Advanced Bash-shell Guide (Version 10) study notes two

Variable substitution
$variable is a shorthand for ${variable}
hello= "A B C D"
Echo $hello # A B C D
echo "$hello" # A B C D
The quotation mark preserves the whitespace inside the variable

1 echo "$uninitialized" # (blank line)
2 Let "uninitialized + = 5" # ADD 5 to it.
3 echo "$uninitialized"
Uninitialized variable is null, but equals 0 in an arithmetic expression
Command substitution
A= ' ls-l ' # assigns result of ' ls-l ' command to ' a '
Echo $a
Or
a=$ (LS-L)
Echo $a

string + Integer equals integer, string equivalent to 0
But when divided by a null variable, the system error.

Variable type
Local variables
Visible in code block
Environment variables
Variables that affect the shell and user interface
Position parameters
Take the last parameter
1 args=$# # Number of args passed.
2 Lastarg=${!args}
Or
lastarg=${!#}
[...] and [[...]]
use [[...]] can block logic errors that are common in scripts compared to []
For example: &&, | |, <, and > Operations

When you do arithmetic, [[...]] Octal and hexadecimal are automatically calculated and []
You can not
5 decimal=15
6 octal=017 # = (decimal)
7 hex=0x0f # = (decimal)
8
9 If ["$decimal"-eq "$octal"]
Ten Then
echo "$decimal equals $octal"
Else
echo "$decimal isn't equal to $octal" # Equal to 017
+ fi # doesn ' t evaluate within [single brackets]!


if [["$decimal"-eq "$octal"]]
Then
echo "$decimal equals $octal" # equals 017
Else
echo "$decimal is not equal to $octal"
FI # evaluates within [[double brackets]]!

((...)) Arithmetic test
If the expression evaluates to 0, its exit status code is 1 or False
If the expression evaluates to non-0, its exit status code is 0 or True
Its exit status and [...] Instead


Echo ${aa##*/} Gets the file name of the variable AA with the/bit delimiter
${filename##*.} ! = "GZ"
filename##*. Gets the filename extension


Infinite Monkeys
AppMakr
String manipulation

Get string length
${#string}
Length of expr $string
Expr "$string": '. * '

Gets the length of a string that matches from the beginning of the string
Expr match "$string" ' $substring '
Expr "$string": ' $substring '
$substring is a regular expression

Gets the position of the substring starting in the string
Expr index $string $substring

Remove substring
${string:position}
${string:position:length}

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Advanced Bash-shell Guide (Version 10) study notes two