bash

Table of Contents Overview Shorcuts Hello, world ! Aliases Command prompt Source Basics Strings Control flow operators File descriptors Function Return code Quickies

Overview

Using bash:

• Google Bash coding conventions
• Bash advanced Guide

Default config file is ~/.bashrc , loaded at terminal start

Shorcuts

Shortcuts

Hello, world !

Create source file helloworld.sh

#!/bin/bash
echo "hello world"

Execute:

• sh helloworld.sh
• sudo chmod +x helloworld.sh; ./helloworld.sh

Aliases

Overview

• list alias
• add alias ALIAS_NAME='COMMAND'
• remove unalias ALIAS_NAME
• run without alias \command
  
• define in terminal:
  • add to specific rc file
  • can be included in .bash_aliases
  • test without closing termninal source .bash_aliases

Command prompt

Alter PS1 variable in .bashrc
PS1='\u \w $ ' => USER_NAME WORKING_DIRECTORY $

To format the prompt, use ASCII escape sequence

Use escape

Open	Body	Close
\[	ESCAPE_SEQUENCE	\]

Here's the codes

Escape sequence

Need	Syntax	Full (use around $foo)
red	\e[31m	\[\e[31m$foo\]
blue	\e[36m	\[\e[36m$foo\
bold	\e[1m	\[\e[1m$foo\
reset	\e[0m	No use with $foo

Always add \[\e[0m\] at end of PS1

Full list of escape codes

To get local-function feedback PS1='$(FUNCTION_NAME) :'

To dynamically return color-code

set_bash_prompt(){
    PS1="[\w] $(COLORCODING_FUNCTION_NAME) \$ "
}
PROMPT_COMMAND=set_bash_prompt

Source

constants
functions
main_function

Basics

Variable

Need	Syntax	Output
Declare variable (optional)	def variable_name
Assign variable	variable_name=Foo
Use variable	echo value of variable_name is $variable_name	value of variable_name is Foo

Strings

Strings

Need	Syntax	Output
Check if string is null	[[ -z "${variable_name}" ]]
Check if string is not null	[[ -n "${variable_name}" ]]

Control flow

Need	Syntax
Test	if [ CONDITION ] then ACTION else ACTION fi
Infinite loop	while true; do foo; sleep 2; done

operators

List:

• -eq : is equal to
• -ne : is not equal to
• -gt : is greater than
• -ge : is greater than or equal to
• -lt : is less than
• -le : is less than or equal to

File descriptors

Description

Name	Number
0	stdin
1	stdout
2	stderr

Use

Need	Syntax
Redirect and append stdout to file FILENAME	1>>filename
Redirect both stdout and stderr to file FILENAME	&>FILENAME
Redirects stderr to stdout.	2>&1

More on this

Function

Function

Need	Syntax
Declare	function function_name() { (..) }
Access parameter	if [[ $1 = "foo" ]]
Call with parameter	$bar = function_name $foo

Return code

Convention:

• successful: 0
• error: 1
• specific error: 2 to 255, see here

List:

• set return code:exit $RETURN_CODE
• get return code: $?

Quickies

List:

• execute a command frequently: while sleep <TIME_SECONDS>; do <COMMAND>; done;