Linux CLI 9 🐧 whatis,apropos and info commands - KandZ

These three commands help you discover and explore commands on your system. They’re complementary tools for finding the right command and learning more about it.

Overview

Command	Purpose	Best For
`whatis`	One-line description of a command	Quick lookup when you know the name
`apropos`	Search by keyword	Finding a command when you don’t know its name
`info`	Detailed hyperlinked documentation	In-depth learning about a command

The `whatis` Command

Displays a brief one-line description of a command.

whatis ls
whatis cp mv
whatis -w 'mv*'
whatis -r 'mv[ab]+'
sudo makewhatis

Command	Description
`whatis ls`	Show one-line description of `ls`
`whatis cp mv`	Show descriptions for multiple commands
`whatis -w 'mv*'`	Wildcard search
`whatis -r 'mv[ab]+'`	Regex search
`whatis -s`	Case-sensitive search
`sudo makewhatis`	Update the `whatis` database

Examples

$ whatis ls
ls (1)               - list directory contents

$ whatis cp mv
cp (1)               - copy files and directories
mv (1)               - move (rename) files

$ whatis mkdir rmdir
mkdir (1)            - make directories
rmdir (1)            - remove empty directories

$ whatis cat
cat (1)              - concatenate files and print on the standard output

Multiple sections for the same name:

$ whatis mkdir
mkdir (1)            - make directories
mkdir (2)            - create a directory

Wildcard search (-w):

$ whatis -w 'mv*'
mv (1)               - move (rename) files
mvfn (1)             - move files with find integration

Regex search (-r):

$ whatis -r 'mv[ab]+'
mv (1)               - move (rename) files

Case-sensitive search (-s):

$ whatis -s LS
LS: nothing appropriate.
$ whatis -s ls
ls (1)               - list directory contents

Updating the `whatis` Database

If you’ve installed new commands and whatis doesn’t find them, update the database:

$ sudo makewhatis

Why? whatis searches a pre-built database of man page descriptions — not the man pages themselves. New installations may not be indexed yet.

Equivalent commands:

sudo mandb              # Common on Debian/Ubuntu
sudo makewhatis          # Common on Red Hat/CentOS

When to use it:

After installing software from source
After installing new man pages
When whatis returns “nothing appropriate” for a command that exists

The `apropos` Command

Searches the manual pages for a keyword — finds commands when you don’t know their names.

apropos directory
apropos "remove file"
apropos -w 'mv*'
apropos -r 'mv[ab]+'
apropos -a remove directory
apropos -s 1,2 directory

Command	Description
`apropos directory`	Search for “directory”
`apropos "remove file"`	Search for multiple keywords
`apropos -w 'mv*'`	Wildcard search
`apropos -r 'mv[ab]+'`	Regex search
`apropos -a remove directory`	Must include all keywords
`apropos -s 1,2 directory`	Search in specific sections

Examples

Single keyword:

$ apropos directory
basename (1)         - strip directory and suffix from filenames
chdir (2)            - change working directory
dirname (1)          - strip last component from file name
mkdir (1)            - make directories
mkdir (2)            - create a directory
pwd (1)              - print name of current/working directory
rmdir (1)            - remove empty directories
...

Multiple keywords (default OR):

$ apropos "remove file"
# Finds commands related to removing files
rm (1)               - remove files or directories
unlink (1)           - call the unlink function to remove the specified file
...

Note: By default, apropos treats space-separated words as alternatives (OR) — it matches commands whose description contains any of the keywords.

All keywords required (-a):

$ apropos -a remove directory
rmdir (1)            - remove empty directories
# Only matches descriptions containing BOTH "remove" AND "directory"

Wildcard search (-w):

$ apropos -w 'mv*'
mv (1)               - move (rename) files
mvn (1)              - Maven - Project Object Model
...

Regex search (-r):

$ apropos -r 'mv[ab]+'
mv (1)               - move (rename) files

Search specific sections (-s):

$ apropos -s 1,2 directory
mkdir (1)            - make directories
mkdir (2)            - create a directory
chdir (2)            - change working directory
pwd (1)              - print name of current/working directory
...

Sections: 1 = user commands, 2 = system calls, 3 = library functions, etc.

Practical Use: Finding the Right Command

You want to rename multiple files but don’t know the command:

$ apropos rename
rename (1)           - rename files
rename (2)           - change the name or location of a file
mv (1)               - move (rename) files
...

You want to search inside files but don’t remember grep:

$ apropos "search text"
grep (1)             - print lines that match patterns
rgrep (1)            - print lines that match patterns
...

You want to compress files but don’t know the tools:

$ apropos compress
bzip2 (1)            - a block-sorting file compressor
gzip (1)             - compress or expand files
tar (1)              - an archiving utility
xz (1)               - compress or decompress .xz and .lzma files
zip (1)              - package and compress (archive) files
...

