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These three commands answer different questions: which is commonly used to locate an executable, whereis searches for related binary, source, and manual files, and whatis gives a brief description from local manual-page indexes. In Bash, start with type -a name when you need to know what the shell will actually resolve, because that can include aliases, functions, and builtins—not just executable files.
Which command answers your question?
| Question | Command to try | What it tells you |
|---|---|---|
| Which definition will Bash use for this name? | type -a name |
Shows matching aliases, functions, builtins, and executable commands that Bash knows about. Bash checks functions and builtins before searching PATH for an executable. GNU Bash manual: command lookup; GNU Bash manual: type |
| Where might related files be installed? | whereis name |
Looks for binary, source, and manual files in standard locations, with lookup influenced by PATH, MANPATH, and system configuration. It may not find every installed copy. whereis(1) |
| What does the name do, briefly? | whatis name |
Searches local manual-page name indexes for short descriptions. Results depend on installed manual pages and index data. whatis(1) |
The tools are not interchangeable. A shell can resolve a name to a function or builtin even when an executable with the same name exists elsewhere on PATH. Conversely, finding a manual page does not establish which implementation your shell will run.
Find what Bash will run
Use type for shell-aware lookup
Run type -a name to see the kinds of matches Bash reports, including multiple matches. For example, replace name with ls:
type -a ls
If you specifically need an executable-path search rather than Bash’s broader command resolution, use type -P name. The distinction matters when a function, alias, or builtin takes precedence over a program file.
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Where which fits
which name is commonly used to look up an executable, but its behavior depends on which which implementation is installed. It is not a dependable way to explain every shell function, alias, or builtin. For a Bash-specific answer about what the shell knows under a name, prefer type. The GNU Bash manual describes Bash’s command lookup and its type builtin: command search and execution and Bash builtins.
Find related files with whereis
Use whereis when you want to look for a command’s associated binary, source, or manual files:
whereis lschecks for the supported file types.whereis -b lslimits the search to binaries.whereis -m lslimits it to manual pages.whereis -s lslimits it to source files.
Options -B, -M, and -S constrain the directories searched for binaries, manuals, and source; -f separates those directory lists from the names being checked. Use whereis -l to print effective lookup paths. These options and the search behavior are documented by util-linux’s whereis(1); exact locations depend on the installed version and environment.
Treat the output as a set of search results, not a complete inventory of every copy on the machine. A result can vary across distributions and configurations, so check the paths on your own system.
Get a short description with whatis
whatis name searches manual-page names and returns brief descriptions. For example:
whatis printf
Depending on your installed manual pages and index, the output may include a shell builtin entry and one or more manual entries. whatis consults local manual-page index data maintained by mandb; it is not an internet search and cannot describe entries absent from the local index. If an entry is missing or the database is stale, results can be incomplete. The whatis(1) manual documents -w for wildcard matching and -r for regular-expression matching; quote patterns as needed to prevent the shell from expanding them first.
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For instructions beyond a one-line description, open the relevant manual page with man name. A short whatis result is a starting point, not a substitute for that page.
A practical lookup sequence
- Check Bash resolution: run
type -a nameto see what kinds of definitions Bash finds. - Check for an executable path: if that is the question, use
type -P name;which nameis another common but implementation-dependent executable lookup. - Look for associated files: run
whereis name, adding-b,-m, or-sto narrow the file type. - Read a brief description: run
whatis name; for fuller usage information, runman name.
Replace name with the command you are investigating, and interpret the results in light of your shell, installed packages, and local manual-page configuration.
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