While I am going through the following, the youtube talk and it’s associated presentation, my hand-ons were recorded here. It is recommended to go through the basics first. You can also refer to the Bash Ref Manual for more information.

Parameters

There are Positional parameters such as $1, $2 and so on. if you use above 9, you must use braces such as ${11}. You can find about special parameters from the documentation.

set -- first second
echo $*
echo $2 and $1

first second
second and first

If the command returns 0 when successful, otherwise number 1 -255. Use the $? to capture the return value. Many programs signal different types of failure with different return values.

whoami
echo $?

ojitha
0

Use special variables such as $*, $@ and so on has been explained in the following code snippet:

bash <<'EOF' -s -- Ojitha "How are you" 

echo "$2 $1 ?"
echo arguement with *: $*
echo arguement with @: $@
echo number of arguments $#

j=0
for i in $@
do
  echo "$j element: $i"
  (( j = j+1 ))
done

EOF


How are you Ojitha ?
arguement with *: Ojitha How are you
arguement with @: Ojitha How are you
number of arguments 2
0 element: Ojitha
1 element: How
2 element: are
3 element: you

function login(){
    if [[ ${@} =~ "-user" && ${@} =~ "-pass" ]]; then
        echo "login as $2"
    else
        echo "Bad Syntax. Usage: -user [username] -pass [password] required"
    fi
}

login -user "Ojitha" -pass "two"
login -pass "two" -user "one" 
login "one" "two"

login as Ojitha
login as two
Bad Syntax. Usage: -user [username] -pass [password] required

There are compound commands

  • iterations: such as while, until, for, and select.
  • conditions: if and case
  • command groups: (list) and {list;}`
i=0
while [ $i -lt 3 ]
do
  echo i: $i
  ((i=i+1))
done


i: 0
i: 1
i: 2

Another example

The until is the opposite of the while.

seq 1 3 > a
while read e; do
    printf '%d\n' $(( e ** 2 ))
done < a    

1
4
9

i=0

until [ $i -gt 3 ]
do
  echo i: $i
  ((i=i+1))
done


i: 0
i: 1
i: 2
i: 3

Following is C lang style For loop

for (( i=0 ; i < 3 ; i++ ))
do
    echo $i
done    

0
1
2

here the iterator

for i in $(seq 1 3) ; do echo $i; done

1
2
3

or

seq 1 3 | while read line; do  printf '%d\n' $((line)); done

1
2
3

as well as

for i in a b "c and d"
do
 echo "=> $i"
done

=> a
=> b
=> c and d

or

for i in {0..8..2}
do
    echo $i
done    

0
2
4
6
8

The select statement example is hard to show in the notebook because it is interactive.

select choice in one two "no choice"
do
    echo $choice
done

will give you to select the choice

1) one
2) two
3) no choice
#?

You can select 1, 2 or 3 from the stdin.

Expressions

The [] matches any one of the enclosed characters. The new way is [[...]], which returns a status of 0 or 1 depending on the evaluation of the conditional expression.

s1="ojitha"
s2=""
[[ -n $s1 ]]
echo "s1: $?"

# test string is non-empty
[[ -n $s2 ]]
echo "s2: $?"

s1: 0
s2: 1

To test string is empty:

s2=""
[[ -z $s2 ]]
echo "s2: $?"

s2: 0

test the strings are the same

s1="ojitha"
s2="hewa"

# matched
[[ 'Hello' == 'Hello' ]]
echo $?

# unmatched
[[ 'Hello' == 'ojitha' ]]
echo $?

0
1

To match the regular expression.

set -- "the pen is my brother's" # set $1 to sample string

RE='^the' # regex
[[ $1 =~ $RE ]]
echo $?

0

or

a=ojitha
[[ $a == +(oj|aj)itha ]]
echo $?

0

The return value 0 is a successful match.

Expression Description
[[ -e file ]] file exists
[[ -f file ]] file is a regular file
[[ -d file ]] file is a directory
[[ -t fd ]] fd idopen and refer to terminal

In the conditional statement:

if [[ 'Hello' == 'oj' ]] then 
    echo 'true' 
else 
    echo "false" 
fi

false

Use with regex

a=ojitha
if [[ $a == +(oj|aj)itha ]]; then echo yes; else echo no; fi

yes

Pattern Matching

The case is a great way to match patterns. The pipe character between two patterns entails a match on condition OR.

case "ojitha Hello" in
    o) 
        echo 'o'
        ;;
    o*) 
        echo 'o*'
        ;;
    *) 
        echo 'nope'
        ;;
esac    

o*

Command Groups

Evaluate the list of commands in a subshell using (...).

NOTE: To evaluate a list of commands in the current shell, use { list; }. Here the spaces next to the curly braces and trailing semicolon are compulsory.

a="Ojitha"
( # subshell
    # inside    
    a="Disinee" 
    echo "In the subshell $a" 
)
# outside
echo "Outside the subshell $a"


In the subshell Disinee
Outside the subshell Ojitha

If you use the group command

a="Ojitha"
{ # group command
    # inside    
    a="Disinee" 
    echo "In the subshell $a" 
}
# outside
echo "Outside the subshell $a"

In the subshell Disinee
Outside the subshell Disinee

echo b; echo a | sort
echo ------ in the subshell ---
(echo b; echo a) | sort # subshell

b
a
------ in the subshell ---
a
b

Another use of parenthesis:

echo I am $(whoami)
echo I am `(whoami)` #same as above

I am ojitha
I am ojitha

Redirection

If not specified, fd 1 (STDOUT) is assumed when redirecting output. Alternative file descriptors may be specified by prepending the fd number. To direct error to file: 2>file. To redirect to a file descriptor, append & and the fd number: 2>&1 to redirect STDERR to the current target for STDOUT.

