sdiff - Linux


The sdiff command in Linux is a utility for comparing and merging two separate files line by line. It visually contrasts file contents and outputs the differences side by side, making it easier for users to review and merge changes. This tool is particularly useful in programming and system administration, where maintaining or updating source code files and configuration files is frequent.


The basic syntax for sdiff is:

sdiff [options] file1 file2

Where file1 and file2 are the files being compared.


Here’s a list of options available in sdiff:

  • -o, --output=FILE: Specifies a file to output the merged content.
  • -i, --ignore-case: Ignores case differences in file comparisons.
  • -w WIDTH, --width=WIDTH: Sets the width of the output to WIDTH columns.
  • -l, --left-column: Outputs only the left column of common lines.
  • -s, --suppress-common-lines: Does not output common lines.
  • --minimal: Attempts to make the differences smaller for a more fine-grained view.
  • -t, --expand-tabs: Expands tabs to spaces in the output, aligning columns.
  • -H, --speed-large-files: Speeds up comparison of large files by making some trade-offs in differential quality.


  1. Basic file comparison:

    sdiff file1.txt file2.txt
  2. Ignore case differences and output to a file:

    sdiff -i -o output.txt file1.txt file2.txt
  3. Specifying a custom width, suppressing common lines:

    sdiff -w 100 -s file1.txt file2.txt
  4. Generate and use left column only:

    sdiff -l file1.txt file2.txt

Common Issues

  • Encoding Differences: If the files have different encodings, sdiff may fail or behave unexpectedly. Prior conversion to a uniform encoding might be necessary.
  • File Access Permissions: Users may encounter permission errors. Ensure appropriate permissions are set to read the files.


sdiff can be effectively combined with other Unix tools, such as tee, grep, or using pipes and redirects in shell scripts. For instance, to immediately apply the changes and log the output, one might use:

sdiff -o merged.txt file1.txt file2.txt | tee change_log.txt
  • diff: Compare files line by line.
  • cmp: Compare two files byte by byte.
  • patch: Apply a diff file to an original.

Further reading:

This concise guide provides an essential understanding of sdiff and its functionality in Linux for file comparison and merging.