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bash-guide

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A guide to learn bash

// repository documentation

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## 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](#15-process-monitoring-operations)
  2. [Basic Shell Programming](#2-basic-shell-programming)  
    2.1. [Variables](#21-variables)  
    2.2. [Array](#22-array)  
    2.3. [String Substitution](#23-string-substitution)  
    2.4. [Other String Tricks](#24-other-string-tricks)  
    2.5. [Functions](#25-functions)  
    2.6. [Conditionals](#26-conditionals)  
    2.7. [Loops](#27-loops)  
    2.8. [Regex](#28-regex)  
    2.9. [Pipes](#29-pipes)  
  3. [Tricks](#3-tricks)  
  4. [Debugging](#4-debugging)  
  5. [Multi-threading](#5-multi-threading)

# 1. Basic Operations

### a. `export`
Displays all environment variables. If you want to get details of a specific variable, use `echo $VARIABLE_NAME`.  
```bash
export
```
Example:
```bash
$ export
AWS_HOME=/Users/adnanadnan/.aws
LANG=en_US.UTF-8
LC_CTYPE=en_US.UTF-8
LESS=-R

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

### b. `whatis`
whatis shows description for user commands, system calls, library functions, and others in manual pages
```bash
whatis something
```
Example:
```bash
$ whatis bash
bash (1)             - GNU Bourne-Again SHell
```

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

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

### e. clear
Clears content on window.

## 1.1. File Operations
<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-less">less</a></td>
      <td><a href="#m-lpq">lpq</a></td>
      <td><a href="#n-lpr">lpr</a></td>
      <td><a href="#o-lprm">lprm</a></td>
      <td><a href="#p-ls">ls</a></td>
      <td><a href="#q-more">more</a></td>
      <td><a href="#r-mv">mv</a></td>
      <td><a href="#s-rm">rm</a></td>
      <td><a href="#t-tail">tail</a></td>
      <td><a href="#u-touch">touch</a></td>
   </tr>
</table>

### a. `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
cat < file1 > file2 #copy file1 to file2
```

### b. `chmod`
The chmod command stands for "change mode" and allows you to change the read, write, and execute permissions on your files and folders. For more information on this command check this [link](https://ss64.com/bash/chmod.html).
```bash
chmod -options filename
```

### c. `chown`
The chown command stands for "change owner", and allows you to change the owner of a given file or folder, which can be a user and a group. Basic usage is simple forward first comes the user (owner), and then the group, delimited by a colon.
```bash
chown -options user:group filename
```

### d. `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.

### e. `diff`
Compares files, and lists their differences.  
```bash
diff filename1 filename2
```

### f. `file`
Determine file type.  
```bash
file filename
```
Example:
```bash
$ file index.html
 index.html: HTML document, ASCII text
```
### g. `find`
Find files in directory
```bash
find directory options pattern
```
Example:
```bash
$ find . -name README.md
$ find /home/user1 -name '*.png'
```

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

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

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

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

### l. `less`
Shows the contents of a file or a command output, one page at a time. It is similar to [more](#q-more), but has more advanced features and allows you to navigate both forward and backward through the file.  
```bash
less filename
```

