bash-guide/README.md

<p align="center">
  <img src="https://cloud.githubusercontent.com/assets/2059754/24601246/753a7f36-1858-11e7-9d6b-7a0e64fb27f7.png" alt="bash logo"/>
</p>

## İçindekiler
  1. [Basit Komutlar](#1-basic-operations)  
    1.1. [Dosya Komutlar](#11-file-operations)  
    1.2. [Text Komutları](#12-text-operations)  
    1.3. [Dizin Komutları](#13-directory-operations)  
    1.4. [SSH, Sistem Bilgisi & Ağ Komutları(#14-ssh-system-info--network-operations)  
    1.5. [İşlem Gösterme Komutları](#15-process-monitoring-operations)
  2. [Basic Shell Programming](#2-basic-shell-programming)  
    2.1. [Değişkenler](#21-variables)  
    2.2  [Diziler](#22-array)  
    2.3. [String Yerleştirme](#23-string-substitution)  
    2.4. [Fonksiyonlar](#24-functions)  
    2.5. [Koşullar(if)](#25-conditionals)  
    2.6. [Döngüler](#26-loops)  
  3. [İpuçları](#3-tricks)  
  4. [Debugging](#4-debugging)  
  

# 1. Basit Komutlar

### a. `export`
Tüm çevre değişkenlerini göstermek için kullanılır. Özel bir çevre değişkeni özelliğini görüntülemek için, `echo $VARIABLE_NAME` kullanın.
```bash
export
```
Örnek:
```bash
$ export
AWS_HOME=/Users/omergulen/.aws
LANG=en_US.UTF-8
LC_CTYPE=en_US.UTF-8
LESS=-R

$ echo $AWS_HOME
/Users/omergulen/.aws
```

### b. `whatis`
whatis kullanıcı komutları, sistem çağrıları, kütüphane fonksiyonları ve manuel sayfasındaki diğer şeylerin açıklamasını görüntüler.
```bash
whatis bir_sey
```
Örnek:
```bash
$ whatis bash
bash (1)             - GNU Bourne-Again SHell
```

### c. `whereis`
whereis çalıştırabilir dosyaları, kaynak kodlarını ve manuel sayfalarını sistem tarafından otomatik oluşturulan bir veri tabanı ile araştırır.
```bash
whereis isim
```
Örnek:
```bash
$ whereis php
/usr/bin/php
```

### d. `which`
which çalıştırabilir dosyaları belirtilmiş bir PATH (yol) içerisinde arar. Bu komut aranan çalıştırabilirlerin tam yolunu yazdırır.
```bash
which program_adi 
```
Örnek:
```bash
$ which php
/c/xampp/php/php
```

### e. clear
Pencere içeriğini temizler.

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

### a. `cat`
UNIX veya Linux altında, aşağıdaki amaçlar için kullanılır.  
* Text dosyalarını ekranda gösterir
* Text dosyalarını kopyalar 
* Text dosyalarını birleştirir 
* Yeni text dosyaları oluşturur

Örnek:
```bash
cat dosya_adi
cat dosya1 dosya2 
cat dosya1 dosya2 > yeni_birlesmis_dosya
cat < dosya1 > dosya2 #dosya 1'i dosya2'ye kopyalar.
```

