If the script is in the $PATH (e.g. /usr/bin), we can just type my.sh instead of ./my.sh to run it.
# add current working directory to path
PATH=$PATH:$PWD
Best practice to increase portability:
# instead of
#!/usr/local/bin/python3
# use
#!/usr/bin/env python3
Differences between shell functions and other scripts (e.g. python scripts)
Functions are executed in the current shell environment whereas scripts execute in their own process. Thus, functions can modify environment variables, e.g. change your current directory, whereas scripts can’t. Scripts will be passed by value environment variables that have been exported using export
Variables
a=Hello # no spaces!
# a = Hello means run a with = and Hello as arguments
echo $a # Hello
echo "$a" # Hello
echo '$a' # $a
echo a # a
# 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=$((expression))
# operators: **, %, +-*/%
e=5
e+=2 # string concatenation
((e+=2)) # arithmetic
Command substitution
d=$(pwd) # run pwd and put the result in d.
Process substitution
command -a <(CMD) # execute CMD, put output in a temp file,
# replace <() with the temp file name.
# useful when command expects input as a file instead of STDIN
# Example:
mkdir foo bar
# This creates files foo/a, foo/b, ... foo/h, bar/a, bar/b, ... bar/h
touch {foo,bar}/{a..h}
touch foo/x bar/y
# Show differences between files in foo and bar
diff <(ls foo) <(ls bar)
# Outputs
# < x
# ---
# > y
# Note that in python os.system(cmd) uses sh
# to use process substitution, do
# os.system('/bin/bash -c "cmd"')
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
note that when expression succeeds (return value is 0), then clause runs. (not like JavaScript, hwere 0 is falsey)
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 $@; do # $@ is the arguments array
echo $i: $f
(( i +=1 ))
done
}
numberthings $(ls)
Arguments
Anything with space in it needs quotes.
$0: name of the script
$1: first argument
$#: the number of arguments
$@: the arguments array
$?: return code of previous command
$$: PID of current script
$_: last argument of previous command
#!/bin/bash
echo "Starting program at $(date)" # Date will be substituted
echo "Running program $0 with $# arguments with pid $$"
for file in $@; do
grep foobar $file > /dev/null 2> /dev/null
# When pattern is not found, grep has exit status 1
# We redirect STDOUT and STDERR to a null register since we do not care about them
if [[ $? -ne 0 ]]; then
echo "File $file does not have any foobar, adding one"
echo "# foobar" >> "$file"
fi
done
Interact with the User
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
# or you can set -x in a.sh
# set -e => exit script when there is an error
$ 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
Print Colored Text
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