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.

#!/bin/bash

1. Bash Basics

a. export

Displays all environment variables and if you want to get detail of specific variable then use echo $VARIABLE_NAME
Syntax:

export

Example:

$ 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

Finds out where a specific binary is on your system.
Syntax:

whereis `name`

Example:

$ whereis php
/usr/bin/php

c. clear

Clears content on window

1.1. File Commands

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
Syntax:

ls `option`

Example:

$ 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

b. touch

Creates or updates your file
Syntax:

touch `filename`

Example:

$ touch trick.md

c. cat

Places standard input into file. Means that it opens the file in terminal for you to edit
Syntax:

cat > `filename`

d. more

Shows the first part of a file (move with space and type q to quit)
Syntax:

more `filename`

e. head

Outputs the first 10 lines of file
Syntax:

head `filename`

f. tail

Outputs the last 10 lines of file. Use -f to output appended data as the file grows
Syntax:

tail `filename`

g. mv

Moves a file from one location to other
Syntax:

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
Syntax:

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
Syntax:

rm `filename`

j. diff

Compares files, and shows where they differ
Syntax:

diff `filename1` `filename2`

k. wc

Tells you how many lines, words and characters there are in a file
Syntax:

wc `filename`

Example:

$ wc demo.txt
7459   15915  398400 demo.txt

Where 7459 is lines, 15915 is words and 398400 is characters.

l. chmod

Lets you change the read, write, and execute permissions on your files
Syntax:

chmod -options `filename`

m. gzip

Compresses files
Syntax:

gzip `filename`

n. gunzip

Un-compresses files compressed by gzip
Syntax:

gunzip `filename`

o. gzcat

Lets you look at gzipped file without actually having to gunzip it
Syntax:

gzcat `filename`

p. lpr

Print the file
Syntax:

lpr `filename`

q. lpq

Check out the printer queue
Syntax:

lpq

Example:

$ lpq
Rank    Owner   Job     File(s)                         Total Size
active  adnanad 59      demo                            399360 bytes
1st     adnanad 60      (stdin)                         0 bytes

r. lprm

Remove something from the printer queue
Syntax:

lprm `jobnumber`

s. 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.
Syntax:

grep `pattern` `filename`

Example:

$ 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

$ grep -r admin /etc/

And -w to search for words only.

1.2. Directory Commands

a. mkdir

Makes a new directory
Syntax:

mkdir `dirname`

b. cd

Moves you from one directory to other. If you just run

$ cd

Then it will moves you to home. Also this command accepts an optional dirname, which if provided will moves you to that directory.

cd `dirname`

c. pwd

Tells you in which directory you currently are
Syntax:

pwd

1.3. SSH, System Info & Network Commands

a. ssh

ssh (SSH client) is a program for logging into a remote machine and for executing commands on a remote machine.
Syntax:

ssh user@host

This command also accepts an option -p that can to used to connect to specific port.
Syntax:

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 Syntax:

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 whois online

i. finger

Displays information about user
Syntax:

finger `username`

j. uname

Shows kernel information
Syntax:

uname -a

k. man

Shows the manual for specified command
Syntax:

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)
Syntax:

du `filename`

n. last

Lists your last logins of specified user
Syntax:

last `yourUsername`

o. ps

Lists your processes
Syntax:

ps -u `yourusername`

p. kill

Kills (ends) the processes with the ID you gave
Syntax:

kill `PID`

p. killall

Kill all processes with the name
Syntax:

killall `processname`

q. top

Displays your currently active processes

r. bg

Lists stopped or background jobs ; resume a stopped job in the background

s. fg

Brings the most recent job in the foreground.

t. ping

Pings host and outputs results
Syntax:

ping `host`

u. whois

Gets whois information for domain
Syntax:

whois `domain`

v. dig

Gets DNS information for domain
Syntax:

dig `domain`

w. wget

Downloads file
Syntax:

wget `file`

2. Basic Shell Programming

Now lets discuss about some basic of shell programming. Lets start with creating variables first.

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:

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:

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:

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

${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

${#array[@]}

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

${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

${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.

Syntax:

functname() {
    shell commands
}

Example:

#!/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.

Syntax:

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.

Syntax:

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

Expression Examples:

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:

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:

while condition; do
  statements
done

until Syntax:

until condition; do
  statements
done

3. Process Handling

myCommand & # runs job in the background and prompts back the shell

jobs # lists all jobs (use with -l to see associated PID)

kill -l # returns a list of all signals on the system, by name and number kill PID # terminates process with specified PID

ps # prints a line of information about the current running login shell and any processes running under it ps -a # selects all processes with a tty except session leaders

4. Tips and 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

5. Debugging Shell Programs

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.

Syntax:

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

Feedback

Suggestions/improvements welcome!