### m. `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
```

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

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

### p. `ls`
Lists your files. `ls` has many options: `-l` lists files in 'long format', which contains the exact size of the file, who owns the file, 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 -la
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>

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

### r. `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.

Also it can be used for rename a file.
```bash
mv old_name new_name
```

### s. `rm`
Removes a file. Using this command on a directory gives you an error.
`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.
```bash
rm filename
```

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

### u. `touch`
Updates access and modification time stamps of your file. If it doesn't exists, it'll be created.
```bash
touch filename
```
Example:
```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
```
Switch to the previous working directory
```bash
cd -
```

### b. `mkdir`
Makes a new directory.  
```bash
mkdir dirname
```
You can use this to create multiple directories at once within your current directory.
```bash
mkdir 1stDirectory 2ndDirectory 3rdDirectory
```
You can also use this to create parent directories at the same time with the -p (or --parents) flag. For instance, if you wanted a directory named 'project1' in another subdirectory at '/samples/bash/projects/', you could run:
```bash 
mkdir -p /samples/bash/projects/project1
mkdir --parents /samples/bash/projects/project1
```
Both commands above will do the same thing.
If any of these directories did no already exist, they would be created as well.

### 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-jobs">jobs</a></td>
      <td><a href="#j-last">last</a></td>
   </tr>
   <tr>
      <td><a href="#k-man">man</a></td>
      <td><a href="#l-passwd">passwd</a></td>
      <td><a href="#m-ping">ping</a></td>
      <td><a href="#n-ps">ps</a></td>
      <td><a href="#o-quota">quota</a></td>
      <td><a href="#p-scp">scp</a></td>
      <td><a href="#q-ssh">ssh</a></td>
      <td><a href="#r-top">top</a></td>
      <td><a href="#s-uname">uname</a></td>
      <td><a href="#t-uptime">uptime</a></td>
   </tr>
   <tr>
      <td><a href="#u-w">w</a></td>
      <td><a href="#v-wget">wget</a></td>
      <td><a href="#w-whoami">whoami</a></td>
      <td><a href="#x-whois">whois</a></td>
      <td><a href="#y-rsync">sync</a></td>
      <td><a href="#z-curl">curl</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. `jobs`
Lists the jobs running in the background, giving the job number.

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

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

### l. `passwd`
Allows the current logged user to change their password.

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

### n. `ps`
Lists your processes.  
```bash
ps -u yourusername
```
Use the flags ef. e for every process and f for full listing. 
```bash
ps -ef
```

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

### p. `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 target_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
```

### q. `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
```

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

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

### t. `uptime`
Shows current uptime.

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

### v. `wget`
Downloads file.  
```bash
wget file
```

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

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

### y. `rsync`
Does the same job as `scp` command, but transfers only changed files. Useful when transferring the same folder to/from server multiple times.
```bash
rsync source_folder user@host:target_folder
rsync user@host:target_folder target_folder
```

### z. `curl`
Curl is a command-line tool for requesting or sending data using URL syntax. Usefull on systems where you only have terminal available for making various requests.
```bash
curl url
```
Use  `-X` or `--request` to specify which method you would like invoke (GET, POST, DELETE, ...).
Use `-d <data>` or `--data <data>` to POST data on given URL.

## 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
#!/usr/bin/env 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 a variable containing multiple values. There's no maximum limit on the size of array. Arrays 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. Other String Tricks

Bash has multiple shorthand tricks for doing various things to strings.

```bash
${variable,,}    #this converts every letter in the variable to lowercase
${variable^^}    #this converts every letter in the variable to uppercase

${variable:2:8}  #this returns a substring of a string, starting at the character at the 2 index(strings start at index 0, so this is the 3rd character),
                 #the substring will be 8 characters long, so this would return a string made of the 3rd to the 11th characters.
```

Here are some handy pattern matching tricks
```bash
if [[ "$variable" == *subString* ]]  #this returns true if the provided substring is in the variable
if [[ "$variable" != *subString* ]]  #this returns true if the provided substring is not in the variable
if [[ "$variable" == subString* ]]   #this returns true if the variable starts with the given subString
if [[ "$variable" == *subString ]]   #this returns true if the variable ends with the given subString
```

The above can be shortened using a case statement and the IN keyword
```bash
case "$var" in
	begin*)
		#variable begins with "begin"
	;;
	*subString*)
		#subString is in variable
	;;

*otherSubString*)
		#otherSubString is in variable
	;;
esac
```

## 2.5. 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
function name() {
    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.

## 2.6. 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         # you 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
```

## 2.7. Loops

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

Different `for` Syntax:
```bash
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
```

# 2.8. Regex

They are a powerful tool for manipulating and searching text. Here are some examples of regular expressions that use each `metacharacter`:

