Introduction to Bash

Question 1

What is Bash

Answer 1

Bash is a vital tool for managing Linux machines. The name is short for “Bourne Again Shell.”

A shell is a program that commands the operating system to perform actions. You can enter commands in a console on your computer and run the commands directly, or you can use scripts to run batches of commands. Shells like PowerShell and Bash give system administrators the power and precision they need for fine-tuned control of the computers they’re responsible for.
In Unix and Linux, everything is a file. That means you can use the same commands without worrying about whether the I/O stream—the input and output—comes from a keyboard, a disk file, a socket, a pipe, or another I/O abstraction.

Question 2

full syntax for a Bash command

Answer 2

command [options] [arguments]

Question 3

Bash treats the first string it encounters as a command. The following command uses Bash’s ls (for “list”) command to display the contents of the current working directory:

Answer 3

ls

Question 4

Arguments often accompany Bash commands. For example, you can include a path name in an ls command to list the contents of another directory:

Answer 4

ls /etc

Question 5

Most Bash commands have options for modifying how they work. Options, also called flags, give a command more specific instructions. As an example, files and directories whose names begin with a period are hidden from the user and aren’t displayed by ls. However, you can include the -a (for “all”) flag in an ls command and see everything in the target directory:

Answer 5

ls -a /etc

Question 6

You can even combine flags for brevity. For example, rather than enter ls -a -l /etc to show all files and directories in Linux’s /etc directory in long form, you can enter this instead:

Answer 6

ls -al /etc

Question 7

Which options and arguments can be used, or must be used, varies from command to command. Fortunately, Bash documentation is built into the operating system. Help is never more than a command away. To learn about the options for a command, use the man (for “manual”) command. For instance, to see all the options for the mkdir (“make directory”) command, do this:

Answer 7

man mkdir

man is your best friend as you learn Bash. man is how you find the information you need to understand how any command works.

Question 8

Most Bash and Linux commands support the –help option. This shows a description of the command’s syntax and options. To demonstrate, enter mkdir –help. The output looks something like this:

Answer 8

Usage: mkdir [OPTION]… DIRECTORY…
Create the DIRECTORY(ies), if they do not already exist.

Mandatory arguments to long options are mandatory for short options too.
-m, –mode=MODE set file mode (as in chmod), not a=rwx - umask
-p, –parents no error if existing, make parent directories as needed
-v, –verbose print a message for each created directory
-Z set SELinux security context of each created directory
to the default type
–context[=CTX] like -Z, or if CTX is specified then set the SELinux
or SMACK security context to CTX
–help display this help and exit
–version output version information and exit

GNU coreutils online help: http://www.gnu.org/software/coreutils/
Report mkdir translation bugs to http://translationproject.org/team/
Full documentation at: http://www.gnu.org/software/coreutils/mkdir
or available locally via: info ‘(coreutils) mkdir invocation’

Help obtained this way is typically more concise than help obtained with man.

Question 9

Wildcards are symbols that represent one or more characters in Bash commands. The most frequently used wildcard is the asterisk. It represents zero characters or a sequence of characters. Suppose your current directory contains hundreds of image files, but you only want to see the PNG files; the ones whose file names end with .png. Here’s the command to list only those files:

Answer 9

ls *.png

Linux has no formal concept of a file-name extension as other operating systems do. This doesn’t mean that PNG files won’t have a .png extension. It simply means Linux attaches no special significance to the fact that the file names end with .png.

Question 10

Now let’s say the current directory also contains JPEG files. Some end in .jpg, while others end in .jpeg. Here’s one way to list all the JPEG files:

Answer 10

ls *.jpg *.jpeg

And here’s another:

ls .jpg

Question 11

The * wildcard matches on zero or more characters, but the ? wildcard represents a single character. If the current directory contains files named 0001.jpg, 0002.jpg, and so on through 0009.jpg, the following command lists them all:

Answer 11

ls 000?.jpg

Question 12

Yet another way to use wildcards to filter output is to use square brackets, which denote groups of characters. The following command lists all the files in the current directory whose names contain a period immediately followed a lowercase J or P. It lists all the .jpg, .jpeg, and .png files, but not .gif files:

Answer 12

ls .[jp]

Question 13

In Linux, file names and the commands that operate upon them are case-sensitive. So to list all the files in the current directory whose names contain periods followed by an uppercase or lowercase J or P, you could enter this:

