Bash脚本中如何将输出重定向至代表标准流的变量（类Perl实现）

Perl makes it easy to abstract output targets with variables that handle both files and standard streams seamlessly. In Bash 4.2 on RHEL7, you can replicate this behavior with a couple of straightforward approaches—no need to rewrite your echo/printf calls every time you switch between stdout and a file.

Method 1: Using Custom File Descriptors (Best for Script-Wide Use) #

This approach lets you set up a single "stream variable" once at the start of your script, then redirect all relevant output to it without repeating checks.

# Define your target: either a file path or "stdout"
output_target="/tmp/my_output.txt"
# output_target="stdout"  # Uncomment this to switch to stdout

# Link a custom file descriptor (here, fd 3) to your target
if [[ "$output_target" == "stdout" ]]; then
    exec 3>&1  # Map fd 3 to the current stdout
else
    exec 3>"$output_target"  # Map fd 3 to the specified file (overwrites existing)
    # Use exec 3>>"$output_target" if you want to append instead of overwrite
fi

# Now redirect output to fd 3 anywhere in your script
echo "This goes to the target (file or stdout)!" >&3
printf "Formatted output: %s\n" "works too" >&3

# Optional: Clean up the file descriptor when you're done with it
exec 3>&-

Key Notes: #

• File descriptors 3-9 are safe to use (0=stdin, 1=stdout, 2=stderr are reserved).
• This method is efficient for scripts with lots of output—you only check the target once.
• Use >> instead of > in the exec line if you want to append to the file instead of overwriting it.

Method 2: Wrapper Function (Simple & Readable) #

If you prefer a more explicit approach, wrap your output logic in a function that handles the target check for you each time.

# Define a function to send output to the target
send_to_target() {
    local content="$1"
    local target="$2"
    
    if [[ "$target" == "stdout" ]]; then
        echo "$content"
    else
        echo "$content" > "$target"
        # Again, use >> instead of > to append
    fi
}

# Usage example
output_target="stdout"
# output_target="/tmp/another_output.log"

send_to_target "First line of content" "$output_target"
send_to_target "$(date +"[%Y-%m-%d %H:%M:%S] Log message")" "$output_target"

Key Notes: #

• Great for scripts where you only need to send output a few times.
• You can extend the function to support printf by adding a flag or separate logic if needed.

Both methods let you treat your output target as a variable, just like you would in Perl—switch between stdout and a file by changing a single variable value, no need to modify every echo/printf call.

内容的提问来源于stack exchange，提问作者vesperto