bash-guide/README.md

<p align="center">
  <img src="http://i.imgur.com/Vi9HBwR.png"/>
</p>

## Table of Contents
  1. [Basic Operations](#1-basic-operations)  
    1.1. [File Operations](#11-file-operations)  
    1.2. [Text Operations](#12-text-operations)  
    1.3. [Directory Operations](#13-directory-operations)  
    1.4. [SSH, System Info & Network Operations](#14-ssh-system-info--network-operations)  
    1.5. [Process Monitoring Operations (TODO)](#15-process-monitoring-operations)
  2. [Basic Shell Programming](#2-basic-shell-programming)  
    2.1. [Variables](#21-variables)  
    2.3. [String Substitution](#22-string-substitution)  
    2.4. [Functions](#23-functions)  
    2.5. [Conditionals](#24-conditionals)  
    2.6. [Loops](#25-loops)  
  3. [Tricks](#4-tricks)  
  4. [Debugging](#5-debugging)  
  

# 1. Basic Operations

### a. `export`
Displays all environment variables and if you want to get detail of specific variable then use `echo $VARIABLE_NAME`  
```bash
export
```
Example:
```bash
$ export
SHELL=/bin/zsh
AWS_HOME=/Users/adnanadnan/.aws
LANG=en_US.UTF-8
LC_CTYPE=en_US.UTF-8
LESS=-R

$ echo $SHELL
/usr/bin/zsh
```

### b. `whereis`
Whereis search for executables, source files, and manual pages using a database built by system automatically.
```bash
whereis name
```
Example:
```bash
$ whereis php
/usr/bin/php
```

### c. `which`
which search for executables in the directories specified by the environment variable PATH. This command will prints full path of the executable(s).
```bash
which program_name 
```
Example:
```bash
$ which php
/c/xampp/php/php
```

### d. clear
Clears content on window

## 1.1. File Operations
<table>
   <tr>
      <td><a href="#a-ls">ls</a></td>
      <td><a href="#b-touch">touch</a></td>
      <td><a href="#c-cat">cat</a></td>
      <td><a href="#d-more">more</a></td>
      <td><a href="#e-head">head</a></td>
      <td><a href="#f-tail">tail</a></td>
      <td><a href="#g-mv">mv</a></td>
      <td><a href="#h-cp">cp</a></td>
      <td><a href="#i-rm">rm</a></td>
      <td><a href="#j-diff">diff</a></td>
   </tr>
   <tr>
      <td><a href="#k-chmod">chmod</a></td>
      <td><a href="#l-gzip">gzip</a></td>
      <td><a href="#m-gunzip">gunzip</a></td>
      <td><a href="#n-gzcat">gzcat</a></td>
      <td><a href="#o-lpr">lpr</a></td>
      <td><a href="#p-lpq">lpq</a></td>
      <td><a href="#q-lprm">lprm</a></td>
   </tr>
</table>

### a. `ls`
Lists your files. It has a lot of options like `-l` lists files in 'long format', which contains the exact size of the file, who owns the file and who has the right to look at it, and when it was last modified. `-a` lists all files, including hidden files. For more information on this command check this [link](https://ss64.com/bash/ls.html)  
```bash
ls option
```
Example:
<pre>
$ ls -al
rwxr-xr-x   33 adnan  staff    1122 Mar 27 18:44 .
drwxrwxrwx  60 adnan  staff    2040 Mar 21 15:06 ..
-rw-r--r--@  1 adnan  staff   14340 Mar 23 15:05 .DS_Store
-rw-r--r--   1 adnan  staff     157 Mar 25 18:08 .bumpversion.cfg
-rw-r--r--   1 adnan  staff    6515 Mar 25 18:08 .config.ini
-rw-r--r--   1 adnan  staff    5805 Mar 27 18:44 .config.override.ini
drwxr-xr-x  17 adnan  staff     578 Mar 27 23:36 .git
-rwxr-xr-x   1 adnan  staff    2702 Mar 25 18:08 .gitignore
</pre>

### b. `touch`
Creates or updates your file  
```bash
touch filename
```
Example:
```bash
$ touch trick.md
```

### c. `cat`
It can be used for the following purposes under UNIX or Linux  
* Display text files on screen
* Copy text files  
* Combine text files  
* Create new text files  
```bash
cat filename
cat file1 file2 
cat file1 file2 > newcombinedfile
```

### d. `more`
Shows the first part of a file (move with space and type q to quit)  
```bash
more filename
```

### e. `head`
Outputs the first 10 lines of file  
```bash
head filename
```

### f. `tail`
Outputs the last 10 lines of file. Use `-f` to output appended data as the file grows  
```bash
tail filename
```


### g. `mv`
Moves a file from one location to other  
```bash
mv filename1 filename2
```
Where `filename1` is the source path to the file and `filename2` is the destination path to the file.

