diff

diff is a shell command that compares the content of files and reports differences. The term diff is also used to identify the output of the command and is used as a verb for running the command. To diff files, one runs diff to create a diff.
Typically, the command is used to compare text files, but it does support comparing binary files. If one of the input files contains non-textual data, then the command defaults to brief-mode in which it reports only a summary indication of whether the files differ. With the --text option, it always reports line-based differences, but the output may be difficult to understand since binary data is generally not structured in lines like text is.
Although the command is primarily used ad hoc to analyze changes between two files, a special use is for creating a patch file for use with the patch command – which was specifically designed to use a diff output report as a patch file.
POSIX standardized the diff and patch commands including their shared file format.

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