Day 14 of my ALX Software Engineering Journey: Shell Init files, variables and expansions

Today I grasped the concept of initialization files, variables and expansions. Things are getting heated in the room but we’ve been reminded to keep on doing hard things!!

Expansion

Is a measure of the ability to express a command in a different form till the shell program runs it.

For example echo ls *

root@LAPTOP-8DCGIHH5:/alx# ls *
alx-low_level_programming:
0x00-hello_world  README.md

alx-system_engineering-devops:
0x00-shell_basics  0x01-shell_permissions  0x02-shell_redirections  0x03-shell_variables_expansions  README.md
root@LAPTOP-8DCGIHH5:/alx#

Above, the shell expands the * into the name of the files in the current directory after the enter key is pressed. The * was not read as * but the expanded version was read which expands into the entire file in the directory.

Types of Expansions

• Pathname Expansions

A good example of a pathname expansion is the * wild card symbol. It is used in pathnames to modify a command. For example

ls usr/bin/ *.h

The command above lists only the files that end with .h in the directory usr and subdirectory bin.

root@LAPTOP-8DCGIHH5:/# ls ./alx/alx*
./alx/alx-low_level_programming:
0x00-hello_world  README.md

./alx/alx-system_engineering-devops:
0x00-shell_basics  0x01-shell_permissions  0x02-shell_redirections  0x03-shell_variables_expansions  README.md

The command above lists all the files that are in the subdirectory of the alx folder and begin with alx. The * symbol expands into files or folders that begin with alx

• Tilde Expansion (~)

Expands into the name of the user in the home directory. In the absence of a named user, it’s expanded into the directory of the current user.

root@LAPTOP-8DCGIHH5:~# echo ~
/root

• Arithmetic Expansion

This type of expansion allows arithmetic operations to be performed with expansions.

The format for arithmetic expansions : $((expression))

E.g echo $((2+2))

root@LAPTOP-8DCGIHH5:/# echo $((2+2))
4

e.g echo $((((6**3))*3))

root@LAPTOP-8DCGIHH5:/# echo $((((6**3))*3))
648

The above arithmetic expansion raised 6 to the power of 3 and further expands by multiplying the result by 3.

• Brace Expansion

This expansion is used to create multiple text strings from a pattern containing braces.

Patterns that are to be expanded using braces contain a preamble and trailing portion called a postscript. It can also contain a comma to separate integers or single characters.

For example

touch Dillibe{1..3}.txt

root@LAPTOP-8DCGIHH5:/# touch Dillibe{1..3}.txt
root@LAPTOP-8DCGIHH5:/# ls
Betty         Dillibe2.txt  alx  boot  etc   file.txt      home  lib    lib64   life   mnt  proc  run   snap  sys   tmp  var
Dillibe1.txt  Dillibe3.txt  bin  dev   file  filelist.txt  init  lib32  libx32  media  opt  root  sbin  srv   test  usr

Dillibe is the preamble .txt is the postscript.

• Parameter expansion

Is a method for transforming and manipulating the content of variables using the ${...} syntax.

E.g echo $USER

root@LAPTOP-8DCGIHH5:/# echo $USER
root

The command above when typed, displays the name of the current user.

• Command substitution

Allows one to use the output of a command as an expansion.

e.g echo $()

Quoting

Is the process by which the shell suppresses an unwanted expansion

E.g echo my money is $100

root@LAPTOP-8DCGIHH5:~# echo my money is $100
my money is 00

Here's the result using quotes

root@LAPTOP-8DCGIHH5:~# echo my money is '$100'
my money is $100

Double Quotes

When a text is placed in a double quote it loses its special meaning and is treated as an ordinary character.

root@LAPTOP-8DCGIHH5:~# echo my money is "100$"
my money is 100$

A double quotation is important when naming a file and you want to add a space in between. Double quotes will suppress the changes and make the text appear as one.

Single quotes

are used to suppress all expansions as illustrated above.

Variables

Bash stores 2 kinds of variables namely; Global variable and Local variable

Global variables are environmental variables available in all shells.

E.g env printenv displays the environment

Local variable: is only available in the local Shell environment. It uses a set of built-in commands without any options, displays a list of all variables excluding environment variables and functions.

Variables can also be classified based on content:

1. String variables

2. Integer variables

3. Constant variable

4. Array variables

Creating variables

In the shell, variables are usually created using lowercase as a convention.

To set variable VARNAME = “value”

Exporting variables

To export local variables, the “export” built-in command is used.

Variables exported are environment variables.

E.g export VARNAME = “value”

Shell initialization files

Are system or user configuration files. They are stored in the etc directory.

e.g /etc/profile