需编写Shell脚本：仅在文件未被占用时读取（QNX嵌入式环境）

Got it, since you're working with a stripped-down QNX shell that doesn't have access to tools like lsof, we can leverage QNX's native utilities and file system behavior to safely check if a file is in use before reading it. Here are a couple of reliable approaches tailored to your environment:

QNX ships with pidin, a core built-in utility that lists process details—including files opened by each running process. We can use this to verify if our target file is currently in use:

#!/bin/sh

# Replace this with your actual file path
TARGET_FILE="/path/to/your/target.file"

# Resolve symlinks to the real file path (handles cases where target is a soft link)
REAL_FILE=$(readlink -f "$TARGET_FILE" 2>/dev/null || echo "$TARGET_FILE")

# Check if the file is listed in any process's open files
pidin files | grep -q "$REAL_FILE"
if [ $? -ne 0 ]; then
    echo "File is not in use. Reading content:"
    cat "$TARGET_FILE"
else
    echo "Error: File is currently open by another process. Cannot read."
    exit 1
fi

Key notes for this method: #

• pidin is included with all standard QNX installations, so it should be available even in minimal shell environments.
• There's a tiny race condition here: between the pidin check and the cat command, another process could open the file. For most embedded use cases, this is acceptable, but if you need stricter safety, combine it with the next approach.

To avoid the race condition entirely, we can attempt to open the file in exclusive mode first. If the open succeeds, the file is safe to read; if it fails, the file is locked or in use. We'll use shell file descriptor redirection to do this:

#!/bin/sh

TARGET_FILE="/path/to/your/target.file"

# Try to open the file in exclusive write mode (binds to file descriptor 9)
exec 9>"$TARGET_FILE"
if [ $? -eq 0 ]; then
    echo "File is available. Reading content:"
    cat "$TARGET_FILE"
    # Clean up the file descriptor when done
    exec 9>&-
else
    echo "Error: File is locked or in use by another process."
    exit 1
fi

How this works: #

• When we run exec 9>"$TARGET_FILE", the shell tries to open the file for writing with exclusive access. QNX's file system will block this operation if another process has the file open for writing or holds a shared lock.
• This method eliminates race conditions because the access check and subsequent read are tied to the same file handle.

For the most robust solution, combine the pidin quick check with the exclusive open to minimize the window for race conditions:

#!/bin/sh

TARGET_FILE="/path/to/your/target.file"
REAL_FILE=$(readlink -f "$TARGET_FILE" 2>/dev/null || echo "$TARGET_FILE")

# First quick check with pidin to catch obvious in-use cases
pidin files | grep -q "$REAL_FILE"
if [ $? -eq 0 ]; then
    echo "File is in use (detected via pidin). Cannot read."
    exit 1
fi

# Confirm with exclusive open to avoid race conditions
exec 9>"$TARGET_FILE"
if [ $? -ne 0 ]; then
    echo "File was opened by another process after our initial check. Cannot read."
    exit 1
fi

# Safe to read the file now
echo "File is safe to read. Content:"
cat "$TARGET_FILE"

# Clean up the file descriptor
exec 9>&-

内容的提问来源于stack exchange，提问作者abhishek mishra