Bash

Echo

• echo
• pwd
• man

echo Hello World
echo $BASH_VERSION
bash --version

echo and pwd  are external programs that we can run using the shell

man bash

man cd    (does not exist)
help cd

First Shell script

• echo
• chmod
• #!

chmod +x hello_world.sh

#!/usr/bin/env bash

echo "Hello World"

Bash command line parameters

• $1

#!/usr/bin/env bash

echo "Hello $1"

$1 $2 $3
$*      which is $1 $2 $3 ...   but that splits up the spaces that might be inside $1 or $2 ..
"$*"    is the same
$@      is the same but
"$@"    is "$1" "$2" "$3" ...

Bash if parameter

• if

#!/bin/bash

if [ "$1" == "" ]
then
    echo $0 DIRNAME
    exit 1
fi

echo "$1"

Bash while-loop

• while

while true; do (date; sleep 1); done

#!/usr/bin/env bash

while [[ -f /tmp/run ]]
do
  echo "Running"
  sleep 2
done

echo done

Bash for-loop

• for

#!/usr/bin/env bash

FRUITS="Apple Banana Peach Orange Strawberry"
for f in $FRUITS
do
    echo $f
done

Bash for loop on a range of numbers

for ((i=1;i<=10;i++));  do echo $i; done

#!/usr/bin/env bash

for ((i=1;i<=10;i++));
do
    echo $i;
done

Bash for loop on files

• for

#!/usr/bin/env bash

# Ynon

#FILES=examples/*
#for f in $FILES
#do
#   echo "File $f"
#done



for f in examples/*
do
   echo "File $f"
done

Bash for loop on command-line arguments

• $@

If there is no list to go over (there is no "in something" part) then by default it will go over
the values on the command line as if this was written:

for a in "$@"

#!/usr/bin/env bash

# Ynon

for a
do
   echo "Argument $a"
done

# ./examples/for_arguments.sh a bb "c d"

Bash a sequence of number - a range of numbers

• seq

• seq FROM STEP TO

#!/usr/bin/env bash

seq 1 1 5
echo
seq 1 2 7

1
2
3
4
5

1
3
5
7

Arithmetic in Bash

$ x=2
$ echo $x
2

$ (( x = x + 3 ))
$ echo $x
5

$ ((x=x+3))
$ echo $x
8

$ ((x+=1))
$ echo $x
9

$ ((x++))
$ echo $x
10

$ ((x*=2))
$ echo $x
20

Conditionals in Bash

Insted of real conditionals in Bash we only have a way to some command was successful or not and the if will execute code based on this condition.

What does it mean a program is successful or not? Exit code 0 or anything else. $?

• [[ ]]}
• -e}

#!/usr/bin/env bash

filename=$1

if [[ -e $filename ]]; then
   echo "File $filename exists"
fi

Bash exit code

#!/usr/bin/env bash

ls  / > /dev/null 2> /dev/null
echo $?         # 0

ls  /qqrq > /dev/null 2> /dev/null
echo $?         # 1

Bash exit code

#!/usr/bin/env bash

ls  / > /dev/null 2> /dev/null
exit_code=$?
echo $exit_code
if [ $exit_code = 0 ]; then
    echo /
fi

ls  /qqrq > /dev/null 2> /dev/null
exit_code=$?
echo $exit_code
if [ $exit_code = 0 ]; then
    echo /qqrq
fi

Make sure you save the exit code immediaely after the execution of some code if you'd like to use it later, as it always contains the exit code of the most recent statement.

Echo multiline string

#!/bin/bash

echo "
Line 1
Line 2
"

Redirect multiline string

#!/bin/bash

echo "
Line 1
Line 2
" > out

User Input (read)

#!/bin/bash

echo "Please type in your name"
read name
echo "Welcome '$name'"

• Catching user input

Counter

COUNTER=0
echo $COUNTER

COUNTER=$((COUNTER+1))
echo $COUNTER

COUNTER=$((COUNTER+1))
echo $COUNTER

Declare

• declare

• RANDOM

• %

declare -a animals=(cow snail elephant mouse)
king=${animals[$(((RANDOM % 4)))]}

echo $king