### h. `cp`
Copies a file from one location to other  
```bash
cp filename1 filename2
```
Where `filename1` is the source path to the file and `filename2` is the destination path to the file.

### i. `rm`
Removes a file. But if you will apply this command on a directory directory, it will gives you an error
`rm: directory: is a directory`
So in order to remove directory you have to pass `-rf` to remove all the content of the directory recursively  
```bash
rm filename
```

### j. `diff`
Compares files, and shows where they differ  
```bash
diff filename1 filename2
```

### k. `chmod`
Lets you change the read, write, and execute permissions on your files  
```bash
chmod -options filename
```

### l. `gzip`
Compresses files  
```bash
gzip filename
```

### m. `gunzip`
Un-compresses files compressed by gzip  
```bash
gunzip filename
```

### n. `gzcat`
Lets you look at gzipped file without actually having to gunzip it  
```bash
gzcat filename
```

### o. `lpr`
Print the file  
```bash
lpr filename
```

### p. `lpq`
Check out the printer queue  
```bash
lpq
```
Example:
```bash
$ lpq
Rank    Owner   Job     File(s)                         Total Size
active  adnanad 59      demo                            399360 bytes
1st     adnanad 60      (stdin)                         0 bytes
```

### q. `lprm`
Remove something from the printer queue  
```bash
lprm jobnumber
```

## 1.2. Text Operations

<table>
   <tr>
      <td><a href="#a-awk">awk</a></td>
      <td><a href="#b-grep">grep</a></td>
      <td><a href="#c-wc">wc</a></td>
      <td><a href="#d-sed">sed</a></td>
      <td><a href="#e-sort">sort</a></td>
      <td><a href="#f-uniq">uniq</a></td>
      <td><a href="#g-cat">cat</a></td>
      <td><a href="#h-cut">cut</a></td>
      <td><a href="#i-echo">echo</a></td>
      <td><a href="#j-fmt">fmt</a></td>
   </tr>
   <tr>
      <td><a href="#k-tr">tr</a></td>
      <td><a href="#l-nl">nl</a></td>
      <td><a href="#m-egrep">egrep</a></td>
      <td><a href="#n-fgrep">fgrep</a></td>
   </tr>
</table>

### a. `awk`
Awk is most usefull command for handling text files. It operates on entire file line by line. By default it uses whitespace to separate the fields. The most common syntax for awk command is

```bash
awk '/search_pattern/ { action_to_take_if_pattern_matches; }' file_to_parse
```

Lets take following file `/etc/passwd`. Here's the sample data that this file contains.
```
root:x:0:0:root:/root:/usr/bin/zsh
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
sync:x:4:65534:sync:/bin:/bin/sync
```
So now lets get only username from this file. Where `-F` specfies that on which base we are going to separate the fields. In our case it's `:`. `{ print $1 }` means print out the first matching field.
```bash
awk -F':' '{ print $1 }' /etc/passwd
```
After running the above command you will get following output.
```
root
daemon
bin
sys
sync
```
For more detail on how to use `awk`, check following [link](https://www.cyberciti.biz/faq/bash-scripting-using-awk).

### b. `grep`
Looks for text inside files. You can use grep to search for lines of text that match one or many regular expressions, and outputs only the matching lines.  
```bash
grep pattern filename
```
Example:
```bash
$ grep admin /etc/passwd
_kadmin_admin:*:218:-2:Kerberos Admin Service:/var/empty:/usr/bin/false
_kadmin_changepw:*:219:-2:Kerberos Change Password Service:/var/empty:/usr/bin/false
_krb_kadmin:*:231:-2:Open Directory Kerberos Admin Service:/var/empty:/usr/bin/false
```
You can also force grep to ignore word case by using `-i` option. Also `-r` can be used to search all files under the specified directory like
```bash
$ grep -r admin /etc/
```
And `-w` to search for words only. For more detail on `grep`, check following [link](https://www.cyberciti.biz/faq/grep-in-bash).

### c. `wc`
Tells you how many lines, words and characters there are in a file  
```bash
wc filename
```
Example:
```bash
$ wc demo.txt
7459   15915  398400 demo.txt
```
Where `7459` is lines, `15915` is words and `398400` is characters.

