bash-guide/README.md

Table of Contents

1. Basic Operations
   1.1. File Operations
   1.2. Text Operations
   1.3. Directory Operations
   1.4. SSH, System Info & Network Operations
   1.5. Process Monitoring Operations (TODO)
2. Basic Shell Programming
   2.1. Variables
   2.3. String Substitution
   2.4. Functions
   2.5. Conditionals
   2.6. Loops
3. Tricks
4. 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

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

Whereis search for executables, source files, and manual pages using a database built by system automatically.

whereis name

Example:

$ 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).

which program_name 

Example:

$ which php
/c/xampp/php/php

d. clear

Clears content on window

1.1. File Operations

ls	touch	cat	more	head	tail	mv	cp	rm	diff
chmod	gzip	gunzip	gzcat	lpr	lpq	lprm

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

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

touch filename

Example:

$ 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

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)

more filename

e. head

Outputs the first 10 lines of file

head filename

f. tail

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

tail filename

g. mv

Moves a file from one location to other

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

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

rm filename

j. diff

Compares files, and shows where they differ

diff filename1 filename2

k. chmod

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

chmod -options filename

l. gzip

Compresses files

gzip filename

m. gunzip

Un-compresses files compressed by gzip

gunzip filename

n. gzcat

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

gzcat filename

o. lpr

Print the file

lpr filename

p. lpq

Check out the printer queue

lpq

Example:

$ 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

lprm jobnumber

1.2. Text Operations

awk	grep	wc	sed	sort	uniq	cat	cut	echo	fmt
tr	nl	egrep	fgrep

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

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.

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.

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.

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. For more detail on grep, check following link.

c. wc

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

wc filename

Example:

$ 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

mkdir

cd

pwd

a. mkdir

Makes a new directory

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

pwd

1.4. SSH, System Info & Network Operations

ssh	whoami	passwd	quota	date	cal	uptime	w	finger	uname
man	df	du	last	ps	kill	killall	top	bg	fg
ping	whois	dig	wget

a. ssh

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

ssh user@host

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

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

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

finger username

j. uname

Shows kernel information

uname -a

k. man

Shows the manual for specified command

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)

du filename

n. last

Lists your last logins of specified user

last yourUsername

o. ps

Lists your processes

ps -u yourusername

p. kill

Kills (ends) the processes with the ID you gave

kill PID

q. killall

Kill all processes with the name

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

ping host

v. whois

Gets whois information for domain

whois domain

w. dig

Gets DNS information for domain

dig domain

x. wget

Downloads file

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.

#!/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:

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.

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.

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.

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

Contribution

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

License