In the above, $(whoami) is a command substitution. The Process substitution feeds the output of a process (or processes) into the stdin of another process. For example, I want to compare two directories:

First, create directories and files inside the directories.

mkdir first && touch first/f1.txt first/f2.txt first/common.txt
mkdir second && touch second/s1.txt second/s2.txt  second/common.txt

to compare

diff <(ls first) <(ls second)

output

2,3c2,3
< f1.txt
< f2.txt
---
> s1.txt
> s2.txt

NOTE: The file common.txt is not visible because that exists in both of the directories.

Parameter Expansion

Find the documentation.

Following code, if the param is empty, it returns the default (in this case $(whoami)) value but is not set to the variable.

who=""
echo who without default: $who
echo who with default: ${who:-$(whoami)}
echo who without default: $who


who without default:
who with default: ojitha
who without default:

if you want to set the default to the who variable (use :=).

Positional parameters and special parameters may not be assigned in this way.

who=""
echo who without default: $who
echo who with default: ${who:=$(whoami)}
echo who without default: $who

who without default:
who with default: ojitha
who without default: ojitha

Alternate

who=ojitha
echo ${who+aaa}
echo $who

aaa
ojitha

as well as

who=ojitha
echo ${who:+aaa}
echo $who

aaa
ojitha

If a parameter is null or unset, the expansion of the word (or a message to that effect if the word is not present) is written to the standard error and the shell, if it is not interactive, exits. Otherwise, the value of the parameter is substituted.

who=
echo ${who:?variable is null}

Substring

who=ojitha
echo ${who:2}
echo ${who:2,3}
echo ${who#oj} # left edge
echo ${who%ha} # right edge
greedy=ojitha:hewa:kuma
echo ${greedy##*:} # greedy

itha
tha
itha
ojit
kuma

From right to left

who=ojitha
echo ${who:(-1)} 

a

Expand to length

who=ojitha
echo length ${#who}
# indirect expansion
${!who}


length 6

Indirect expansion

surname=lastname; lastname=HEWA
echo ${!surname}

HEWA

List name-matching prefix

v1=""
v2=""
v3=""
v4=""

echo "${!v@}"
echo ${!v*} 

v1 v2 v3 v4
v1 v2 v3 v4

List the keys in an array ${!arr[*]} or ${!arr[@]}

Pattern Substituion

who=abba
echo ${who/b/x}
# greedy
echo ${who//b/x}


axba
axxa

substitute at left edge

who=aabb
echo ${who/#aa/12}


12bb

substitute at the right edge

who=aabb
echo ${who/%bb/12}

aa12

Arrays

There are arrays and new associative arrays. First see the arrays:

# declare the array
a=( 1 4 3 ojitha "Hello World" )
declare -p a
# add to array
a+=("Hewa")
declare -p a
echo "second element: ${a[1]}"
# print array indexes
echo Array indexes ${!a[@]}

declare -a a=([0]="1" [1]="4" [2]="3" [3]="ojitha" [4]="Hello World")
declare -a a=([0]="1" [1]="4" [2]="3" [3]="ojitha" [4]="Hello World" [5]="Hewa")
second element: 4
Array indexes 0 1 2 3 4 5

substitute _ for all the spaces

# declare the array
a=( "This is an example of substituting underscore for spaces" "Hello World" )
a=( "${a[@]// /_}" )
declare -p a

declare -a a=([0]="This_is_an_example_of_substituting_underscore_for_spaces" [1]="Hello_World")

Sub arraying

# declare the array
a=( 1 2 3 ojitha "Hello World" )
a=( ${a[@]:2:3} )
declare -p a

declare -a a=([0]="3" [1]="ojitha" [2]="Hello" [3]="World")

Passing an array argument

#variable
a=(1 2 3 4)

function display(){
    arr=($*)
    for i in ${arr[@]}
    do 
        echo $i 
    done
}
display ${a[@]}

1
2
3
4

Associative arrays

# Declare an associative array
declare -A arr

arr[fruit]=Mango
arr[bird]=Cockatail
arr[flower]=Rose
arr[animal]=Tiger

for i in ${!arr[*]}
do
    echo $i
done

flower
fruit
animal
bird

Arithmatics

((1==1)); echo "1==1 $?"

((1!=1)); echo "1!=1 $?"

((1<2)); echo "1<2 $?"

((1>2)); echo "1>2 $?"
# adding
echo 1+1 = $((1+1))

# counter
count=1
# after evaluate
echo "count=1, count++ after evaluation is $((count++))"
# before evaluate
count=1
echo "count=1, ++count before evaluation is $((++count))"

1==1 0
1!=1 1
1<2 0
1>2 1
1+1 = 2
count=1, count++ after evaluation is 1
count=1, ++count before evaulation is 2

# counter
count=5
# after evaluate
echo "count=5, count++ after evaluation is $((count--))"
# before evaluate
count=5
echo "count=5, ++count before evaluation is $((--count))"

count=5, count++ after evaluation is 5
count=5, ++count before evaluation is 4

Brace Expansion

Generate arbitrary words

echo walk{,e{d,s},ing,ful{l,ly}}

walk walked walkes walking walkfull walkfully

echo {1..5}{0,5}%

10% 15% 20% 25% 30% 35% 40% 45% 50% 55%

Session Portability

Import elements from the current session directly into a new local or remote session.

  • p for parameters
  • f for functions
#variable
who=ojitha

function hello(){
    echo Hello $1
}
# pass a variable and the function to a new shell
bash -c "$(declare -p who; declare -f hello); hello $who "

Hello ojitha

You can pass the session to remote

ssh remote_host "$(declare -p parameter; declare -f func); your code "