### b. `chmod`
```chmod``` komutu "change mode" yani mod değiştir anlamına karşılık gelmektedir ve dosyanın veya dizinin okunabilir, yazılabilir ve çalıştırabilir olmasını değiştirebilir. Daha fazla bilgi için bu linki kontrol edin [link](https://ss64.com/bash/chmod.html).
```bash
chmod -secenekler dosya_adi
chmod +x -w calistir_ama_yazama
```

### c. `chown`
chown komutu "change owner" yani sahibini değiştir anlamına karşılık gelmektedir ve verilen dosya veya dizinin sahibini kullanıcı ve grup olarak değiştirebilir. Basit kullanımı önce kullanıcı:grup adı gelir ve sonrasında dosya veya dizin adı verilir.
```bash
chown -secenekler kullanici:grop dosya_adi
```

### d. `cp`
Dosyaları kopyalar.  
```bash
cp dosya1 dosya2
cp /root/Desktop/file1.txt /var/www/html/file2.php
```
`dosya1` kaynak yolunu `dosya2` ise hedef yolunu belirtir.

### e. `diff`
Dosyaları karşılaştırıp, farklılıklarını gösterir. 
```bash
diff dosya1 dosya2
```

### f. `file`
Dosya türünü belirler.  
```bash
file dosya
```
Örnek:
```bash
$ file index.html
 index.html: HTML document, ASCII text
```
### g. `find`
Dizinde dosya arar.
```bash
find dizin secenek duzen(yapi)
```
Example:
```bash
$ find . -name README.md
$ find /home/ -name '*.png'
```

### h. `gunzip`
Dosyaları ```gzip``` ile çıkartır.  
```bash
gunzip filename
```

### i. `gzcat`
Çıkarmadan gzip ile sıkıştırılmış dosyalara bakmamızı sağlar.  
```bash
gzcat dosya
```

### j. `gzip`
Dosyaları sıkıştırır. 
```bash
gzip dosya
```

### k. `head`
Dosyanın ilk 10 satırını yazdırır. 
```bash
head dosya
```

### l. `lpq`
Yazıcı baskı sırasını yazdırır.  
```bash
lpq
```
Örnek:
```bash
$ lpq
Sıra    Sahip       İş      Dosya(lar)                      Toplam Boyut
active  omergulen   59      deneme                          399360 bytes
1st     omergulen   60      (stdin)                         0 bytes
```

### m. `lpr`
Dosyayı yazıcıdan yazdırır.  
```bash
lpr dosya
```

### n. `lprm`
Yazıcı baskı sırasından bir iş siler.  
```bash
lprm is_numarasi
```

### o. `ls`
Dosyaları listelemek için kullanılır. Hiç bir yol veya dizin göstermezseniz bulunduğunuz dizinde çalışır. ```-l``` gibi listeleyip daha çok bilgi veren ve ```-a``` gibi tüm dosyaları (gizliler dahil) gösteren parametreleri vardır. Daha detaylı bilgi için [link](https://ss64.com/bash/ls.html).  
```bash
ls secenek
```
Örnek:
<pre>
$ ls -la
rwxr-xr-x   33 omergulen  staff    1122 Mar 27 18:44 .
drwxrwxrwx  60 omergulen  staff    2040 Mar 21 15:06 ..
-rw-r--r--@  1 omergulen  staff   14340 Mar 23 15:05 .DS_Store
-rw-r--r--   1 omergulen  staff     157 Mar 25 18:08 .bumpversion.cfg
-rw-r--r--   1 omergulen  staff    6515 Mar 25 18:08 .config.ini
-rw-r--r--   1 omergulen  staff    5805 Mar 27 18:44 .config.override.ini
drwxr-xr-x  17 omergulen  staff     578 Mar 27 23:36 .git
-rwxr-xr-x   1 omergulen  staff    2702 Mar 25 18:08 .gitignore
</pre>

### p. `more`
Dosyanın ilk kısmını gösterir (```Space``` ile hareket eder ve ```q``` ile çıkılır).  
```bash
more dosya
```

### q. `mv`
Dosyayı taşır.  
```bash
mv dosya1 dosya2
```
`dosya1` kaynak yolunu `dosya2` ise hedef yolunu belirtir.

Ayrıca dosyanın ismini değiştirmek için kullanır.
```bash
mv eski_isim yeni_isim
```

### r. `rm`
Dosyaları silmeye yarar. Bunu bir dizin üzerinde kullanmaya çalışmak hataya yol açar.
`rm: directory: is a directory`
To remove a directory you have to pass `-r` which will remove the content of the directory recursively. Optionally you can use `-f` flag to force the deletion i.e. without any confirmations etc.
Dizin silmek için `-r` kullanmalısınız. Silinmeyen dosyaları zorla silmek için de `-f` komutu kullanılmaktadır.
```bash
rm dosya
```

### s. `tail`
Outputs the last 10 lines of file. Use `-f` to output appended data as the file grows.  
Dosyanın son 10 satırını yazdırır. `-f` parametresiyle çıktıyı sona ekler dosya büyüdükçe.
```bash
tail dosya
```

### t. `touch`
Yetki ve zaman etiketini günceller. Böylelikle dosya var olmamışsa yaratır.
```bash
touch dosya
```
Örnek:
```bash
$ touch trick.md
```

## 1.2. Text Operations

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

### a. `awk`
awk is the most useful command for handling text files. It operates on an 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` specifies 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. `cut`
Remove sections from each line of files

*example.txt*
```bash
red riding hood went to the park to play
```

*show me columns 2 , 7 , and 9 with a space as a separator*
```bash
cut -d " " -f2,7,9 example.txt
```
```bash
riding park play
```

### c. `echo`
Display a line of text

*display "Hello World"*
```bash
echo Hello World
```
```bash
Hello World
```

*display "Hello World" with newlines between words*
```bash
echo -ne "Hello\nWorld\n"
```
```bash
Hello
World
```

### d. `egrep`
Print lines matching a pattern - Extended Expression (alias for: 'grep -E')

*example.txt*
```bash
Lorem ipsum
dolor sit amet, 
consetetur
sadipscing elitr,
sed diam nonumy
eirmod tempor
invidunt ut labore
et dolore magna
aliquyam erat, sed
diam voluptua. At
vero eos et
accusam et justo
duo dolores et ea
rebum. Stet clita
kasd gubergren,
no sea takimata
sanctus est Lorem
ipsum dolor sit
amet.
```

*display lines that have either "Lorem" or "dolor" in them.*
```bash
egrep '(Lorem|dolor)' example.txt
or
grep -E '(Lorem|dolor)' example.txt
```
```bash
Lorem ipsum
dolor sit amet,
et dolore magna
duo dolores et ea
sanctus est Lorem
ipsum dolor sit
```

### e. `fgrep`
Print lines matching a pattern - FIXED pattern matching  (alias for: 'grep -F')

*example.txt*
```bash
Lorem ipsum
dolor sit amet,
consetetur
sadipscing elitr,
sed diam nonumy
eirmod tempor
foo (Lorem|dolor) 
invidunt ut labore
et dolore magna
aliquyam erat, sed
diam voluptua. At
vero eos et
accusam et justo
duo dolores et ea
rebum. Stet clita
kasd gubergren,
no sea takimata
sanctus est Lorem
ipsum dolor sit
amet.
```

*Find the exact string '(Lorem|dolor)' in example.txt*
```bash
fgrep '(Lorem|dolor)' example.txt
or
grep -F '(Lorem|dolor)' example.txt
```
```bash
foo (Lorem|dolor) 
```

### f. `fmt`
Simple optimal text formatter

*example: example.txt (1 line)*
```bash
Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.
```

*output the lines of example.txt to 20 character width*
```bash
cat example.txt | fmt -w 20
```
```bash
Lorem ipsum
dolor sit amet,
consetetur
sadipscing elitr,
sed diam nonumy
eirmod tempor
invidunt ut labore
et dolore magna
aliquyam erat, sed
diam voluptua. At
vero eos et
accusam et justo
duo dolores et ea
rebum. Stet clita
kasd gubergren,
no sea takimata
sanctus est Lorem
ipsum dolor sit
amet.
```

### g. `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. `-r` can be used to search all files under the specified directory, for example:
```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).

