Basic Bash Scripting

Begin with !/ and the bash location

!/usr/bash

extension .sh

Two possibility to run a bash script:

#1
bash script.sh

#2
./script.sh

bash script.sh One Two Three
#One, Two and Three are arguments. You can use then in the script
echo $1     #print first argument, One
echo $3     #print third argument, Three
echo $*     #print all the arguments
echo $#     #print arguments count, 3

Variables

var1="hello"     #create a variable (no space around =)
$var1     #reference the variable
echo "good morning, $var1 and hi"

#Variable in variable
var2="Hi equal to $(var1)"

Array

myarray=(1 2 3 4 5)
echo ${myarray[@]}     #return all elements
echo ${myarray[@]:1:3}     #return three elements starting from second 
echo ${#myarray[@]}     #return length
echo ${myarray[2]}     #return tird element (zero-indexing)
myarray[0]=10     #change first element
myarray+=(8)     #add a new element at the end

IF statement

if [ condition ]; then
[do something...]
else
[do something else...]
fi

if [ $x == "hello" ]; then
    echo "hello!"
else
    echo "hi!"
fi

-eq -> equal to
-ne -> not equal to
-gt -> greater then
-lt -> less then
-ge -> greater thenor equal to
-le -> less then or equal to
-e -> if file exists
-s -> if file exists and has size greater then 0
-r -> if file exists and is readable
-w -> if file exists and is writable
|| -> OR
&& -> AND

FOR loops

for x in {[START]..[STOP]..[INCREMENT]} [or] (([start];[end];[increment]))
do
[do something...]
done

for x in {1..5..2}     #{START..STOP..INCREMENT}
do
    echo $x
done

for ((x=2;x<=8;x+=2))     #((start;end;increment))
do
    echo $x
done

#list all file in dir with the py extension
for file in dir/*.py
do
    echo $file
done

#list all file in dir with the word hello in filename
for file in $(ls dir/ | grep -i 'hello')
do
    echo $file
done

WHILE statements

x = 1
while [ $x -le 10]
do
    echo $x
    ((x+=1))
done

CASE statements

case 'VAR' in
PATTERN1)
COMMAND1 ;;
PATTERN2)
COMMAND2 ;;
*)
DEFAULT COMMAND ;;
esac

# move file with name hello, hi in their folder, remove ciao and hola files and move all the other file in another folder
case $(cat $1) in
    *hello*)
    mv $1 hello/ ;;
    *hi*)
    mv $2 hi/ ;;
    *ciao*|*hola*)
    rm $1 ;;
    *)
    mv $1 others/ ;;
esac

Basic Functions

function_name () {
[function code]
return [something]
}

alternative syntax:

function function_name {
[function code]
return [something]
}

call the function:
function_name

Scheduling scripts with Cron

cron: time-based job scheduler

echo "* * * * * python create_model.py" | crontab

echo scheduler command into cromtab

5 stars to set, one for each time unit:

#run every minutes forever
* * * * * bash script.sh

#run every sunday at 13:30
30 13 * * 7 bash script.sh

#run every 15, 30 and 45 minutes
15,30,45 * * * * bash script.sh

#run every 15 minutes
*/15 * * * * bash script.sh

#see job scheduled with cron
crontab -l

#edit your cronjob list
crontab -e