Unix Shell Wine

Quoting Command-Line Arguments in Shell Scripts

When writing shell scripts, especially for applications like WINE that interact with Windows file structures, quoting command-line arguments correctly is crucial. A common problem encountered is the mishandling of these arguments, which can lead to errors when paths contain spaces or special characters. In this post, we’ll discuss how to handle and resolve such quoting issues effectively.

The Problem

Consider a scenario where you have a shell script designed to execute a WINE application. The script takes in a list of arguments, converts Unix paths into Windows paths, and then invokes the executable. Unfortunately, due to improper quoting in the command-line arguments, an error occurs:

wine: cannot find ''/home/chris/.wine/drive_c/Program'

Key Issues Observed:

  1. Path Chopping: The path to the executable is truncated at the first space, despite being enclosed in single quotes.
  2. Backslash Interpretation: A backslash followed by ’t’ (\t) is interpreted as a tab character rather than a literal string.

Understanding the Quoting Issue

These problems arise from how the shell interprets quotes and special characters in command substitutions. Here’s why these issues happen:

  • When arguments are passed to the shell, any spaces can cause the command to break apart unless they are properly quoted.
  • Variables expanded into strings may contain escaped characters, which also need to be treated with care to avoid unwanted transformations.

For instance, if you echo a variable that contains escape sequences, they can get misinterpreted. This can look like the following:

Y='y\ty'
Z="z${Y}z"
echo $Z   # Outputs: zy	 yz (not zy  yz)

Such behavior can lead to debugging challenges and unexpected results in scripts.

The Solution

To resolve these issues with quoting, especially in our shell script example, we can utilize the built-in eval command. This command re-evaluates the arguments provided to it, allowing correct parsing of the command string before execution.

Step-by-Step Fix

  1. Modify the Last Line of the Script: Instead of executing the command directly, wrap it in eval:

    eval "$CMD"

  2. Advantages of Using eval:

    • Spaces in Paths: It allows paths with spaces (like Program Files) to be treated correctly because it evaluates the entire string within quotations as a single command.
    • Escape Handling: Using eval can help ensure that escape sequences are handled appropriately, reducing the risk of unintentional character translations like the \t converting to a tab.

  3. Testing: After making this adjustment, test your script with various inputs, especially those with spaces and escape characters, to ensure everything functions as expected.

Final Script Example

Here’s how your revised shell script might look:

#! /bin/sh

if [ "${1+set}" != "set" ]
then 
  echo "Usage; winewrap EXEC [ARGS...]"
  exit 1
fi

EXEC="$1"
shift

ARGS=""

for p in "$@"; do
  if [ -e "$p" ]; then
    p=$(winepath -w "$p")
  fi
  ARGS="$ARGS '$p'"
done

CMD="wine '$EXEC' $ARGS"
echo $CMD
eval "$CMD"

Conclusion

Quoting command-line arguments in shell scripts is a critical skill, particularly when dealing with complex application pathways in environments like WINE. By using eval, you can circumvent common errors related to space and escape character misinterpretation, ensuring your scripts run smoothly and efficiently.

Feel free to experiment with your scripts and share any further insights or questions in the comments below!