<table>
   <tr>
      <td><a href="#a-dot">`.`(dot)</a></td>
      <td><a href="#b-asterisk">`*`(asterisk)</a></td>
      <td><a href="#c-plus">`+`(plus)</a></td>
      <td><a href="#d-question_mark">`?`(question mark)</a></td>
      <td><a href="#c-plus">`|`(pipe)</a></td>
      <td><a href="#c-plus">`[]`(character class)</a></td>
      <td><a href="#c-plus">`[^]`(negated character class)</a></td>
      <td><a href="#c-plus">`()`(grouping)</a></td>
      <td><a href="#c-plus">`{}`(quantifiers)</a></td>
      <td><a href="#c-plus">`\`(escape)</a></td>
   </tr>
</table>

### a. `.` (dot)
Matches any single character except newline.  
```bash
grep h.t file.txt
```
Output:
```bash
hat
hot
hit
```

### b. `*` (asterisk)
Matches zero or more occurrences of the preceding character or group.
```bash
grep ab*c file.txt
```
Output:
```bash
ac
abc
abbc
abbbc
```

### c. `+` (plus)
Matches one or more occurrences of the preceding character or group.
```bash
grep ab+c file.txt
```
Output:
```bash
abc
abbc
abbbc
abbbbc
```

### d. `?` (question mark)
Matches zero or one occurrence of the preceding character or group.
```bash
grep ab?c file.txt
```
Output:
```bash
ac
abc
```

### e. `|` (pipe)
Matches either the pattern to the left or the pattern to the right.
```bash
egrep "cat|dog" file.txt
```
Output:
```bash
cat
dog
```

### f. `[]` (character class)
Matches any character inside the brackets.
```bash
[aeiou] will match any vowel
[a-z] will match any lowercase letter
```

### g. `[]` (negated character class)
Matches any character not inside the brackets.
```bash
[^aeiou] will match any consonant
[^a-z] will match any non-lowercase letter
```

### h. `()` (grouping)
Groups multiple tokens together and creates a capture group.
```bash
egrep "(ab)+" file.txt
```

Output:
```bash
ab
abab
ababab
```

### i. `{}` (quantifiers)
Matches a specific number of occurrences of the preceding character or group.
```bash
egrep "a{3}" file.txt
```

Output:
```bash
aaa
aaaa
aaaaa
```

### j. `\` (escape)
Escapes the next character to match it literally.
```bash
egrep "a\+" file.txt
```

Output:
```bash
a+
```
=======
## 2.9. Pipes

Multiple commands can be linked together with a pipe, `|`. A `|` will send the standard-output from command A to the standard-input of command B.
Pipes can also be constructed with the `|&` symbols. This will send the standard-output **and** standard-error from command A to the standard-input of command B.

# 3. Tricks

## Set an alias

Run `nano ~/.bash_profile` and add the following line:

```bash
alias dockerlogin='ssh www-data@adnan.local -p2222'  # add your alias in .bash_profile
```

## To quickly go to a specific directory

Run `nano ~/.bashrc` and add the following line:

```bash
export hotellogs="/workspace/hotel-api/storage/logs"
```

Now you can use the saved path:

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

## Re-execute the previous command

This goes back to the days before you could rely on keyboards to have an "up" arrow key, but can still be useful. 
To run the last command in your history
```bash
!!
```
A common error is to forget to use `sudo` to prefix a command requiring privileged execution. Instead of typing the whole command again, you can:
```bash
sudo !!
```
This would change a `mkdir somedir` into `sudo mkdir somedir`.

## 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
```

# 5. Multi-threading
You can easily multi-threading your jobs using `&`. All those jobs will then run in the background simultaneously and you can see the processes below are running using `jobs`.

```bash
sleep 15 & sleep 5 &
```

The optional `wait` command will then wait for all the jobs to finish.

```bash
sleep 10 & sleep 5 &
wait
```

## 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

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## License

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