For each pathname given via the command-line or from a file via --stdin, check whether the file is excluded by .gitignore (or other input files to the exclude mechanism) and output the path if it is excluded.
By default, tracked files are not shown at all since they are not subject to exclude rules; but see --no-index.
By default, any of the given pathnames which match an ignore pattern will be output, one per line. If no pattern matches a given path, nothing will be output for that path; this means that path will not be ignored.
If --verbose is specified, the output is a series of lines of the form:
<source> <COLON> <linenum> <COLON> <pattern> <HT> <pathname>
<pathname> is the path of a file being queried, <pattern> is the matching pattern, <source> is the patterns source file, and <linenum> is the line number of the pattern within that source. If the pattern contained a ! prefix or / suffix, it will be preserved in the output. <source> will be an absolute path when referring to the file configured by core.excludesFile, or relative to the repository root when referring to .git/info/exclude or a per-directory exclude file.
If -z is specified, the pathnames in the output are delimited by the null character; if --verbose is also specified then null characters are also used instead of colons and hard tabs:
<source> <NULL> <linenum> <NULL> <pattern> <NULL> <pathname> <NULL>
If -n or --non-matching are specified, non-matching pathnames will also be output, in which case all fields in each output record except for <pathname> will be empty. This can be useful when running non-interactively, so that files can be incrementally streamed to STDIN of a long-running check-ignore process, and for each of these files, STDOUT will indicate whether that file matched a pattern or not. (Without this option, it would be impossible to tell whether the absence of output for a given file meant that it didnt match any pattern, or that the output hadnt been generated yet.)
Buffering happens as documented under the GIT_FLUSH option in git(1). The caller is responsible for avoiding deadlocks caused by overfilling an input buffer or reading from an empty output buffer.