The `info` Command

Provides more detailed, hyperlinked documentation than man pages.

info cp
info -k "copy"
info -f /path/to/file.info

Command	Description
`info cp`	Show info page for `cp`
`info -k "copy"`	Search for a keyword in all info files
`info -f file`	Use a specific info file

Examples

Basic info:

$ info cp
# Displays a hyperlinked manual for cp

Search info files:

$ info -k "copy"
"copy" has the following matches:
- cp
- cpio
- dd
- install
- ...

Use a specific info file:

$ info -f /usr/share/info/coreutils.info

Info Navigation Shortcuts

The info command uses its own pager with hyperlinked navigation:

Key	Action
`Space`	Scroll ahead
`Backspace`	Scroll back
`n`	Next node
`p`	Previous node
`u`	Go one level up
`Enter`	Follow a link
`l`	Go back in history
`r`	Go forward in history
`q`	Quit info

Info page structure:

File: coreutils.info,  Node: cp invocation,  Next: dd invocation,  Prev: cat invocation,  Up: Basic operations

'cp': Copy files and directories
================================

'cp' copies files...

File — which info file you’re in
Node — the current section
Next / Prev — sibling nodes
Up — the parent node

Man vs Info

Aspect	man	info
Format	Plain text	Hyperlinked
Navigation	Linear (scroll)	Tree (nodes)
Depth	Moderate	Deep
Structure	Single page	Multiple nodes
Best for	Quick reference	In-depth learning
Coverage	Everything	GNU tools mainly

Key insight: For GNU tools (like cp, ls, bash), info often has more information than man. For other tools, man may be the only option.

Complete Example Session

# ============================================
# PART 1: WHATIS — QUICK DESCRIPTIONS
# ============================================

# Single command
$ whatis ls
ls (1)               - list directory contents

# Multiple commands
$ whatis cp mv mkdir
cp (1)               - copy files and directories
mv (1)               - move (rename) files
mkdir (1)            - make directories

# Multiple sections for same name
$ whatis mkdir
mkdir (1)            - make directories
mkdir (2)            - create a directory

# Wildcard
$ whatis -w 'm*'
mkdir (1)            - make directories
mkdir (2)            - create a directory
mknod (1)            - make block or character special files
mknod (2)            - create a special or ordinary file
more (1)             - file perusal filter for crt viewing
mount (8)            - mount a filesystem
mv (1)               - move (rename) files

# Update database
$ sudo makewhatis
# (no output — updates the database silently)

# ============================================
# PART 2: APROPOS — SEARCH BY KEYWORD
# ============================================

# Single keyword
$ apropos directory
basename (1)         - strip directory and suffix from filenames
dirname (1)          - strip last component from file name
mkdir (1)            - make directories
pwd (1)              - print name of current/working directory
rmdir (1)            - remove empty directories

# Multiple keywords (OR by default)
$ apropos "remove file"
rm (1)               - remove files or directories
unlink (1)           - call the unlink function to remove the specified file

# All keywords required (-a)
$ apropos -a remove directory
rmdir (1)            - remove empty directories

# Wildcards
$ apropos -w 'mv*'
mv (1)               - move (rename) files
mvn (1)              - Maven - Project Object Model

# Regex
$ apropos -r 'mv[ab]+'
mv (1)               - move (rename) files

# Specific sections
$ apropos -s 1,2 directory
chdir (2)            - change working directory
mkdir (1)            - make directories
mkdir (2)            - create a directory
pwd (1)              - print name of current/working directory

# Practical: find a compression tool
$ apropos compress
bzip2 (1)            - a block-sorting file compressor
gzip (1)             - compress or expand files
tar (1)              - an archiving utility
xz (1)               - compress or decompress .xz and .lzma files
zip (1)              - package and compress (archive) files

# ============================================
# PART 3: INFO — DETAILED DOCUMENTATION
# ============================================

$ info cp
# Shows hyperlinked manual for cp

# Search info pages
$ info -k "copy"
"copy" has the following matches:
- cp
- cpio
- dd
- install
- ...

# Using specific info file
$ info -f /usr/share/info/coreutils.info

# Navigation (in info)
# Press Space → scroll ahead
# Press n → next node
# Press p → previous node
# Press u → go up
# Press Enter → follow link
# Press q → quit

Quick Reference

`whatis` Options

Option	Description
(none)	Show description for a command
`-w`	Wildcard search
`-r`	Regex search
`-s`	Case-sensitive search
`makewhatis`	Update the database (with `sudo`)

`apropos` Options

Option	Description
(none)	Search by keyword
`-a`	Require all keywords
`-w`	Wildcard search
`-r`	Regex search
`-s N`	Search in specific sections

`info` Options

Option	Description
(none)	Show info page for a command
`-k keyword`	Search all info files
`-f file`	Use specific info file

`info` Navigation

Key	Action
`Space`	Scroll ahead
`Backspace`	Scroll back
`n`	Next node
`p`	Previous node
`u`	Up one level
`Enter`	Follow link
`q`	Quit

