newswim / learnyoubash_notes Goto Github PK

IMPORTANT NOTES ABOUT BASH

File set-up

First line of the file should contain #!/usr/bin/env bash. This tells the system what program to use (in this case, Bash).

To make a file executable, change its read/write values..

chmod +x file.sh

Variables

Only two data types (or more appropriately, zero data types) -- Strings and Numbers

• To declare a variables, use = with no spaces.
• To display a variable, use the echo command.
• To delete a variable, use the unset command.

foo="Andy"
echo $foo       # > Andy
unset foo       # deleted

Single Quotes vs. Double Quotes

• Variables contained within double quotes will be expanded / evaluated.
• This is not the case with single quoted strings.

NAME="Denys"
echo "My name is $NAME"   # > My name is Denys
echo 'My name is $NAME'   # > My name is $NAME

Environmental Variable

To expose a variable beyond the file scope, declare its value with export.

export MY_GLOBAL="see/me/from/anywhere"

note: There are a lot of global variables in bash. You will meet these variables fairly often, so here is a quick lookup table with the most practical ones:

Positional Parameters

Allocated when the function is evaluated. Here's a list:

In this example, ./script.sh foo bar

# the parameter values would be...

# $0='./script.sh'
# $1='foo'
# $2='bar'

Default parameter values

Variables/parameters can have default values defined with {VAL:-'defaultValue'} syntax.

# Assign FOO value 'default' if it's empty
FOO=${FOO:-'default'}