Why which and whereis for commands in Linux CLI? - Purpose & Use Cases

The Big Idea

Discover how to instantly find any command on your system without endless searching!

The Scenario

Imagine you want to run a program on your computer, but you don't know where it is installed or which version will run if you type its name. You try typing the command, but sometimes it doesn't work or runs a different program than you expected.

The Problem

Manually searching for a program's location means digging through folders one by one. This is slow and confusing, especially when many programs have similar names or multiple versions exist. You might run the wrong program by mistake, causing errors or wasted time.

The Solution

The which and whereis commands quickly tell you where a program lives on your system. which shows the exact program that will run when you type its name, while whereis finds all related files like executables, source code, and manuals. This saves time and avoids confusion.

Before vs After

✗ Before

ls /usr/bin | grep program_name
find / -name program_name

✓ After

which program_name
whereis program_name

What It Enables

You can instantly find and verify the exact program you want to run, making your work faster and more reliable.

Real Life Example

When installing new software, you can use which to check if the command is ready to use, or whereis to find its documentation and binaries without guessing.

Key Takeaways

which shows the command that will run from your PATH.

whereis locates all files related to a command.

Both save time and reduce errors when working with commands.

Practice

1. What does the which command do in Linux?

easy

A. Lists all files related to a command including source and man pages

B. Displays the manual pages of a command

C. Shows the full path of the executable that runs when you type a command

D. Shows the current directory contents

Solution

1. Step 1: Understand the purpose of which

   The which command searches your PATH environment to find the executable file that runs when you enter a command.

2. Step 2: Compare with other options

   whereis lists multiple locations including source and man files, not just the executable path. Manual pages are shown by man. Directory contents are shown by ls.

3. Final Answer:

   Shows the full path of the executable that runs when you type a command -> Option C

4. Quick Check:

   which = executable path [OK]

Hint: Remember: which = executable path only [OK]

Common Mistakes:

• Confusing which with whereis
• Thinking which shows manual pages
• Assuming which lists all related files

2. Which of the following is the correct syntax to find the location of the ls command using which?

easy

A. which -l ls

B. which ls

C. which --all ls

D. which ls -a

Solution

1. Step 1: Recall basic which syntax

   The basic syntax to find the path of a command is simply which command_name. No extra flags are needed for a single path.

2. Step 2: Analyze options

   Options like -l or --all are not standard or needed for basic usage. Adding -a or extra flags incorrectly after the command name is invalid.

3. Final Answer:

   which ls -> Option B

4. Quick Check:

   Simple syntax: which + command [OK]

Hint: Use just 'which command' to find executable path [OK]

Common Mistakes:

• Adding unnecessary flags to which
• Placing flags after command name
• Using incorrect or unsupported options

3. What will be the output of the command whereis bash on a typical Linux system?

medium

A. bash: /bin/bash /usr/share/man/man1/bash.1.gz

B. bash: command not found

C. /usr/bin/bash

D. /bin/bash

Solution

1. Step 1: Understand whereis output format

   whereis shows multiple locations: the binary executable path, source files if any, and manual page locations.

2. Step 2: Analyze typical output for bash

   On most systems, whereis bash returns the binary path like /bin/bash and the man page location like /usr/share/man/man1/bash.1.gz.

3. Final Answer:

   bash: /bin/bash /usr/share/man/man1/bash.1.gz -> Option A

4. Quick Check:

   whereis = binary + man files [OK]

Hint: whereis shows binary and man page locations [OK]

Common Mistakes:

• Expecting only one path from whereis
• Confusing whereis output with which output
• Thinking whereis returns error if command exists

4. You run which python3 but get no output. What is the most likely reason?

medium

A. Python3 is installed but hidden by a firewall

B. You typed the command incorrectly

C. which command is broken on your system

D. Python3 is not installed or not in your PATH

Solution

1. Step 1: Interpret empty output from which

   If which returns nothing, it means it did not find the executable in any directory listed in your PATH environment variable.

2. Step 2: Consider common causes

   Most often, this means the program is not installed or its executable directory is not in PATH. Typing errors usually cause an error message, not empty output. The command itself rarely breaks, and firewalls do not hide executables.

3. Final Answer:

   Python3 is not installed or not in your PATH -> Option D

4. Quick Check:

   No output from which = command missing or PATH issue [OK]

Hint: No output means command not found in PATH [OK]

Common Mistakes:

• Assuming which is broken
• Blaming firewall for missing executable
• Ignoring PATH environment variable

5. You want to find all locations related to the gcc compiler including binary, source, and manual files. Which command should you use?

hard

A. whereis gcc

B. which gcc

C. locate gcc

D. find / -name gcc

Solution

1. Step 1: Understand the purpose of each command

   which shows only the executable path. locate searches a database for filenames but may show unrelated files. find searches the entire filesystem but is slow and may produce too many results.

2. Step 2: Identify the command that shows binary, source, and manual files

   whereis is designed to show all related locations for a command including binary, source, and man pages, making it the best choice.

3. Final Answer:

   whereis gcc -> Option A

4. Quick Check:

   whereis = binary + source + man files [OK]

Hint: Use whereis to find all related command files [OK]

Common Mistakes:

• Using which to find all files
• Using locate without filtering results
• Running find without knowing its slowness