Shell Parameter and Variable Expansion
The
"$" character introduces parameter expansion, command substitution, or arithmetic expansion.
e.g
[root@localhost ~]# echo $SHELL
/bin/bash
[root@localhost ~]# echo ${SHELL}
/bin/bash
The basic form of parameter expansion is
"${PARAMETER}". The value of
"PARAMETER" is substituted. The braces are required when
"PARAMETER" is a positional parameter with
more than one digit, or when
"PARAMETER" is
followed by a character that is not to be interpreted as part of its name.
Length of Variable
[root@localhost ~]# echo $SHELL
/bin/bash
[root@localhost ~]# echo ${#SHELL}
9
[root@localhost ~]# declare -i a=123
[root@localhost ~]# declare -p a
declare -i a="123"
[root@localhost ~]# echo ${#a}
3
Indirect Expansion
If the first character of
"PARAMETER" is an exclamation point, Bash uses the value of the variable formed from the rest of
"PARAMETER"
as the name of the variable; this variable is then expanded and that
value is used in the rest of the substitution, rather than the value of
"PARAMETER" itself. This is known as
indirect expansion.
[root@localhost ~]# echo ${!SH*}
SHELL SHELLOPTS SHLVL
Command Substitution
$(COMMAND) or `COMMAND`
[root@localhost ~]# date
Thu Jan 8 17:01:43 PST 2015
[root@localhost ~]# echo date
date
[root@localhost ~]# echo `date`
Thu Jan 8 17:01:51 PST 2015
[root@localhost ~]# echo This is the date: `date`
This is the date: Thu Jan 8 17:01:56 PST 2015
Arithmetic Expansion
BASH only supports interger arithmetic
[root@localhost ~]# product=$((3.6*4.9))
-bash: 3.6*4.9: syntax error: invalid arithmetic operator (error token is ".6*4.9")
Interpret as command expansion
[root@localhost ~]# product=$("36*49")
-bash: 36*49: command not found
[root@localhost ~]# product=$(36*49)
-bash: 36*49: command not found
[root@localhost ~]# product=$((36*49))
[root@localhost ~]# echo $product
1764
[root@localhost ~]# expr 4 + 5
9
[root@localhost ~]# a=$(expr 4 + 5)
[root@localhost ~]# echo $a
9
Arithmetic for decimal number
- bc
[root@localhost ~]# echo "scale=100;3.222553*8.77777" | bc
28.28682904681
[root@localhost ~]# echo "3.222553*8.77777" | bc
28.286829
[root@localhost ~]# echo "scale=2;3/8" | bc
.37
[root@localhost ~]# echo "scale=5;3/8" | bc
.37500
[root@localhost ~]# echo "scale=1;3/8" | bc
.3
[root@localhost ~]# echo "scale=2;3/8" | bc
.37
[root@localhost ~]# echo "3/8" | bc
0
