shell

Basic

Variable

Convention: all caps for system variables, lower case for user defined variables.

echo $BASH
echo $BASH_VERSION
echo $HOME
echo $PWD

For user defined var, see the code below. Note that there is no space between var name and “=” and its value.

name=Mark
echo The name is $name

Array

Separate each element with space, not comma.

os=('ubuntu' 'windowns' 'mac')
os[3]='kali'  # add/set an element
echo "${os[@]}"  # print all elements
echo "${!os[@]}"  # print the indices (may not be consecutive)
echo "${#os[@]}"  # print the length of array
unset os[3]  # remove an element 

# string
string=dasfsd
## the following all prints the full string
echo "${os[@]}"
echo "${os[0]}"
echo "${os[1]}"

Logical Operator

And

age=25
if [ $age -gt 18 ] && [ $age -lt 30 ]
# Alternatively, we can use
# if [ $age -gt 18 -a $age -lt 30 ]
# if [[ $age -gt 18 && $age -lt 30 ]]
then 
    echo "valid age"
fi

Or

replace the second with -o

Arithmetic Operation

Notice the space between parenthesis and condition.

# integer
num1=10
num2=5
echo $(( num1+num2 ))
echo $(expr $num1 + $num2 )  # this does not work for *

# float
num1=20.5
num2=5
echo "20.5/5" | bc #4
echo "scale=2;20.5/5" | bc #4.10
echo "$num1+$num2" | bc
echo "scale=3;sqrt($num1)" | bc -l  # -l calls the library
echo "scale=3;3^3" | bc -l

Control Flow

IF

Note the space between condition and “[]”.

integer comparison

-eq, -ne, -gt, -ge, -lt, -le: typical with [condition]

<, <=, >, >=: typical with ((condition))

string comparison

== (equivalent to =), !=: typical with [condition]

<, >, -z- (string is null): typical with [[condition]]

# integer comparison 
cnt=10
if [ $cnt -eq 10 ]
then 
    statement
fi

# string comparison
word=abc
if [ $word == 'abc' ]
then 
    statement
elif [[ $word < 'zzz' ]]
then
    statement
else
    statement
fi

Case

[a-z], [A-Z], [0-9]

? matches one letter character

if [a-z] also match upper case letter, execute LANG=C.

case expression in
    pattern1 )
        statement ;;
    pattern2 )
        statement ;;
    * ) # * matches any strings
        echo "unknown" ;;
esac

While

Pattern.

while [ condition ]
do 
    command1
    command2
done

We can sleep and open terminal.

n=1
while (( n <= 10))
do
    echo "$n"
    (( n++ ))
    sleep 1
    gnome-terminal & 
    xterminal
done

Until

Equivalent to while (!condition).

For

for var in 1 2 3 4 5  # in {1..5..1}, start, end, step
for var in file1 file2 file3
for (( exp1; exp2; exp3 ))

Select

Iterate the list and give user a menu-like prompt.

select name in mark john tom ben
do 
    echo "$name selected"
done

Usually combine with case.

select name in mark john tom ben
do 
    case $name in 
    mark )
        echo "mark selected" ;;
    john )
        echo "john selected" ;;
    tom )
        echo "tom selected" ;;     
    * )
        echo "please provide no.1-4"
    esac
done

Function

When we call the function, no parenthesis is needed.

function name(){
    command
}

function print(){
    echo $1 $2 $3
}
print Hello World Again

Default, the variable is global. If we want to declare local variable, use local keyword.

function print(){
    name=$1
    # local name=$1
    echo "the name is $name"
}
name="Tom"
print Max
echo "The name is $name: After"
# will be Tom if local keyword is used; Max if not. 

usage() {
    echo "You need to provide an argumet"
    echo "usage: $0 file_name"
}
file_exist() {
    local file="$1"
    [[ -f "$file" ]] && return 0 || return 1
}
[[ $# eq 0 ]] && usage
if ( file_exist "$1" )
then
    echo "file found"
else
    echo "file not found"
fi

IO

read inputs

echo "Enter Personal Info"
read name age email 
echo "name: $name, age: $age, email: $email"

If we want user to enter info in the same line.

read -p 'username: ' user_var
echo 'username is' $user_var

If we want to silence the input (not displaying it).

read -sp 'password:' pw
echo  # let next message appear in a new line
echo 'password is' $pw

We can save all inputs in an array.

echo "enter names:"
read -a names 
echo "Names include: ${names[0]}, ${names[1]}"

Using while loop.

while read line
do 
    echo $line
done < content.txt

# an alternative way
cat content.txt | while read p
do
    echo $p
done

while IFS=' ' read -r line  # -r will skip escape sign
do 
    echo $line
done < content.txt

Pass Arguments

Print the argument. ‘$0’ represent the shell script name.

echo $1 $2 $3

We can store the arguments in an array. The shell script name will not be parsed into the array, thus ${args[0]} is in fact $1.

args=("$@")
echo ${args[0]}
echo $@  # print out all arguments 
echo $#  # print out the number of arguments 

File test operators

-e check whether the file exists

echo -e "enter filename: \c"
read filename
if [-e $filename]
then 
    echo "$filename found"
else
    echo "$filename not found"
fi

Append output

-f check whether the path is a file

-w check write permission

> overwrite a file

>> append to a file

echo -e "enter filename: \c"
read filename
if [ -f $filename ]
then 
    if [ -w $filename ]
    then 
        echo "I want to append"
        cat >> $filename
    else
        echo "no permission"
    fi
fi