1. Command Substitution
Syntax:
`command`
Ex:
#!/bin/sh
DATE=`date`
echo "Date is $DATE"
USERS=`who | wc -l`
echo "Logged in user are $USERS"
UP=`date ; uptime`
echo "Uptime is $UP"
$./test.sh
Date is Thu Jul 2 03:59:57 MST 2009
Logged in user are 1
Uptime is Thu Jul 2 03:59:57 MST 2009
03:59:57 up 20 days, 14:03, 1 user, load avg: 0.13, 0.07, 0.15
2. Variable Substitution
| Form | Description |
|---|---|
| ${var} | Substitue the value of var. |
| ${var:-word} | If var is null or unset, word is substituted for var. The value ofvar does not change. |
| ${var:=word} | If var is null or unset, var is set to the value of word. |
| ${var:?message} | If var is null or unset, message is printed to standard error. This checks that variables are set correctly. |
| ${var:+word} | If var is set, word is substituted for var. The value of var does not change. |
Ex:
#!/bin/sh
echo ${var:-"Variable is not set"}
echo "1 - Value of var is ${var}"
echo ${var:="Variable is not set"}
echo "2 - Value of var is ${var}"
unset var
echo ${var:+"This is default value"}
echo "3 - Value of var is $var"
var="Prefix"
echo ${var:+"This is default value"}
echo "4 - Value of var is $var"
echo ${var:?"Print this message"}
echo "5 - Value of var is ${var}"
$./test.sh
Variable is not set
1 - Value of var is
Variable is not set
2 - Value of var is Variable is not set
3 - Value of var is
This is default value
4 - Value of var is Prefix
Prefix
5 - Value of var is Prefix