5.1 Given a scenario, deploy and execute basic BASH scripts

Question 1

What is the PATH variable used for?

Answer 1

Scripts should be located in one of the directories defined by the $PATH variable

Question 2

export

Answer 2

The export command can also be used to display all environment variables:

To convert an existing local variable to an environment variable, use the export command:

[student@OCS ~]$ echo $NAME
Sarah
[student@OCS ~]$ export NAME

Question 3

env

Answer 3

The env command displays environment variables in the current shell. Local variables are not displayed when the env command is executed

Question 4

set

Answer 4

The set command displays all shell variables and values when executed with no arguments. The output also includes any functions that have been declared within the shell.

The set command can also be used to modify the behavior of the shell

Question 5

printenv

Answer 5

Like the env command, the printenv command is used to display environment variables; however, the env command has additional features, whereas the printenv command is used solely to display environment variables

Question 6

echo

Answer 6

The echo command is used to display information. Typically, it is used to display the value of variables

Question 7

!/bin/bash

Answer 7

The first line of a script should include the path to the interpreter. For BASH shell scripts, this should be the path to the executable bash command. This path can be discovered by executing the following command:

[root@OCS ~]$ which bash
/bin/bash

Question 8

!/bin/bash

Answer 8

The first line of a script should include the path to the interpreter. For BASH shell scripts, this should be the path to the executable bash command. This path can be discovered by executing the following command:

[root@OCS ~]$ which bash
/bin/bash

Question 9

What is meant by sourcing scripts?

Answer 9

Most commands are executed as separate processes that have their own environment. The source command allows you to execute a bash script as if the commands within the script were executed directly on the command line

source ./functions.sh

Use the . command to perform the same function as the source command, as shown next:

[root@OCS ~]$ . ./functions.sh

Question 10

File Blobbing

Answer 10

A file glob (also called a wildcard) is any character provided on the command line that represents a portion of a filename. The following globs are supported:

Glob

Description

*

Matches zero or more characters in a filename.

?

Matches any single character in a filename.

[ ]

Matches a single character in a filename as long as that character is represented within the [ ] characters.

Question 11

File Blobbing

Answer 11

A file glob (also called a wildcard) is any character provided on the command line that represents a portion of a filename. The following globs are supported:

Glob

Description

*

Matches zero or more characters in a filename.

?

Matches any single character in a filename.

[ ]

Matches a single character in a filename as long as that character is represented within the [ ] characters.

Question 12

How would you copy all files that end in .txt

Answer 12

[student@OCS ~]$ cp *.txt ~

Question 13

What command would you use to remove all files that are four characters long?

Answer 13

[student@OCS ~]$ rm ????

Question 14

What command would you use to view the file type for all files in the current directory that begin with a, b, or c

Answer 14

[student@OCS ~]$ file [abc]*

or

[student@OCS ~]$ file [a-c]*

Question 15

What does this shell expansion do: ${}

Answer 15

With the variable expansion BASH feature, you can return a different value. For example, the following will return “Bob” if the $name variable isn’t set:

$ echo “Hello, ${name:-Bob}”
Hello, Bob$
echo “Hello, $name”
Hello,

Question 16

What does this shell expansion do: $() or cmd

Answer 16

Command substitution is the process of executing a subcommand within a larger command. This is typically used to gather data and store it in a variable. For example, the following command stores the output of the date command in the $today variable:

today=$(date)
Command substitution can be performed by using one of two methods:

$(cmd)

‘cmd’

Question 17

What are exit codes?

Answer 17

When a command executes, it returns a value of success (0) or failure (>1) to the shell or script from which it was executed. This status is stored in the $? variable

Question 18

What are meta characters?

Answer 18

Metacharacters are special characters to the BASH shell. For example, file globbing characters (*, [ ], and ?) are considered metacharacters

Question 19

Positional Parameters

Answer 19

Your script can be run with arguments, like so:

./example.sh arg1 arg2 arg3
Each of the arguments is stored in $1, $2, $3, and so forth. $0 is the name of the script itself.

The special BASH variable $# contains the number of arguments passed to the script

Question 20

Escaping Characters

Answer 20

To escape a character is to have a special character (aka, a metacharacter) treated as a plain character. For example, the following disables the special meaning of the redirection character, using double quotes:

$ echo “this | that”
this | that

Here are some escaping character essentials:

Double quotes disable the special meaning of redirection characters (, and |) and globbing characters (*, ?, [, and ]).

Use single quotes to disable all metacharacters, including redirection, globbing, command substitution (the ‘ ‘ and $() characters), and variable substitution (the $ character).

Place a \ character (backslash character) before any metacharacter to make it a plain character for the BASH shell.