### h. `nl`
Number lines of files

*example.txt*
```bash
Lorem ipsum
dolor sit amet,
consetetur
sadipscing elitr,
sed diam nonumy
eirmod tempor
invidunt ut labore
et dolore magna
aliquyam erat, sed
diam voluptua. At
vero eos et
accusam et justo
duo dolores et ea
rebum. Stet clita
kasd gubergren,
no sea takimata
sanctus est Lorem
ipsum dolor sit
amet.
```

*show example.txt with line numbers*
```bash
nl -s". " example.txt 
```
```bash
     1. Lorem ipsum
     2. dolor sit amet,
     3. consetetur
     4. sadipscing elitr,
     5. sed diam nonumy
     6. eirmod tempor
     7. invidunt ut labore
     8. et dolore magna
     9. aliquyam erat, sed
    10. diam voluptua. At
    11. vero eos et
    12. accusam et justo
    13. duo dolores et ea
    14. rebum. Stet clita
    15. kasd gubergren,
    16. no sea takimata
    17. sanctus est Lorem
    18. ipsum dolor sit
    19. amet.
```

### i. `sed`
Stream editor for filtering and transforming text

*example.txt*
```bash
Hello This is a Test 1 2 3 4
``` 

*replace all spaces with hyphens*
```bash
sed 's/ /-/g' example.txt
```
```bash
Hello-This-is-a-Test-1-2-3-4
```

*replace all digits with "d"*
```bash
sed 's/[0-9]/d/g' example.txt
```
```bash
Hello This is a Test d d d d
```

### j. `sort`
Sort lines of text files

*example.txt*
```bash
f
b
c
g
a
e
d
```

*sort example.txt*
```bash
sort example.txt
```
```bash
a
b
c
d
e
f
g
```

*randomize a sorted example.txt*
```bash
sort example.txt | sort -R
```
```bash
b
f
a
c
d
g
e
```

### k. `tr`
Translate or delete characters

*example.txt*
```bash
Hello World Foo Bar Baz!
```

*take all lower case letters and make them upper case*
```bash
cat example.txt | tr 'a-z' 'A-Z' 
```
```bash
HELLO WORLD FOO BAR BAZ!
```

