Bash Script

Basics

The first line of the script is

#!/bin/bash

If the script is in the $PATH (e.g. /usr/bin), we can just type my.sh instead of ./my.sh to run it.

Variables

a=Hello # no spaces!
echo $a # Hello
echo "$a" # Hello
# add attributes to variables
declare -i d=123 # d is an integer
declare -r e=456 # e is read-only
declare -l f="LOLCats" # f is lolcats
declare -u g="LOLCats" # g is LOLCATS
# Built-in varibles
# $HOME $PWD $MACHTYPE $HOSTNAME $RANDOM
# $BASH_VERSION related: bash --version
# $0 name of the script
# $SECONDS time since this bash session had run / since the script started
# env: shows all environment variables
d=$(pwd) # run pwd and put the result in d.
# Arithmetic (only integers are supported)
val=$((expression))
# operators: **, %, +-*/%
e=5
e+=2 # string concatenation
((e+=2)) # arithmetic

Comparisons

[[ expression ]] # Note the spaces
# false/failure: returns 1
# true/success: returns 0
[[ "cat" == "cat" ]]
echo $? # $? stores the previous value
# comparison for integers
# 20 > 100 is true because its string comparison
[[ 20 -lt 100 ]]
# -lt less than, -gt, -le less than or equal to, -ge, -eq, -ne
# &&, ||, ! logic operators
#string is null value (empty string)
[[ -z $a ]] # is null?
# -n => is not null?

Strings

# concatenation
a=Hello
b=World
c=$a$b
echo $c # HelloWorld
# get string's length
echo ${#a} # 5
# substring
# start index, length
echo ${c:3:4} # olWo
# last four characters. Note the space before dash.
f=${c: -4} # orld
# replace
fruit="apple banana banana cherry"
# replace the first occurance
echo ${fruit/banana/durian} # apple durian banana cherry
# replace all occurance
echo ${fruit//banana/durian} # apple durian durian cherry
# replace only if banana is the beginning of the string
echo ${fruit/#banana/durian}
# replace only if banana is the end of the string
echo ${fruit/%banana/durian}
# replace regex matches
echo ${fruit/b*/durian} # apple durian

Reading and Writing Text Files

echo "Some text" > file.txt # If already exsits, override, else, create.
> file.txt # clear content of a file
echo "Some text" >> file.txt # append to end
# read
cat < file.txt
# read line by line
while read f; do
echo $f
done < file.txt
# Example: set a list of instructions
# create a file ftp.txt with commands
ftp -n < ftp.txt

Control Structures

if

if [ expression ] # or if [[ $a =~ regex ]] (( integer comparison )) or no brackets at all
if expression; then
...
if expression
then
echo "true"
elif
...
else
...
fi

while/until

i=0
while [ $i -le 10 ]; do
echo i;$i
((i+=1))
done
j=0
until [ $j -ge 10 ]; do
echo j:$j
((j+=1))
done

for loop

for i in 1 2 3 # or 1 in {1..100}
do
echo $i
done
# C style for loop
for (( i=1; i<=10; i++ ))
do
...
done
# array
arr=("apple" "banana")
for i in ${arr[@]}
do
...
done
# associative array (BASH 4.0+)
declare -A arr
arr["name"]="scott"
arr["id"]="123"
for i in "${!arr[@]}" # use quote to handle spaces in keys
do
echo "$i: ${arr[$i]}"
done
# command substitution
for i in $(ls)
do
echo "$i"
done

Case (switch in C)

a="dog"
case $a in
cat) echo "Feline";;
dog|puppy) echo "Canine";; # dog or puppy
*) echo "No match!";; # default
esac # case spelled backwards

Array

a=() # empty array
b=("apple" "banana" "cherry")
echo ${b[2]} # cherry
b[5]="kiwi"
b+=("mango") # append
echo ${b[@]} # the whole array
echo ${b[@]: -1} # last element
# Associate arrays (BASH4.0+)
declare -A myarray
myarray[color]=blue
# if key/value has spaces, use quotes

Functions

function greet {
echo "Hi"
}
# invoke
greet
# with arguments
function sayHi {
echo "Hi, $1"
}
sayHi David
function numberThings {
i=1
for f in [email protected]; do # [email protected] is the arguments array
echo $i: $f
(( i +=1 ))
done
}
numberthings $(ls)

Interact with the User

Arguments

Anything with space in it needs quotes.

