

```plaintext
MemAvailable
SwapFree
Cached
Buffers
Slab
SReclaimable
SUnreclaim
```

Use the following criteria:

- A high *used* value does not necessarily indicate a problem because Linux uses free memory for the page cache.

- The key is to check whether *MemAvailable* is too low.

- The system is under genuine memory pressure only when *MemAvailable* is low, swap usage continues to increase, and the system becomes sluggish.

## Check for OOM events or memory warnings

```plaintext
dmesg -T | grep -i -E "oom|out of memory|killed process|memory allocation failure"
journalctl -k | grep -i -E "oom|out of memory|killed process"
```

If the following log messages appear, the system has experienced an out-of-memory (OOM) condition.

```plaintext
Out of memory
Killed process xxx
```

## Identify the processes using the most memory

```plaintext
ps aux --sort=-rss | head -20
```

Or:

```plaintext
ps -eo pid,ppid,comm,rss,vsz,%mem --sort=-rss | head -30
```

The fields are defined as follows:

| **Field** | **Meaning** |
| --- | --- |
| RSS | Physical memory actually in use. |
| VSZ | Virtual address space, not actual memory usage. |
| %MEM | Percentage of total memory. |

You can also use:

```plaintext
top
```

After the interface opens, press **M** to sort processes by memory usage.

```plaintext
M
```

## View detailed memory information for a process

Assume that the process ID (PID) is *1234*:

```plaintext
cat /proc/1234/status | grep -E "VmRSS|VmSize|VmData|VmSwap|Threads"
```

For more detailed information:

```plaintext
cat /proc/1234/smaps_rollup
```

Pay particular attention to these fields:

```plaintext
Rss
Pss
Private_Clean
Private_Dirty
Swap
```

If *Private_Dirty* continues to increase, it usually indicates a memory leak.

# What should I do if the system freezes?

Determine the severity of the system freeze by checking whether the keyboard, network, or serial port still responds.

- Responsive (soft lockup): For example, SSH remains accessible, the serial port still accepts input, or the SysRq key still works. This indicates that the kernel is still running but one or more processes are unresponsive. Diagnose the issue before attempting a forced restart to avoid losing diagnostic information.

- Completely unresponsive (hard lockup): The keyboard indicators do not flash, the network connection is completely lost, and the serial port produces no output. This condition is typically caused by a kernel panic, a hardware lockup, or a fatal interrupt storm. Recovery requires a hardware reset or power cycle.