When to Use Which

Situation	Use
Know the name, want a quick description	`whatis cmd`
Don’t know the name, want to find it	`apropos keyword`
Want to search by keyword	`apropos -a word1 word2`
Want detailed docs on a GNU tool	`info cmd`
Want to search all docs for a phrase	`info -k "phrase"`
Multiple commands’ descriptions at once	`whatis cmd1 cmd2 cmd3`

Best Practices

✅ Do This:

# Quick lookup when you know the name
whatis gzip

# Find commands by keyword
apropos "compress"

# Require multiple keywords
apropos -a "remove" "directory"

# Search specific section
apropos -s 1 "network"

# Get detailed docs
info coreutils

# Update whatis database after installs
sudo makewhatis

# Search info by keyword
info -k "extract"

❌ Don’t Do This:

# Don't forget to update whatis database
whatis mynewcmd     # ⚠️ "nothing appropriate"
sudo makewhatis      # ✅ Update first

# Don't rely on apropos with common words
apropos "file"      # ⚠️ Returns hundreds of results
apropos -a "remove" "directory"  # ✅ Narrow search

# Don't forget info uses its own navigation
info ls
# Use Space, n, p, u, q — not arrow keys!

# Don't confuse sections in man vs info
man 2 mkdir          # ✅ Man section 2
info mkdir           # ✅ Info page (no sections)

Common Pitfalls

Pitfall	Problem	Solution
`whatis` finds nothing	Database not updated	Run `sudo makewhatis`
`apropos` too many results	Common keyword	Use `-a` for all keywords
Can’t navigate `info`	Wrong keys	Use Space, n, p, u, q
Confusing `man` and `info`	Different navigation	Remember: `man` is linear, `info` is hyperlinked
Wrong section for `whatis`	Multiple entries	Use section number: `man 2 mkdir`

Visual: Choosing the Right Tool

┌──────────────────────────────────────────┐
│  What do you want to do?                 │
└──────────────────┬───────────────────────┘
                   │
        ┌──────────┴──────────┐
        │                     │
  Know the name?        Don't know the name?
        │                     │
        ▼                     ▼
  ┌──────────┐          ┌──────────────┐
  │ whatis   │          │  apropos     │
  │  cmd     │          │  keyword     │
  └────┬─────┘          └──────┬───────┘
       │                       │
       └───────────┬───────────┘
                   │
                   │ Need more detail?
                   ▼
             ┌──────────┐
             │  info    │
             │  cmd     │
             └──────────┘

Real-World Examples

1. Discovering Compression Tools

$ apropos compress
bzip2 (1)            - a block-sorting file compressor
gzip (1)             - compress or expand files
pigz (1)             - parallel implementation of gzip
tar (1)              - an archiving utility
xz (1)               - compress or decompress .xz and .lzma files
zip (1)              - package and compress (archive) files

2. Getting Details on a Known Command

$ whatis tar
tar (1)              - an archiving utility

$ info tar
# Opens the detailed info page

3. Finding All File-Removal Commands

$ apropos -a remove file
rm (1)               - remove files or directories
unlink (1)           - call the unlink function to remove the specified file

4. Updating the Database

# After installing a new tool
$ sudo apt install mytool
$ whatis mytool
mytool: nothing appropriate.
$ sudo makewhatis
$ whatis mytool
mytool (1)           - a great new tool

5. Detailed Information on Bash

$ info bash
# Opens the complete Bash manual
# Navigate with n, p, u, Space, Enter, q

Summary

Command	Purpose	Example
`whatis`	One-line description	`whatis ls`
`apropos`	Search by keyword	`apropos "remove file"`
`info`	Detailed hyperlinked docs	`info cp`

Key takeaways:

whatis — quick lookup when you know the name — whatis cmd
apropos — search by keyword when you don’t know the name — apropos keyword
info — the deepest documentation, especially for GNU tools — info cmd
sudo makewhatis — update the whatis/apropos database after installing software
apropos -a — require all keywords (narrows results)
apropos -s N — restrict to specific manual sections
info -k — search all info files for a keyword
info navigation — uses nodes and links: n (next), p (prev), u (up), Enter (follow), q (quit)

Remember: These three commands form your discovery toolkit. When you know a command’s name, use whatis for a quick description. When you don’t, use apropos to search by keyword. And when you want the deepest documentation, use info — especially for GNU tools, where info often has content that man doesn’t!

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Overview

The whatis Command

Examples

Updating the whatis Database

The apropos Command

Examples

Practical Use: Finding the Right Command

The info Command

Examples

Info Navigation Shortcuts

Man vs Info

Complete Example Session

Quick Reference

whatis Options

apropos Options

info Options

info Navigation

When to Use Which

Best Practices

Common Pitfalls

Visual: Choosing the Right Tool

Real-World Examples

1. Discovering Compression Tools

2. Getting Details on a Known Command

3. Finding All File-Removal Commands

4. Updating the Database

5. Detailed Information on Bash

Summary

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The `whatis` Command

Updating the `whatis` Database

The `apropos` Command

The `info` Command

`whatis` Options

`apropos` Options

`info` Options

`info` Navigation