Bash CheatSheet

Reference Lynda.com - Up and running Bash Scripting

Bash script syntax

To run like bash script.

#!/bin/bash

Comments

# comment

Use extension .sh for shell and bash files.

To run a bash file use:
bash myscript.sh

To make the script executable without the bash command use:

chmod +x myscript.sh
./myscript.sh

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Echo

To print empty line.

  echo

To print all as strings. Escape special characters with a \.

  greeting="hello"
  # No quotes
  echo $greeting, world \(planet\)!  # hello, world (planet)!
  # Single quotes
  echo '$greeting, world (planet)!'  # $greeting, world (planet)!
  # Double quotes
  echo "$greeting, world (planet)!"  # $hello, world (planet)!
  # Double quotes escaped variables
  echo "\$greeting, world (planet)!"  # $greeting, world (planet)!

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Variables

Must not have whitespaces next to the equal sign.

Call them with a $.

  a=Hello
  b="Good Morning"
  c=16
  echo $a
  echo $b
  echo $c
  echo "$b! I have $c apples."

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Special attributes

Declare vars with special attributes

  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

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Special Variables

  echo $PWD # Returns current directory
  echo $HOME  # Returns home directory
  echo $MACHTYPE  # Returns machine type eg. Mac x86_64-apple-darwin12
  echo $HOSTNAME # Host name
  echo $BASH_VERSION
  echo $SECONDS # seconds of the session open

Run commands inside variables.

  a=$(pwd) 
  echo $a # current directory

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Arithmetic

To do arithmetic operations we need to wrap into (( )).

Only integers are supported.

  echo $((2+3)) # 5
  d=2
  e=$((d+2)) # 4
  ((e++)) # 5
  ((e+=4))

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Comparison operations

[[ expression ]] to run comparison expressions

Result is

• 0: TRUE
• 1: FALSE

Non-numeric comparison

The following work for non-numeric values:

Compare strings

#!/bin/bash
# this is a bash script
echo working with Comparison operators
[[ "cat" == "cat" ]]
echo $? 
# 0

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Integer comparison

For working with Integers use the following:

Compare integers

[[ 10 -gt 2 ]]
echo $? 
# 0

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Logic Operators

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String Null Value

Check if string is null

echo is string null?
a=""
b="cat"
[[ -z $a ]]
echo $?
[[ -z $b ]]
echo $?

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File operators

More operators

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Working with Strings

Concat strings

#!/bin/bash
# this is a bash script
echo Working with Strings
echo
echo Concat strings
a="hello"
b="world"
c=$a$b
echo $c # helloworld

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Length of strings

echo 'Length of strings use #'
echo ${#a} # 5
echo

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Substring

echo Get the substring 
d=${c:3} # specify the starting point for the substring
echo $d # loworld

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With certain length

echo Get substring with certain length
e=${c:3:4} # in this case 4 is the length of the substring
echo $e # lowo

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Starting from the end

echo Get substring starting from the end of the string
f=${c: -4} # get the last 4 chars of the string
echo $f # orld
echo Now get the first 3 letters of the last 4 letters
g=${c: -4:3} 
echo $g # orl
echo

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Replace string

echo Replace string
fruits="apple banana kiwi cherry"
echo ${fruits/banana/melon} # replace banana with melon

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Coloring and Styling text

Styled text tput

tput is a command that allows styling and coloring text

Styles

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Colors

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Example script

#!/bin/bash
# this is a bash script
echo 'Colors & Styles'
echo
flashred=$(tput setab 7; tput setaf 1; tput blink)
red=$(tput setaf 1)
none=$(tput sgr0)
echo $flashred"ERROR:"$none$red" Something went wrong."$none

To see a list of tput commands type man terminfo

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Date

Date is not part of bash but it ships with Unix

#!/bin/bash
# this is a bash script
echo 'DATE'
echo
echo 'Use the operator + to specify the date format'
date +"%d-%m-%Y" # 30-may-1980 (day month year)
date +"%H-%M-%S" # 20-40-24 (hour minutes seconds)

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Printf

It is like echo but with more features
Printf comes with Unix, not bash, so you can save it with -v to a variable and print it with echo.

#!/bin/bash
# this is a bash script
echo 'Printf'
echo
printf "Name:\t%s\nID:\t%04d\n" "Jorge" "27"
# Name:   Jorge
# ID:     0027
echo
echo
# %s and %04d indicate where to print the values Jorge and 27
today=$(date +"%d-%m-%Y")
time=$(date +"%H:%M:%S")
printf -v d "Current User:\t%s\nDate:\t\t%s @ %s\n" $USER $today $time
echo "$d"
# Current User: user
# Date:         20-03-2015 @ 10:23:42

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Arrays

Arrays don’t need commas.
They are initialized with () and called with [].

These arrays behave like JavaScript where the size is flexible.

#!/bin/bash
# This is a bash script
echo 'Arrays'
echo
a=()
b=("apples" "bananas" "lemons")
echo ${b[2]} # lemons
b[5]="kiwi"
echo ${b[@]} # apples bananas lemons kiwi
b+=("oranges") # add at the end of the array
echo ${b[@] : -1} # show the last element in the array

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Associative Arrays

This type of arrays is more like the object literals in JavaScript where the keys are strings instead of numbers.

