Bash - Do More With Less

In This Guide...
After this guide you will:
Be more efficient with the bash shell.
Know how to execute previous bash commands.
Know how to clear bash command history.
Know how to shortern commands with aliases.
Know how to customize the terminal colours.
Know how to create patterns to match file and folder names.
Be able to create bash pipelines to do more with less.
Know how to execute multiple commands with one command.
Reinforce your knowledge with a quick challenge.
Prerequisites: know the basics of bash.
This content is from the Complete Raspberry Pi 24/7 Hosting Course.
Time Travel - Bash History
Here you will learn how to go back in time and execute previous commands. This is how....
The history command lets us see previously executed commands.
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$ history
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    1  ls
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    2  echo "Testing123"
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    3  ls
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    4  history
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We can then execute them with !<command_number>.
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$ !2
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echo Testing123
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Testing123
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How to Clear Bash History
Sometimes history may get messy, and mine had 500+ previous commands, before making this tutorial.
Syntax
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$ history -c; history -w;
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.bash_history stores bash history.
.bashrc
.bashrc is a Bash shell script that Bash runs whenever it is started interactively.
Hence when .bashrc looks like this...
~/.bashrc
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echo "Testing123"
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Then we reload the terminal with bash we get this...
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$ [email protected]:~$ bash
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Testing123
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Every time the bash terminal is loaded, .bashrc is executed.
Customize the Terminal - Command Prefix
.bashrc can be used for many things including customizing the bash command prefix.
This is how I did it:
~/.bashrc
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PS1='[\033[1;31m]\u[\033[m]@[\033[1;32m]\h[\033[m]:[\033[1;36m]\w[\033[m]$[\033[m] '
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Now the prefix has colours!
Shorten Commands - Bash Aliases
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alias <name>="<command>"
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Example
~/.bashrc
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alias testing123="echo 'It Worked!'"
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$ bash # or manually reopen the terminal
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$ testing123
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It Worked!
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Use .bash_aliases for aliases
In the default folder (~), we can create special file called .bash_aliases. This is a conventional file to contain bash aliases. 
This is optional, but the code in .bashrc below should load the aliases in the file.
~/.bashrc
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if [ -f $HOME/.bash_aliases ]
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then
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  . $HOME/.bash_aliases
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fi
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Wildcards
Wildcard characters are used to define patterns for searching text on data in bash.
The find command is used for finding files or folders within a folder.
Type man find for more info.
Grep - Match Patterns in Strings
The code below will find the line containing '-e ', and output it.
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$ man grep | grep -e '-e '
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Anything - Star or Asterisk (*)
Used to search for any character, zero or more times.
Example 1
Example 2
Example 3
This will find everything in the current directory.
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$ find *
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1.txt
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2.txt
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3.txt
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a.png
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b.png
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c.png
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error.log
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log.log
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log.txt
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20-04-21-213100.log
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This is also useful if you want to find anything that matches a file extension.
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$ find *.png
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This is used to find a file with a specific name and any file extension.
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$ find log.*
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log.log
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log.txt
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Unknown - Question Mark (?)
Used to search for a fixed number of characters. '?' can indicate an unknown character.
Example 1
Example 2
Example 3
This will find any .txt file with a 1 character name.
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$ find ?.txt
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1.txt
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2.txt
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3.txt
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This will find any file name, with a 3 letter file extension, with 'x' in the middle.
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$ find *.?x?
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1.txt
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2.txt
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3.txt
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log.txt
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In this example, this operator can be useful for finding log files at a specific hour.
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$ find 20-04-21-21????.log
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20-04-21-213100.log
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Range - Square Brackets ([])
Used to search for a range of characters (i.e. 0-9, a-z).
Ranges are case sensitive. Therefore, [a-z] will not match 'Z', but [A-Z] will.
Example 1
Example 2
Example 3
This is useful for finding files that start with any number from 0-9. This is called a range.
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$ find [0-9].txt
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1.txt
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2.txt
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3.txt
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This also works with the alphabet!
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$ find [a-z].png
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a.png
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b.png
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c.png
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The square bracket can combine characters to make a set. The character can only be any character within the set. 
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$ find [23ac].???
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2.txt
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3.txt
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a.png
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c.png
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Piping
What is a pipeline?
In computer science, a pipeline is a series of elements, where each one is connected to the next.
The Command Pipeline
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$ echo "Hello Earth"
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Hello Earth
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[0] Standard Input - typed command and its arguments; stdin.
[1] Standard Output - normal output from a command; stdout.
[2] Standard Error - error output from a command; stderr.
We Can Control Command Output
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$ echo "Hello Earth" 0>> echo
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$ Hello Earth
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This command puts the output of the first command into the second command.
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$ echo "Hello Earth" 1>> output.txt
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$ cat output.txt
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Hello Earth
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This puts the output of the first command into a file (output.txt).
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$ cat '' 2>> error.txt
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$ cat error.txt
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cat: '': No such file or directory
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This puts the error from the cat command, into a file (error.txt).
Logical Operators Can Help Too
Logical operators are used for command pipelines.
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A ; B     # Run A and then B, regardless of success of A
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A && B    # Run B if A succeeded
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A || B    # Run B if A failed
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A &       # Run A in background.
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More on Logical Operators
Brace Expansion
Brace expansion is used to generate strings.
Combinations
Prefix/Suffix
List Command Example
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$ echo {a..z}
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a b c d e f g h i j k l m n o p q r s t u v w x y z
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$ echo {z..w}
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z y x w
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$ echo {1..10}
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1 2 3 4 5 6 7 8 9 10
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$ echo {3..-1}
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3 2 1 0 -1
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$ echo a{a..f}
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aa ab ac ad ae af
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$ echo {1..3} && echo {99,100}
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1 2 3 99 100
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$ ls {Desktop,Documents}
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Desktop:
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'Visual Studio Code.lnk'*   desktop.ini
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​
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Documents:
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'My Music'@  'My Pictures'@  'My Videos'@   desktop.ini
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Nested Brace Expansion
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$ echo {a,{a..c},{a..c}}
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a a b c a b c
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Make sure there are no spaces around commas, otherwise brace expansion will not work.
Multiple Brace Expansion
Brace yourself for a basic maths lesson.
32=3∗3=93^2=3*3=932=3∗3=9
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$ echo {1..3} # 3*1 = 3 combinations
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1 2 3
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$ echo {1..3}{1..3} # 3*3 = 9 combinations
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11 12 13 21 22 23 31 32 33
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$ echo {1..3}{1..3}{1..3} # 3*3*3 = 27 combinations
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111 112 113 121 122 123 131 132 133 211 212 213 221 222 223 231 232 233 311 312 313 321 322 323 331 332 333
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​Challenge - Make a Today Command​
Doing is the most effective way of learning, and will help reinforce your knowledge and understanding.
This content is from an upcoming Raspberry Pi hosting course.