103.1 Lesson 1

While staring at the flashing rectangle of a command line prompt, your first question will probably be “Where am I?” Or, more precisely, “Where in the Linux filesystem am I right now and if, say, I created a new file, where would it live?” What you are after here is your present work directory, and the pwd command will tell you what you want to know:

Assuming that Frank is currently logged in to the system and he is now in his home directory: /home/frank/. Should Frank create an empty file using the touch command without specifying any other location in the filesystem, the file will be created within /home/frank/. Listing the directory contents using ls will show us that new file:

Besides your location in the filesystem, you will often want information about the Linux system you are running. This might include the exact release number of your distribution or the Linux kernel version that is currently loaded. The uname tool is what you are after here. And, in particular, uname using the -a (“all”) option.

Here, uname shows that Frank’s machine has the Linux kernel version 4.18.0 installed and is running Ubuntu 18.04 on a 64-bit (x86_64) CPU.

You will often come across documentation talking about Linux commands with which you are not yet familiar. The command line itself offers all kinds of helpful information on what commands do and how to effectively use them. Perhaps the most useful information is found within the many files of the man system.

As a rule, Linux developers write man files and distribute them along with the utilities they create. man files are highly structured documents whose contents are intuitively divided by standard section headings. Typing man followed by the name of a command will give you information that includes the command name, a brief usage synopsis, a more detailed description, and some important historical and licensing background. Here is an example:

man only works when you give it an exact command name. If, however, you are not sure about the name of the command you are after, you can use the apropos command to search through the man page names and descriptions. Assuming, for instance, that you cannot remember that it is uname that will give you your current Linux kernel version, you can pass the word kernel to apropros. You will probably get many lines of output, but they should include these:

If you do not need a command’s full documentation, you can quickly get basic data about a command using type. This example uses type to query four separate commands at once. The results show us that cp (“copy”) is a program that lives in /bin/cp and that kill (change the state of a running process) is a shell builtin — meaning that it is actually a part of the Bash shell itself:

Notice that, besides being a regular binary command like cp, uname is also “hashed.” That is because Frank recently used uname and, to increase system efficiency, it was added to a hash table to make it more accessible the next time you run it. If he would run type uname after a system boot, Frank would find that type once again describes uname as a regular binary.

Sometimes — particularly when working with automated scripts — you will need a simpler source of information about a command. The which command that our previous type command traced for us, will return nothing but the absolute location of a command. This example locates both the uname and which commands.

You will often carefully research the proper usage for a command and successfully run it along with a complicated trail of options and arguments. But what happens a few weeks later when you need to run the same command with the same options and arguments but cannot remember the details? Rather than having to start your research over again from scratch, you will often be able to recover the original command using history.

Typing history will return the most recent commands you have executed with the most recent of those appearing last. You can easily search through those commands by piping a specific string to the grep command. This example will search for any command that included the text bash_history:

Here a single command is returned along with is sequence number, 1605.

And speaking of bash_history, that is actually the name of a hidden file you should find within your user’s home directory. Since it is a hidden file (designated as such by the dot that precedes its filename), it will only be visible by listing the directory contents using ls with the -a argument:

What is in the .bash_history file? Take a look for yourself: you will see hundreds and hundreds of your most recent commands. You might, however to surprised to find that some of your most recent commands are missing. That is because, while they are instantly added to the dynamic history database, the latest additions to your command history are not written to the .bash_history file until you exit your session.

You can leverage the contents of history to make your command line experience much faster and more efficient using the up and down arrow keys on your keyboard. Hitting the up key multiple times will populate the command line with recent commands. When you get to the one you would like to execute a second time, you can run it by pressing Enter. This makes it easy to recall and, if desired, modify commands multiple times during a shell session.