#!/bin/bash
# This is a bash script
echo 'Associative Arrays'
echo
# NOTE: This just works for Bash version > 4.0
declare -A peopleAges
peopleAges[John]=50
peopleAges["John Wayne"]=79
echo ${peopleAges["John Wayne"]}

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Reading and writing text files

#!/bin/bash
# This is a bash script
echo 'Text Files'
echo
echo "Some Text" > file.txt # creates or overwrites file.txt with the string "Some Text"
echo "Add this text" >> file.txt # Appends the string to the end of file.txt
> file.txt  # empties file.txt
echo "Add this text" >> file.txt # Appends the string to the end of file.txt
echo "text2" >> file.txt 
echo "text3" >> file.txt 
# this loop reads each line in file.txt
while read f; do
    echo $f
done < file.txt

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Using a text file as an input for a command

ftp.txt

open mirrors.xmission.com
user anonymous nothinghere
ascii
cd gutenberg
get GUTINDEX.00

Using the file.txt as input

#!/bin/bash
# This is a bash script
echo 'Text File as input for a command'
echo
echo 'You can use cat to read a file'
cat < file.txt
echo
echo 'You can use any command and use input from a file.'
ftp -n < ftp.txt 
ls # you should see the GUTINDEX.00 file in your current directory

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Here document

You can specify the start and end of an text as input to a command with a keyword.
In the following script you can see that it uses a text blob for cat and then another one for ftp.

 #!/bin/bash
# This is a bash script
echo 'Here documents'
echo
echo 'Using here document to know until when to print cat.'
cat << EndOfText
This is a 
multiline 
text string
EndOfText
# cat will use input until it sees EndOfText #
echo
echo 'Using tabs with the dash flag'
ftp -n <<- DoneWithTheUpdate
    open mirrors.xmission.com
    user anonymous nothinghere
    ascii
    cd gutenberg
    get GUTINDEX.01
    bye
DoneWithTheUpdate
# this marks the end of the input for the command ftp #

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The if statement

#!/bin/bash
# The if statement
echo 'if statement'
a=2000
if [ $a -gt 1000 ]
then
    echo $a is greater than 1000!
else
    echo $a is not greater than 1000
fi

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The if statement using regular expressions.

echo 'if statement using regular expressions'
s="This is 1 string!"
if [[ $s =~ [0-9]+ ]]; then     # Note the double square brackets are used because Regex also uses them.
    echo "There are some numbers in the string: $s"
else
    echo "There are no numbers in the string: $s"
fi

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The while loop

#!/bin/bash
# The while loop
i=0
while [ $i -le 9 ] ; do
    echo i:$i
    ((i+=1))
done
# result
# i:0
# i:1
# i:2
# i:3
# i:4
# i:5
# i:6
# i:7
# i:8
# i:9

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The until loop

The oposite of the while loop.

# the Until loop
j=0
until [ $j -ge 10 ]; do
    echo j:$j
    ((j+=1))
done
# result
# j:0
# j:1
# j:2
# j:3
# j:4
# j:5
# j:6
# j:7
# j:8
# j:9

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The for loop

for i in 1 2 3
do
    echo $i
done

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With brace expansion

for j in {1..20}
do
    echo $j
done

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Like in C languages

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

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Looping through an array

arr=("apple" "banana" "lemon")
for i in ${arr[@]}
do
    echo $i
done

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Use the output of a command

for i in $(ls)
do
    echo $i
done

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Switch case

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

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Functions

#!/bin/bash
# Function
function greet {
    echo "Hello $1 good $2"
}
echo "And now meeting"
greet Jorge morning
You can exit the function with `return` or exit the script with `exit 0`

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With unspecified parameters

function listallthings {
    i=0
    for f in $@; do
        echo $i: $f
        ((i+=1))
    done
}
listallthings $(ls)

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Arguments

for i in $@; do
    echo $i
done
echo "there are $# arguments"

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Flags

The colon after the flag means that the flag is required.
The first colon means that any unknown flag will be passed too.
In the following case it says that the flags u and p are required. The a and b are optional.

#!/bin/bash
# Flags
while getopts :u:p:ab option; do
    case $option in
        u) user=$OPTARG;;
        p) pass=$OPTARG;;
        a) echo "Got the A flag";;
        b) echo "Got the B flag";;
        ?) echo "Unknown flag";;
    esac
done
echo "User $user password $pass"

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Input during execution

More flads for read command

#!/bin/bash
echo "What is your name?"
read name
echo "What is your password?"
read -s pass  # -s is secret mode so it won't display the input
read -p "What is your favorite animal? " animal  # -p   allows to use inline input
echo name: $name pass: $pass animal: $animal

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Input with Select

Similar to HTML Select/option. It only accepts numbers as input.

#!/bin/bash
# Select
select option in "cat" "dog" "quit"
do
    case $option in
        cat) echo "Cats like to sleep";;
        dog) echo "Dogs like to play";;
        quit) break;;
        *) echo "Unknown command";;
    esac
done

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Ensuring a response

Patterns to ensure a response from the user

Requiring a minimum of arguments

if [ $# -lt 3 ] ; then
    cat <<- EOM
    This command requires three arguments:
    username, userid and favorite number.
    EOM
else
    # the program goes here
    echo "Username: $1"
    echo "UserID: $2"
    echo "Favorite Number: $3"
fi

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Requiring an input

#!/bin/bash
# Ensuring - pattern 2
read -p "Favorite animal? " a
while [[ -z $a ]] ; do
    read -p "I need an answer! " a
done
echo "$a was selected"

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Validating input with regular expressions

#!/bin/bash
# Ensuring - pattern 3
read -p "What year? [nnnn] " a
while [[ ! $a =~ [0-9]{4} ]]; do
    read -p "A year, please! [nnnn] " a
done
echo "Selected year: $a"

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Import files

In this case we are importing config/colors.sh

source config/colors.sh

Find directories

find / -type d -name 'httpdocs

The first parameter / is where to look

-name could be -iname to ignore case

also -type is not mandatory

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