Answer 13

ls .[jpJP]

Question 14

Expressions in square brackets can represent ranges of characters. For example, the following command lists all the files in the current directory whose names begin with a lowercase letter:

Answer 14

ls [a-z]*

Question 15

This command, by contrast, lists all the files in the current directory whose names begin with an uppercase letter:

Answer 15

ls [A-Z]*

Question 16

And this one lists all the files in the current directory whose names begin with a lowercase or uppercase letter:

Answer 16

ls [a-zA-Z]*

Question 17

If you need to use one of the wildcard characters as an ordinary character, you make it literal or “escape it” by prefacing it with a backslash. So, if for some reason you had an asterisk as part of a file name—something you should never do intentionally—you could search for it by using a command such as:

Answer 17

$ ls *

Question 18

ls lists the contents of your current directory or the directory specified in an argument to the command. By itself, it lists the files and directories in the current directory:

Answer 18

ls

Question 19

Files and directories whose names begin with a period are hidden by default. To include these items in a directory listing, use an -a flag:

Answer 19

ls -a

Question 20

To get even more information about the files and directories in the current directory, use an -l flag:

Answer 20

ls -l

Question 21

Suppose you want to see what’s inside a file. You can use the cat command for that. The output won’t make much sense unless the file is a text file. The following command shows the contents of the os-release file stored in the /etc directory:

Answer 21

cat /etc/os-release

Question 22

This is a useful command because it tells you which Linux distribution you’re running:

Answer 22

NAME=”Ubuntu”
VERSION=”18.04.2 LTS (Bionic Beaver)”
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME=”Ubuntu 18.04.2 LTS”
VERSION_ID=”18.04”
HOME_URL=”https://www.ubuntu.com/”
SUPPORT_URL=”https://help.ubuntu.com/”
BUG_REPORT_URL=”https://bugs.launchpad.net/ubuntu/”
PRIVACY_POLICY_URL=”https://www.ubuntu.com/legal/terms-and-policies/privacy-policy”
VERSION_CODENAME=bionic
UBUNTU_CODENAME=bionic

The /etc directory is a special one in Linux. It contains system-configuration files. You don’t want to delete any files from this directory unless you know what you’re doing.

Question 23

Some Bash commands can only be run by the root user; a system administrator or superuser. If you try one of these commands without sufficient privileges, it fails. For example, only users logged in as a superuser can use cat to display the contents of /etc/at.deny:

Answer 23

cat /etc/at.deny

Question 24

at.deny is a special file that determines who can use other Bash commands to submit jobs for later execution.

You don’t want to run as root most of the time; it’s too dangerous. To run commands that require admin privilege without logging in as a superuser, you’ll preface the commands with sudo:

Answer 24

sudo cat /etc/at.deny

sudo stands for “superuser do.” When you use it, you’re telling the shell that for this one command, you’re acting with the root-user level of permission.

Question 25

cd stands for “change directory,” and it does exactly what the name suggests: it changes the current directory to another directory. It enables you to move from one directory to another just like its counterpart in Windows. The following command changes to a subdirectory of the current directory named orders:

Answer 25

cd orders

Question 26

You can move up a directory by specifying .. as the directory name:

Answer 26

cd ..

Question 27

This command changes to your home directory; the one you land in when you first sign in:

Answer 27

cd ~

Question 28

You can create directories by using the mkdir command. The following command creates a subdirectory named orders in the current working directory:

Answer 28

mkdir orders

Question 29

If you want to create a subdirectory and another subdirectory under it with one command, use the –parents flag:

Answer 29

mkdir –parents orders/2019

Question 30

The rmdir command deletes (removes) a directory, but only if it’s empty. If it’s not empty, you get a warning instead. Fortunately, you can use the rm command to delete directories that aren’t empty in combination with the -r (recursive) flag. The command would then look like so, rm -r.

The rm command is short for “remove.” As you’d expect, rm deletes files. So this command puts an end to 0001.jpg:

Answer 30

rm 0001.jpg

Question 31

And this command deletes all the files in the current directory:

Answer 31

rm *

Question 32

Be wary of rm. It’s a dangerous command.

Running rm with a -i flag lets you think before you delete:

Answer 32

rm -i *

Make it a habit to include -i in every rm command, and you might avoid falling victim to one of Linux’s biggest blunders. The dreaded rm -rf / command deletes every file on an entire drive. It works by recursively deleting all the subdirectories of root and their subdirectories. The -f (for “force”) flag compounds the problem by suppressing prompts. Don’t do this.

