bash.md

bash

Bourne Again Shell

Kernel and Shell

• Kernel is the Core of the OS, OSX uses the Mach Kernel
• Shell is the outer layer of the OS, the interface to communicate with the Kernel, OSX uses the bash shell. You can switch between shells, it's just the interface - not like the OS

Commands

• Echo - echo <statement> print to stdout

  • echo hello $name is ok, but special chars must be escaped
  • echo 'hello $name' will print literally, not interpreting the variable
  • echo "hello $name" will interpret the var and escape characters
  • echo (without args) prints an empty line
  • echo -e \<escaped_sequence>m - for colors and styles
    • e.g. color echo -e '\033[34;42mColored Text\033[0m' - 34 means blue font, 42 means green for background
    • e.g. underline echo -e '\033[4;31mError\033[0m' - 4 is underline, 0 at the end resets styles
    • you can use tput for a more verbose way of defining styles, e.g. echo -e $(tput setaf 1)"red string"
      • tput sgr0 resets styles

• Viewing files

  • more <file> - browse file page by page
  • head <file> - print first lines
  • tail <file> - print last lines

• Expansions

  • brace expansion: touch file_{0..1000} creates 1001 files, echo {1..10..2} - prints ints from 1 to 10, counting by 2s (1,3,5,7,9)

• Piping - pipe takes the result of one command and pipes it into another one, e.g. ls | more displays list of files with pagination

• Redirection - >, works with stdin, stdout, and stderr. stdout and stderr can be represented by 1 and 2,

  • i.e.: cp -v * ../somefolder 1>../success.txt 2> ../error.txt copies everything from current dir (with verbosity), but prints stdout and stderr to files, not to terminal - it's redirecting
  • 1 and 2 can be together represented by &
  • redirecting to /dev/null trashes the output

• grep - use as grep <pattern> <file>, option -i makes it case-insensitive

Scripting

• start with hashbang aka shebang - #!/bin/bash - which specifies the path to bash executable
• commented lines start with #
• chmod +x <filename> to make the script executable
• assign the result of a command to a variable - where=$(pwd)

Variables

• name=Walpurgia, town="Little Bindle", len=134 declare a variable
• declare -i d=32 marks that variable as an integer
• declare -r d=32 marks that variable as read-only
• declare -l d=LsadfW # => lsadfw lowercases
• declare -u d=Lssdfw # => LSSDFW uppercases
• $name calls a variable

Special variables

• $HOME, $PWD, $PATH, $HOSTNAME, $BASH_VERSION
• $MACHTYPE returns the machine type
• $SECONDS returns no. of seconds the bash session has been running (in script, from when the script started)
• $0 returns the name of the script

Math

• bash only works with integers, but you can use bc program to get floats - echo 1/3 | bc -l
• arithmetic operations must be enclosed as ((expression))
• assign to a variable - four=$((3+1))
• Operators:
  • /, *, +, -, %
  • ** is exponentiation
  • ++, --, +=, -=, *=, /=

Comparison operations

• [[ expression ]] return 1 or 0, 0 means success, 1 is failure
• [[ $a < $b ]], and others: >, >=, <=, ==, !=
• logic - [[ $a && $b ]] and ||, and [[ ! $b ]]
• [[ -z $a ]] - is it null?
• [[ -n $a ]] - is it not null?
• [[ $string =~ [<regexp>] ]] - regex

String operations

• concatenation: $c=$a$b
• character count: ${#c}
• substring:
  • d=${c:3} - d is c from 3rd char to end
  • e=${c:3:10} - chars 3 to 10
• replace: ${<stringname>/<searchterm>/<replacewith>}, e.g. ${name/Bogdan/Peter}
  • to replace all instances double the first /
  • replace only if search term is at the beginning - add # before search term
  • replace only if at the end - % before search term
  • wildcards - ${name/B*/Nikolai}

Handy helpers

• date formatting - date +"<format string>", e.g. date +"%d, %m, %Y"
• printf -v d assigns the output to variable d
• printf - lets you print out data with a particular format - e.g. printf "Name:\t%s\nID:\t%04d\n" "Adam" "43" will print

Name:	Adam
ID:	  0043

Arrays

• fruits=("banana" "apple" "pear"),
  • retrieve: echo ${fruits[0]} (whole array - echo ${fruits[@]}, last elem. - echo ${fruits[@]: -1})
  • set: fruits[1]="orange"
  • push: fruits+="kiwi"
  • get length: echo ${#fruits[@]}
• Associative arrays (key-value pairs) - bash version >=4

declare -A myarray
myarray[color]=blue
myarray["Home Address"]="12 James Street"
echo ${myarray[color]}

Files, but not binaries

• write to file echo "Some text" > file.txt
• read a file cat < file.txt
• zero out a file > file.txt
• append to file echo "Some more text" >> file.txt

Control structures

• 'if' statement

if <expression>
then
      echo "True"
elif <expression 2>; then
      echo "First was false, this is true"
fi

• 'while' loop, can be reversed, as with 'until' instead of while

while <expression>; do
      # ...
done

• 'for in' and 'for' loops

for <variable> in <collection>
do
    # ...
done

for (( i=0; i<=10; i++ ))
do
    # ...
done

# with an associative aray:
for key in "${!myarray[@]}"
do
    echo "$key has value ${myarray[$key]}"
done

• case

a="dog"
case $a in
      cat) echo "Feline";;
      dog|puppy) echo "Canine";;
      *) echo "No match";;
esac

Functions

function greet {
  echo "Hello, $1"
}
greet Pedro # Hello, Pedro

• $@ is an array with all arguments

User input

• $1, $2, etc. are arguments passed to the script
• $# contains the number of arguments

Flags

# here colons after flags mean we expect the values fo be there, they are not optional
# colon at the beginning of flags list means they can be additional flags, then represented by '?)' in case
while getopts u:p: option
do
  case $option in
    u) user=$OPTARG;;
    p) pass=$OPTARG;;
  esac
done
echo "User: $user, password: $pass"

Input - read keyword

echo "What is your name?"
read name
echo "$name, that's a nice name"

• read -s password, -s is 'silent' flag
• in one line: read -p "Why me?" reason

Select

select animal in "cat" "dog" "bird"
do
  echo "You've selected $animal"
  break
done

Prompting and defaults

if [ $# -lt 3 ]; then
  echo "This command requires three arguments"
else
  echo "All right, carry on"
fi

read -p "Favourite writer? [Vonnegut] " a
# here we test for not null, but we can test regex or sth else as well
while [[ -z "$a" ]]; do
  a="Vonnegut"
done
echo "$a was selected"