which and file

Two small but handy commands round off your searching skills: which and file.

Where does a command live? which

When you type ls, Linux runs a program stored somewhere on disk. The which command tells you exactly where that program is:

which ls

/bin/ls

which grep

/usr/bin/grep

This is useful when you want to know whether a command is installed at all: if which prints nothing, the command was not found.

What kind of file is this? file

A file's name does not always tell you what is inside it. The file command inspects the content and reports the type:

file notes.txt

notes.txt: ASCII text

file photo.png

photo.png: PNG image data

file looks at the actual bytes, so it is right even when the extension is missing or wrong.

Mission: Unknown Data Probe

A routine scan found a file called mystery.dat in your home directory. The extension is unfamiliar and nobody knows what it contains. Investigate it, then file it away properly.

1. Confirm that the basic listing tool (ls) is actually installed on this system.
2. Identify what kind of data is really inside mystery.dat.
3. It turns out to be plain readable text. Display its contents to see the message.
4. Create a folder called identified and copy the file into it under the clearer name readme.txt.

Expected result

You see where ls lives, the inspection reveals mystery.dat is plain ASCII text despite its suspicious name, you read the message, and a copy sits in identified/readme.txt under a name that finally makes sense.