*take all spaces and make them into newlines*
```bash
cat example.txt | tr ' ' '\n'
```
```bash
Hello
World
Foo
Bar
Baz!
```

### l. `uniq`
Report or omit repeated lines

*example.txt*
```bash
a
a
b
a
b
c
d
c
```

*show only unique lines of example.txt (first you need to sort it, otherwise it won't see the overlap)*
```bash
sort example.txt | uniq
```
```bash
a
b
c
d
```

*show the unique items for each line, and tell me how many instances it found*
```bash
sort example.txt | uniq -c
```
```bash
    3 a
    2 b
    2 c
    1 d
```

### m. `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.

## 1.3. Directory Operations

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

### a. `cd`
Moves you from one directory to other. Running this  
```bash
$ cd
```
moves you to home directory. This command accepts an optional `dirname`, which moves you to that directory.
```bash
cd dirname
```

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

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

## 1.4. SSH, System Info & Network Operations

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

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

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

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

### d. `df`
Shows disk usage.

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

### f. `du`
Shows the disk usage of files or directories. For more information on this command check this [link](http://www.linfo.org/du.html)
```bash
du [option] [filename|directory]
```
Options:
- `-h` (human readable) Displays output it in kilobytes (K), megabytes (M) and gigabytes (G).
- `-s` (supress or summarize) Outputs total disk space of a directory and supresses reports for subdirectories. 

Example:
```bash
du -sh pictures
1.4M pictures
```

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

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

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

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

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

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

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

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

### o. `scp`
Transfer files between a local host and a remote host or between two remote hosts.

*copy from local host to remote host*
```bash
scp source_file user@host:directory/target_file
```
*copy from remote host to local host*
```bash
scp user@host:directory/source_file target_file
scp -r user@host:directory/source_folder farget_folder
```
This command also accepts an option `-P` that can be used to connect to specific port.  
```bash
scp -P port user@host:directory/source_file target_file
```

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

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

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

### s. `uptime`
Shows current uptime.

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

### u. `wget`
Downloads file.  
```bash
wget file
```

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

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

## 1.5. Process Monitoring Operations

<table>
   <tr>
      <td><a href="#a-kill">kill</a></td>
      <td><a href="#b-killall">killall</a></td>
      <td><a href="#c-&">&amp;</a></td>
      <td><a href="#d-nohup">nohup</a></td>
   </tr>
</table>

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

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

### c. &
The `&` symbol instructs the command to run as a background process in a subshell.
```bash
command &
```

### d. `nohup`
nohup stands for "No Hang Up". This allows to run command/process or shell script that can continue running in the background after you log out from a shell.
```bash
nohup command
```
Combine it with `&` to create background processes 
```bash
nohup command &
```

# 2. Basic Shell Programming


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

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

## 2.1. Variables

Creating variables in bash is similar to other languages. There are no data types. A variable in bash can contain a number, a character, a string of characters, etc. 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. The value of variable is retrieved by putting the `$` in the beginning of variable name.

Example:
```bash
echo $str   # hello world
```
## 2.2. Array
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
```

If no index is supplied, array element 0 is assumed. To find out how many values there are in the array use the 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
```

## 2.3 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
```

## 2.4. 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 run the 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 functions, are treated in the same manner as arguments given to the script.

## 2.5. 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  # at least one of the statements 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
-s 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
```

## 2.6. 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"

```bash
source ~/.bashrc
cd $hotellogs
```

## Exit traps

Make your bash scripts more robust by reliably performing cleanup.

```bash
function finish {
  # your cleanup here. e.g. kill any forked processes
  jobs -p | xargs kill
}
trap finish EXIT
```

## Saving your environment variables

When you do `export FOO = BAR`, your variable is only exported in this current shell and all its children, to persist in the future you can simply append in your `~/.bash_profile` file the command to export your variable
```bash
echo export FOO=BAR >> ~/.bash_profile
```

## Accessing your scripts

You can easily access your scripts by creating a bin folder in your home with `mkdir ~/bin`, now all the scripts you put in this folder you can access in any directory.

If you can not access, try append the code below in your `~/.bash_profile` file and after do `source ~/.bash_profile`.
```bash
    # set PATH so it includes user's private bin if it exists
    if [ -d "$HOME/bin" ] ; then
        PATH="$HOME/bin:$PATH"
    fi
```

# 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 [How to](https://help.github.com/articles/creating-an-issue/)
- Open pull request with improvements [How to](https://help.github.com/articles/about-pull-requests/)
- 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/)