Resolving the “Syntax error: redirection unexpected” in Linux

by
Syntax error: redirection unexpected
LinkedIn

When working with shell scripts in Linux, encountering a “Syntax error: redirection unexpected” error can be a frustrating experience. This error generally surfaces when a script attempts to execute a command involving redirection, but the shell interprets the syntax as incorrect or unexpected. Such issues are most commonly found when scripts crafted on one system are run on another, or when a script uses features not supported by the shell it’s executed in. Let’s dive into how to diagnose and resolve this issue, with real-time examples to guide you.

Understanding the Issue

The error message essentially tells us that the shell (the command interpreter in Linux) cannot correctly interpret one or more lines in the script, particularly around redirection operators (like >, >>, or |). Redirection is a powerful feature in Unix-like systems, allowing you to direct the output of a command to a file or another command.

Example

Imagine you have written a script example_script.sh intended to back up a directory’s contents by compressing it and then redirecting the output to a log file. The script might include a line like this:

tar -czf /backup/mybackup.tar.gz /home/test_user/data > /backup/backup.log

If this script throws the “Syntax error: redirection unexpected” when executed, it’s a sign that something is wrong in the syntax or compatibilty issue between different OS

Replicate & Resolve the issue

It would be always preferred to replicate the issue to understand things better before resolving it.

1. Create a Shell Script with Bash-specific syntax

Let’s create a script that uses some Bash-specific features, which might not be recognized by other shells like dash or when the script has incorrect line endings.

Open a text editor and create a new file named example_script.sh.

Add the following Bash-specific syntax to the file:

#!/bin/bash
echo "Current Directory Files" > >(tee directory_files.txt)

This script attempts to list files in the current directory and uses process substitution, which is a feature not supported by all shells.

2. Force the Script to Run in a Non-Bash Shell

Even though our script specifies “#!/bin/bash“, forcing it to run in a shell that does not support Bash-specific features, like dash, can reproduce the issue.

You can replicate this by explicitly invoking the script with a different shell (In this case I am using dash):

~ % dash example_script.sh

example_script.sh: 2: Syntax error: redirection unexpected

This command ignores the shebang line and attempts to run the script with “dash”, which might not support the Bash-specific redirection used in the script.

When executed in a non-Bash shell (dash), the script will throw the “Syntax error: redirection unexpected” error as shown above, this is due to the shell’s inability to interpret Bash-specific redirection or process substitution.

3. Resolve the Issue

To resolve the issue that we have replicated, ensure the script is run with a shell that supports the syntax used, such as Bash, and check that the file has the correct line endings.

bash example_script.sh

Convert line endings if necessary: If the script was edited on a Windows system, convert the line endings to Unix format using dos2unix or a similar tool/command will be necessary.

dos2unix example_script.sh

Common troubleshooting steps

1. Check the Shell Used in the Script

First, ensure the script’s shebang line matches the intended shell. If your script uses bash-specific features, the first line should be #!/bin/bash.

2. Convert File Format

Scripts edited or created on Windows might have CRLF line endings, which Unix systems don’t like. You can convert the file using:

dos2unix example_script.sh

Or with sed:

sed -i 's/\r$//' example_script.sh

3. Check for Unsupported Redirects

Advanced redirection features such as process substitution (>(...) or <(...)), are not supported by all shells. Ensure your script doesn’t use features unsupported by its designated shell.

4. Syntax Verification

Manually inspect your script for syntax errors, especially around the parts that involve redirection.

5. Explicitly Invoke the Intended Shell

If your script is bash-specific, run it with bash explicitly:

bash example_script.sh

6. Ensure Script Permissions

Make sure the script is executable:

chmod +x example_script.sh

7. Use Echo for Debugging

Temporarily replace commands involving redirection with echo to make sure they are formed correctly.

8. Opt for More Compatible Syntax

For scripts intended to run on multiple shell environments, stick to syntax that is widely supported, or use conditionals to detect the shell and adjust accordingly.

9. Debugging

Enable script debugging to see each command before it’s executed, which can help pinpoint the issue:

% set -x

+update_terminal_cwd:5> local url_path=''                                                                                                                          
+update_terminal_cwd:10> local i ch hexch LC_CTYPE=C LC_COLLATE=C LC_ALL='' LANG=''
+update_terminal_cwd:11> i = 1

Conclusion

In conclusion, “Syntax error: redirection unexpected” can typically be resolved by ensuring compatibility between the script’s syntax and the executing shell, along with making sure the script is in the correct format and has the appropriate permissions. By methodically checking each potential source of error and employing debugging techniques, you can ensure your scripts run smoothly across different environments.

Happy Learning!!

Jerry Richard
Follow me
Latest posts by Jerry Richard (see all)

Was this post helpful?

Yes
No
Thanks for your feedback!
LinkedIn

Leave a Comment