Fix: Sed output “char X: unterminated `s’ command”

Sed, short for Stream Editor, is a powerful text processing tool in Linux that allows you to manipulate text data in files and streams. It is particularly popular for its ability to easily perform search and replace operations on large files. However, users often encounter the “char X: unterminated `s’ command” error when working with Sed. This error can be confusing, and it can sometimes be confusing what causes it. More detail regarding the working of the Sed command can be found in this article

This article will not only explain the main causes of this error but also provides solutions for fixing it by discussing the following content.

  • Reason: Missing Terminating Delimiter
  • Other Ways to Fix the “char X: unterminated `s’ command” Error

Reason: Missing Terminating Delimiter

The “char X: unterminated `s’ command” error occurs when sed encounters a substitution command that is not properly terminated. The substitution command replaces a specific pattern in the text with another string. It starts with the letter “s” followed by the pattern to be replaced, the replacement string, and the delimiters used to separate these values, as shown below:

sed 's/apple/orange' file.txt

The main reason for this error is that the substitution command is not properly terminated; sed will output the “char 14: unterminated `s’ command” error. 


This error can be resolved by placing the proper delimiter at the end, as shown below:

$ sed 's/apple/orange/g' file.txt

Code Explanation

  • The letter “s” indicates a substitution command
  • The “apple” is the pattern to be replaced, whereas the “orange” is the replacement string
  • The “g” is a flag that specifies that the replacement should be done globally in the file.

Some Quick Fixes

To fix the “char X: unterminated `s’ command” error, you can follow these steps:

1. Check for Missing Delimiters

The first step is to check if the “s” command has the correct syntax with the terminating delimiter. If the delimiter is missing, add it to the command.

2. Check for Escaped Delimiters

If the pattern or replacement string contains the delimiter character, check if it is properly escaped with a backslash. If not, escape the delimiter.

3. Check for Multi-Line Commands

If the “s” command is not terminated within a single line, check if the command is split across multiple lines. If so, either add the missing delimiter or terminate the command within a single line.

4. Check for Other Syntax Errors

Finally, check for other syntax errors in the sed command, such as missing semicolons between commands.


The “char X: unterminated `s’ command” error in sed can be caused by a variety of factors, including missing delimiters, escaped delimiters within the pattern or replacement string, multi-line commands, and other syntax errors. By understanding these causes and following the steps to fix them, you can effectively troubleshoot this error and successfully use sed for your text transformation needs.