Using Numeric Operators and Variable Assignment in Shell Scripts

The expr command provides basic arithmetic in shell scripts; its syntax is expr variable1 operator variable2 and requires spaces around the operator.

Examples:

A=10<br/>B=20<br/>expr $A + $B   # outputs 30<br/>expr $A - $B   # outputs -10<br/>expr $A \* $B  # outputs 200 (asterisk must be escaped)<br/>expr $A / $B   # outputs 0<br/>expr $A % $B   # outputs 10

Results can be stored in a variable using command substitution: abc=$(expr $A + $B)<br/>echo $abc # 30 Command substitution also allows extracting data, such as an IP address:

IPADDR=$(ifconfig eth0 | grep "inet" | awk '{print $2}')<br/>echo $IPADDR   # e.g., 10.141.113.216

Quoting rules are demonstrated: double quotes preserve spaces and allow variable expansion, while single quotes treat everything literally and require escaping for embedded single quotes.

webserver="nginx 1.11"<br/>echo $webserver   # nginx 1.11<br/>webserver='nginx 1.11'<br/>echo $webserver   # nginx 1.11

Backticks (``) perform command substitution as well, useful for nesting commands, though nesting can become cumbersome:

pwd<br/>which pwd<br/>rpm -qf $(which pwd)   # coreutils-8.22-15.el7.x86_64<br/>rpm -q `rpm -qf \\`which pwd\\``   # demonstrates nesting difficulty