In This Guide...

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.

$ history
    1  ls
    2  echo "Testing123"
    3  ls
    4  history

We can then execute them with !<command_number>.

$ !2
echo Testing123
Testing123

How to Clear Bash History

Sometimes history may get messy, and mine had 500+ previous commands, before making this tutorial.

Syntax

$ history -c; history -w;

.bashrc

.bashrc is a Bash shell script that Bash runs whenever it is started interactively.

Hence when .bashrc looks like this...

~/.bashrc
echo "Testing123"

Then we reload the terminal with bash we get this...

$ [email protected]:~$ bash
Testing123

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
PS1='[\033[1;31m]\u[\033[m]@[\033[1;32m]\h[\033[m]:[\033[1;36m]\w[\033[m]$[\033[m] '

Shorten Commands - Bash Aliases

alias <name>="<command>"

Example

~/.bashrc
alias testing123="echo 'It Worked!'"

$ bash # or manually reopen the terminal
$ testing123
It Worked!

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
if [ -f $HOME/.bash_aliases ]
then
  . $HOME/.bash_aliases
fi

Wildcards

Wildcard characters are used to define patterns for searching text on data in bash.

Grep - Match Patterns in Strings

The code below will find the line containing '-e ', and output it.

$ man grep | grep -e '-e '

Anything - Star or Asterisk (*)

Used to search for any character, zero or more times.

$ find *
1.txt
2.txt
3.txt
a.png
b.png
c.png
error.log
log.log
log.txt
20-04-21-213100.log

$ find *.png

$ find log.*
log.log
log.txt

Unknown - Question Mark (?)

Used to search for a fixed number of characters. '?' can indicate an unknown character.

$ find ?.txt
1.txt
2.txt
3.txt

$ find *.?x?
1.txt
2.txt
3.txt
log.txt

$ find 20-04-21-21????.log
20-04-21-213100.log

Range - Square Brackets ([])

Used to search for a range of characters (i.e. 0-9, a-z).

$ find [0-9].txt
1.txt
2.txt
3.txt

$ find [a-z].png
a.png
b.png
c.png

$ find [23ac].???
2.txt
3.txt
a.png
c.png

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

$ echo "Hello Earth"
Hello Earth

[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

$ echo "Hello Earth" 0>> echo
$ Hello Earth

This command puts the output of the first command into the second command.

$ echo "Hello Earth" 1>> output.txt
$ cat output.txt
Hello Earth

This puts the output of the first command into a file (output.txt).

$ cat '' 2>> error.txt
$ cat error.txt
cat: '': No such file or directory

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.

A ; B     # Run A and then B, regardless of success of A
A && B    # Run B if A succeeded
A || B    # Run B if A failed
A &       # Run A in background.

Brace Expansion

Brace expansion is used to generate strings.

$ echo {a..z}
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
$ echo {z..w}
z y x w
$ echo {1..10}
1 2 3 4 5 6 7 8 9 10
$ echo {3..-1}
3 2 1 0 -1

$ echo a{a..f}
aa ab ac ad ae af
$ echo {1..3} && echo {99,100}
1 2 3 99 100

$ ls {Desktop,Documents}
Desktop:
'Visual Studio Code.lnk'*   desktop.ini
​
Documents:
'My Music'@  'My Pictures'@  'My Videos'@   desktop.ini

Nested Brace Expansion

$ echo {a,{a..c},{a..c}}
a a b c a b c

Multiple Brace Expansion

Brace yourself for a basic maths lesson.

$ echo {1..3} # 3*1 = 3 combinations
1 2 3
$ echo {1..3}{1..3} # 3*3 = 9 combinations
11 12 13 21 22 23 31 32 33
$ echo {1..3}{1..3}{1..3} # 3*3*3 = 27 combinations
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

​Challenge - Make a Today Command​