### d. `sed`
TODO

### e. `sort`
TODO

### f. `uniq`
TODO

### g. `cat`
TODO

### h. `cut`
TODO

### i. `echo`
TODO

### j. `fmt`
TODO

### k. `tr`
TODO

### l. `nl`
TODO

### m. `egrep`
TODO

### n. `fgrep`
TODO

## 1.3. Directory Operations

<table>
   <tr>
      <td><a href="#a-mkdir">mkdir</a></td>
      <td><a href="#b-cd">cd</a></td>
      <td><a href="#c-pwd">pwd</a></td>
   </tr>
</table>

### a. `mkdir`
Makes a new directory  
```bash
mkdir dirname
```

### b. `cd`
Moves you from one directory to other. If you just run  
```bash
$ cd
```
Then it will moves you to home. Also this command accepts an optional `dirname`, which if provided will moves you to that directory.
```bash
cd dirname
```

### c. `pwd`
Tells you in which directory you currently are  
```bash
pwd
```

## 1.4. SSH, System Info & Network Operations

<table>
   <tr>
      <td><a href="#a-mkdir">ssh</a></td>
      <td><a href="#b-cd">whoami</a></td>
      <td><a href="#c-pwd">passwd</a></td>
      <td><a href="#d-quota">quota</a></td>
      <td><a href="#e-date">date</a></td>
      <td><a href="#f-cal">cal</a></td>
      <td><a href="#g-uptime">uptime</a></td>
      <td><a href="#h-w">w</a></td>
      <td><a href="#i-finger">finger</a></td>
      <td><a href="#j-uname">uname</a></td>
   </tr>
   <tr>
      <td><a href="#k-man">man</a></td>
      <td><a href="#l-df">df</a></td>
      <td><a href="#m-du">du</a></td>
      <td><a href="#n-last">last</a></td>
      <td><a href="#o-ps">ps</a></td>
      <td><a href="#p-kill">kill</a></td>
      <td><a href="#q-killall">killall</a></td>
      <td><a href="#r-top">top</a></td>
      <td><a href="#s-bg">bg</a></td>
      <td><a href="#t-fg">fg</a></td>
   </tr>
   <tr>
      <td><a href="#u-ping">ping</a></td>
      <td><a href="#v-whois">whois</a></td>
      <td><a href="#w-dig">dig</a></td>
      <td><a href="#x-wget">wget</a></td>
   </tr>
</table>

### a. `ssh`
ssh (SSH client) is a program for logging into a remote machine and for executing commands on a remote machine.  
```bash
ssh user@host
```
This command also accepts an option `-p` that can to used to connect to specific port.  
```bash
ssh -p port user@host
```

### b. `whoami`
Return current logged in username

### c. `passwd`
Allows the current logged user to change his password

### d. `quota`
Shows what your disk quota is  
```bash
quota -v
```

### e. `date`
Shows the current date and time

### f. `cal`
Shows the month's calendar

### g. `uptime`
Shows current uptime

### h. `w`
Displays who is online

### i. `finger`
Displays information about user  
```bash
finger username
```

### j. `uname`
Shows kernel information  
```bash
uname -a
```

### k. `man`
Shows the manual for specified command  
```bash
man command
```

### l. `df`
Shows disk usage

### m. `du`
Shows the disk usage of the files and directories in filename (du -s give only a total)  
```bash
du filename
```

### n. `last`
Lists your last logins of specified user  
```bash
last yourUsername
```

### o. `ps`
Lists your processes  
```bash
ps -u yourusername
```

### p. `kill`
Kills (ends) the processes with the ID you gave  
```bash
kill PID
```

### q. `killall`
Kill all processes with the name  
```bash
killall processname
```

### r. `top`
Displays your currently active processes

### s. `bg`
Lists stopped or background jobs ; resume a stopped job in the background

### t. `fg`
Brings the most recent job in the foreground.

### u. `ping`
Pings host and outputs results  
```bash
ping host
```

### v. `whois`
Gets whois information for domain  
```bash
whois domain
```

### w. `dig`
Gets DNS information for domain  
```bash
dig domain
```

### x. `wget`
Downloads file  
```bash
wget file
```


# 2. Basic Shell Programming


This is first line that you will in bash script files called `shebang`. The shebang line in any script determines the script's ability to be executed like an standalone executable without typing sh, bash, python, php etc beforehand in the terminal.

```bash
#!/bin/bash
```

## 2.1. Variables

Creating variable in bash is similar to other language. There are no data types. A variable in bash can contain a number, a character, a string of characters. You have no need to declare a variable, just assigning a value to its reference will create it.

Example:
```bash
str="hello world"
```

The above line creates a variable `str` and assigns "hello world" to it. Then the value of variable is retrieved by putting the `$` in the beginning of variable name.

Example:
```bash
echo $str   # hello world
```

Also like other languages bash has also arrays. An array is variable containing multiple values. There's no maximum limit on the size of array. Array in bash are zero based. The first element is indexed with element 0. There are several ways for creating arrays in bash. Which are given below.

