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linux:intro-to-bash-part-2

Intro to Bash, 2

Date: Wed Sep 5 13:01:37 EDT 2018

Job control

Note: the dollar sign in the examples below simply indicate the Bash prompt. Don't type those in if you follow along.

Bash provides tools to work with multiple utilities. For example, open the nano text editor like so:

$ nano

And then press the Ctrl-z combo to background the process. Now open vim, and then repeat with the Ctrl-z combo to background that process. Repeat with htop:

$ vim

Ctrl-z

$ htop

Ctrl-z

To get a list of jobs, run the jobs command:

$ jobs

Each job will have an ID number. To foreground the second job, use the fg command:

$ fg 2

It's best not to kill a job, i.e., with the kill command. Instead, try to close it out normally (such as Ctrl-x in nano). But if you want, run jobs like so to get the PID (process ID) number for the job:

$ jobs -l

That will return the PID number for the backgrounded jobs. To kill a runaway process/job, you'd do:

$ kill -9 30125

Assuming that 30125 is the PID for the misbehaving job.

Note: It's good to know about job control and the jobs command, but I generally use apps called screen, tmux, or more often, a window manager called i3 to help manage separate jobs. [Demo screen]

Environmental Variables

To print environmental variables, run the following command:

$ printenv

This is useful because it will give you information about how variables have been set in places like .profile, .bashrc, and elsewhere. Notice, for example,
the $PATH variable (listed in printenv output with the dollar sign). That tells you where Bash looks for executables when you run them. If you want to add new directories to the $PATH variable, then you can do that in your .bashrc file.

Redirection

A command like ls normally prints output to the screen, which in many cases is what we call the standard output, or stdout, for short. We can redirect stdout to file using the right angle bracket:

$ ls > directory_listing.txt

Using only a single right angle bracket will overwrite the file, here named directory_listing.txt, if it already exists. To append (or add) stdout to the file, use two angle brackets:

$ ls -ahlt >> directory_listing.txt

We can redirect in the reverse order. That is, we can take input from a file instead of the keyboard -- thus making the file function as stdin rather than the keyboard. To do so, we reverse the angle bracket. This is usually helpful in Bash scripts. For example, when I created the user accounts for our server, I used the following conditional in Bash that took a file name as stdin. Here's an abbreviated form of that command, where the variable "$user_list" is a file containing a single column of usernames. This is not the full command that I use, so if you try it, it won't work the way you'd like:

$ while read i ; do useradd "$i" ; done <"$user_list"

Another example -- creating a bunch of empty files:

$ while read j ; do touch "$i" ; done<greek-letters

Another way to redirect stdin is to use what's called process substitution. This is what's happening in the ls commands in the parentheses below. Process substitution is where the "input or output of a process ... appear as a temporary file":

Create a tmp directory and change to it:

$ mkdir tmp ; cd tmp/

Create two new subdirectories in tmp/:

$ mkdir A ; mkdir B

Create three empty files in the tmp/A/ directory:

$ cd A ; touch one two three

Create three empty files in the tmp/B/ directory:

$ cd ../B ; touch one two four

Compare the differences with process substitution:

$ cd .. ; sdiff <(ls A/) <(ls B/)

File permissions

All files in Linux have permissions. See, e.g., below, where I have one regular, empty file called newfile.txt and one directory called workspace:

$ ls -lh
-rw-r--r--  1 sean sean    0 Sep  6 00:28 newfile.txt
drwxr-xr-x 16 sean sean 4.0K Jun 21 18:35 workspace

For now, we're going to concentrate on the three octals represented after the initial dash or the initial d in the above list. The initial dash can indicate a number of file attributes, and I'll cover some of that a bit later in this video and more of it later on in the course.

Each of the letters in the list of file permissions represent a number. The r for read access is represented by the number 4. The w for write access is represented by the number 2. And the x for execute is represented by the number 1. You can read more about this in man page for chmod.

Additionally, the rwx values can be repeated up to three times in order to indicate permissions for the user who owns the file, other users in the file's group, and the last for all users, which is important to set if the file is to be accessible on a website.

Thus, the newfile.txt listed above indicates that the user has read and write access (rw-), that the group sean has read access (r--), and that all users have read access (r--). If I want to make sure that only I have read and write access, then I use chmod like so:

$ chmod 600 newfile.txt

Since r indicates 4 and w indicates 2, then the six is simply the sum of those two numbers. If I want a group to have read and write access to that file, then I simply repeat the sum of those numbers in the second set of octals:

$ chmod 660 newfile.txt

Or, if I want the owner of the file to have read and write access but the group to have only read access, and all users to have no access, then:

$ chmod 640 newfile.txt

If we want to be able to cd into a directory, then the x, or execute, bit must be set. For a regular file, the x bit indicates that the file is a script, application, or program of some sort (could be a binary program or a script file, like Bash, Python, etc.).

The chown command changes who owns the file, for either or both the user or group. The following command changes the owner of the file to a user named sam::

$ chown sam newfile.txt

If I want to also change the group, then I state both the owner's name and the group's name. Here the group's name is wildcats (assuming the group wildcats already exists on the system):

$ chown sam:wildcats newfile.txt

More on creating new groups later.

I've already covered in a previous video how to create hard and symbolic links, but here are some refresher commands:

Let's create a directory called tmp2 and change to that directory:

$ mkdir tmp2 ; cd tmp2/

Create an empty file called a.txt:

$ touch a.txt

Link a.txt to a file called b.txt, which will be created when I make the link:

$ ln a.txt b.txt

Look at the inode for each file (i.e., its metadata), note that they are the same because this is a hard link:

$ stat a.txt ; stat b.txt

Add info to a.txt and cat file b.txt, and you'll note that the content is the same.

To create a symbolic link, which is useful if the symbolic link exists on a separate file system, using the -s option for the ln command:

$ ln -s a.txt c.txt

Note that these files, although the content is updated whenever one file is updated, are two separate files:

$ stat a.txt ; stat c.txt

linux/intro-to-bash-part-2.txt · Last modified: 2019/01/21 11:05 by seanburns