Working with flags

# Case one: get u and p values
# my.sh
while getopts u:p: option; do
case $option in
u) user=$OPTARG;;
p) pass=$OPTARG;;
esac
done
# in shell, call with ./my.sh -u scott -p 123
# Case two: boolean flags
while getopts :u:p:ab option; do # ab has no colon after them. They are boolean flags.
# `:` before u => we care about unknown flags
case $option in
u) user=$OPTARG;;
p) pass=$OPTARG;;
a) echo "A";;
b) echo "B";;
?) echo "I don't know $OPTARG";;
esac
done

Get input during execution

echo "What is your name?"
read name # wait for user to input, and store in name
read -s pass # silent. Don't print the input password on screen
read -p "What's your age?" age # -p prompt
select option in "cat" "dog" "bird" "quit"
do
case $option in
cat) echo "you selected $animal";;
quit) break;;
*) echo "I don't understand"
esac
done

Ensuring a response

What if user just press enter?

if [ $# -lt 3 ]; then
cat <<- EOM
some error message
EOM
else
# the program here
fi
read -p "Name? " n
while [[ -z "$n" ]]; do
read -p "Need a name! " n
done
# provide a default answer
read -p "Name? [David] " n
while [[ -z "$n" ]]; do
n=David
done
# input validation by regex
read -p "What year? [nnnn] " a
while [[ ! $a =~ [0-9]{4} ]]; do
read -p "A year, please! [nnnn] " a
done

Date and Printf

date
date +"%d-%m-%Y"
date +"%H:%M:%S"
printf "Name:\t%s\nID:\t%04d\n" "Name" "12" # 4 digit with padding zeros
printf -v d "strings" # store the string in variable d
#command substitution strips out the newlines

Advanced Topics

Debug Mode

# run script in normal mode
$ bash a.sh
# run in debug mode
# prints out each command as it is run
$ bash -x a.sh

Brace Expansion

~ $ echo {apple,banana}
apple banana
~ $ echo {apple, banana}
{apple, banana} # does not work with spaces
~ $ touch {apple,banana} # creates two files
~ $ echo {1..10}
1 2 3 4 5 6 7 8 9 10
~ $ echo {A..z}
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z [ ] ^ _ ` a b c d e f g h i j k l m n o p q r s t u v w x y z # uppercase letters are before lowercases
~ $ echo {a..Z}
a ` _ ^ ] [ Z # reverse order
~ $ echo {apple,banana}_{1..3}
apple_1 apple_2 apple_3 banana_1 banana_2 banana_3 # combinations
# in bash 4+
~ $ echo {01..1000} # add padding zeros
~ $ echo {1..10..2} # specify intervals

ANSI escape codes: e.g. -e can enable escape sequences (start + string to print + end).

Start: '\033[number1;number2m'. Number1 and number 2 are foreground and background colors. m indicates the end of sequence. A style number (followed by ;) before number1 is optional.

End: usually it's '\033[0m', to clear all the formatting.

Color

Foreground

Background

Black

30

40

Red

31

41

Green

32

42

Yellow

33

43

Blue

34

44

Magenta

35

45

Cyan

36

46

White

37

47

Style table

Style

Value

No Style

0

Bold

1

Low Intensity

2

Underline

4

Blinking

5

Reverse

7

Invisible

8

echo -e '\033[34;42mColor Text\033[0m'
# store formatting in variables
flashred="\033[5;31;40m"
end="\033[0m"
echo -e $flashred"Error"$end

Alternative: use tput.

Here Documents

cat << endoftext
this is a
multiline
text
endoftext
# with <<-, bash will strips off leading tabs
cat <<- endoftext2 > log.txt
this is a
multiline
text
endoftext2
ftp -n <<- label
commands
label

References