Examples:
```bash
array[0] = val
array[1] = val
array[2] = val
array=([2]=val [0]=val [1]=val)
array(val val val)
```
To display a value at specific index use following syntax

```bash
${array[i]}     # where i is the index
```

One thing to note that if no index is supplied, array element 0 is assumed. To find out how many values there are in the array check following syntax

```bash
${#array[@]}
```

Bash has also support for the ternary conditions. Check some examples below.

```bash
${varname:-word}    # if varname exists and isn't null, return its value; otherwise return word
${varname:=word}    # if varname exists and isn't null, return its value; otherwise set it word and then return its value
${varname:+word}    # if varname exists and isn't null, return word; otherwise return null
${varname:offset:length}    # performs substring expansion. It returns the substring of $varname starting at offset and up to length characters
```

## 2.2 String Substitution

Check some of the syntax on how to manipulate strings

```bash
${variable#pattern}         # if the pattern matches the beginning of the variable's value, delete the shortest part that matches and return the rest
${variable##pattern}        # if the pattern matches the beginning of the variable's value, delete the longest part that matches and return the rest
${variable%pattern}         # if the pattern matches the end of the variable's value, delete the shortest part that matches and return the rest
${variable%%pattern}        # if the pattern matches the end of the variable's value, delete the longest part that matches and return the rest
${variable/pattern/string}  # the longest match to pattern in variable is replaced by string. Only the first match is replaced
${variable//pattern/string} # the longest match to pattern in variable is replaced by string. All matches are replaced
${#varname}     # returns the length of the value of the variable as a character string
```

## 2.3. Functions
As in almost any programming language, you can use functions to group pieces of code in a more logical way or practice the divine art of recursion. Declaring a function is just a matter of writing function my_func { my_code }. Calling a function is just like calling another program, you just write its name.

```bash
functname() {
    shell commands
}
```

Example:
```bash
#!/bin/bash
function hello {
   echo world!
}
hello

function say {
    echo $1
}
say "hello world!"
```

When you will run above example the `hello` function will output "world!". The above two functions `hello` and `say` are identical. The main difference is function `say`. This function, prints the first argument it receives. Arguments, within funtions, are treated in the same manner as arguments given to the script.

## 2.4. Conditionals

The conditional statement in bash is similar to other programming languages. Conditions have many form like the most basic form is `if` expression `then` statement where statement is only executed if expression is true.

```bash
if [expression]; then
    will execute only if expression is true
else
    will execute if expression is false
fi
```

Sometime if conditions becoming confusing so you can write the same condition using the `case statements`.

```bash
case expression in
    pattern1 )
        statements ;;
    pattern2 )
        statements ;;
    ...
esac
```

Expression Examples:

```bash
statement1 && statement2  # both statements are true
statement1 || statement2  # one of the statement is true

str1=str2       # str1 matches str2
str1!=str2      # str1 does not match str2
str1<str2       # str1 is less than str2
str1>str2       # str1 is greater than str2
-n str1         # str1 is not null (has length greater than 0)
-z str1         # str1 is null (has length 0)

-a file         # file exists
-d file         # file exists and is a directory
-e file         # file exists; same -a
-f file         # file exists and is a regular file (i.e., not a directory or other special type of file)
-r file         # you have read permission
-r file         # file exists and is not empty
-w file         # your have write permission
-x file         # you have execute permission on file, or directory search permission if it is a directory
-N file         # file was modified since it was last read
-O file         # you own file
-G file         # file's group ID matches yours (or one of yours, if you are in multiple groups)

file1 -nt file2     # file1 is newer than file2
file1 -ot file2     # file1 is older than file2

-lt     # less than
-le     # less than or equal
-eq     # equal
-ge     # greater than or equal
-gt     # greater than
-ne     # not equal
```

## 2.5. Loops

There are three types of loops in bash. `for`, `while` and `until`.

Different `for` Syntax:
```bash
for x := 1 to 10 do
begin
  statements
end

for name [in list]
do
  statements that can use $name
done

for (( initialisation ; ending condition ; update ))
do
  statements...
done
```

`while` Syntax:
```bash
while condition; do
  statements
done
```

`until` Syntax:
```bash
until condition; do
  statements
done
```

# 3. Tricks

## set an alias
Open `bash_profile` by running following command `nano ~/.bash_profile`
> alias dockerlogin='ssh www-data@adnan.local -p2222'  # add your alias in .bash_profile

## to quickly go to a specific directory
nano ~/.bashrc
> export hotellogs="/workspace/hotel-api/storage/logs"

source ~/.bashrc
cd hotellogs


# 4. Debugging
You can easily debug the bash script by passing different options to `bash` command. For example `-n` will not run commands and check for syntax errors only. `-v` echo commands before running them. `-x` echo commands after command-line processing.

```bash
bash -n scriptname
bash -v scriptname
bash -x scriptname
```

## Contribution

- Report issues
- Open pull request with improvements
- Spread the word

## License

[![License: CC BY 4.0](https://img.shields.io/badge/License-CC%20BY%204.0-lightgrey.svg)](https://creativecommons.org/licenses/by/4.0/)