Question 33

If you want to delete a subdirectory named orders that isn’t empty, you can use the rm command this way:

Answer 33

rm -r orders

This deletes the orders subdirectory and everything in it, including other subdirectories.

Question 34

The cp command copies not just files, but entire directories (and subdirectories) if you want. To make a copy of 0001.jpg named 0002.jpg, use this command:

Answer 34

cp 0001.jpg 0002.jpg

Question 35

If 0002.jpg already exists, Bash silently replaces it. That’s great if it’s what you intended, but not so wonderful if you didn’t realize you were about to overwrite the old version.

Fortunately, if you use the -i (for “interactive”) flag, Bash warns you before deleting existing files. This is safer:

Answer 35

cp -i 0001.jpg 0002.jpg

Question 36

You can use wildcards to copy several files at once. To copy all the files in the current directory to a subdirectory named photos, do this:

Answer 36

cp * photos

Question 37

To copy all the files in a subdirectory named photos into a subdirectory named images, do this:

Answer 37

cp photos/* images

Question 38

This assumes that the images directory already exists. If it doesn’t, you can create it and copy the contents of the photos directory by using this command:

Answer 38

cp -r photos images

The -r stands for “recursive.” An added benefit of the -r flag is that if photos contains subdirectories of its own, they too are copied to the images directory.

Question 39

The ps command gives you a snapshot of all the currently running processes. By itself, with no arguments, it shows all your shell processes; in other words, not much. But it’s a different story when you include a -e flag:

Answer 39

ps -e

-e lists all running processes, and there are typically many of them.

Question 40

For a more comprehensive look at what processes are running in the system, use the -ef flag:

Answer 40

ps -ef

This flag shows the names of all the running processes, their process identification numbers (PIDs), the PIDs of their parents (PPIDs), and when they began (STIME). It also shows what terminal, if any, they’re attached to (TTY), how much CPU time they’ve racked up (TIME), and their full path names.

Question 41

w command

Answer 41

To find out who’s on your servers, Linux provides the w (for “who”) command. It displays information about the users currently on the computer system and those users’ activities. w shows user names, their IP addresses, when they logged in, what processes they’re currently running, and how much time those processes are consuming. It’s a valuable tool for sysadmins.

Question 42

Bash I/O operators

Answer 42

< for redirecting input to a source other than the keyboard
> for redirecting output to destination other than the screen
» for doing the same, but appending rather than overwriting
| for piping output from one command to the input of another

Question 43

Suppose you want to list everything in the current directory but capture the output in a file named listing.txt. The following command does just that:

Answer 43

ls > listing.txt

Question 44

If listing.txt already exists, it gets overwritten. If you use the&raquo_space; operator instead, the output from ls is appended to what’s already in listing.txt:

Answer 44

ls&raquo_space; listing.txt

Question 45

The piping operator is powerful (and often used). It redirects the output of the first command to the input of the second command. Let’s say you use cat to display the contents of a large file, but the content scrolls by too quickly for you to read. You can make the output more manageable by piping the results to another command such as more. The following command lists all the currently running processes. But once the screen is full, the output pauses until you select Enter to show the next line:

Answer 45

ps -ef | more

Question 46

You can also pipe output to head to see just the first several lines:

Answer 46

ps -ef | head

Question 47

Or suppose you want to filter the output to include only the lines that contain the word “daemon.” One way to do that is by piping the output from ps to Linux’s useful grep tool:

Answer 47

ps -ef | grep daemon

Question 48

You can also use files as input. By default, standard input comes from the keyboard, but it too can be redirected. To get input from a file instead of the keyboard, use the < operator. One common sysadmin task is to sort the contents of a file. As the name suggests, sort sorts text in alphabetical order:

Answer 48

sort < file.txt

Question 49

To save the sorted results to a new file, you can redirect input and output:

Answer 49

sort < file.txt > sorted_file.txt

Question 50

You can use I/O operators to chain Linux commands as needed. Consider the following command:

Answer 50

cat file.txt | fmt | pr | lpr

The output from cat goes to fmt, the output from fmt goes to pr, and so on. fmt formats the results into a tidy paragraph. pr paginates the results. And lpr sends the paginated output to the printer.