Macintosh Terminal Pocket Guide

Variables

We described shell variables in Shell variables:

➜ MYVAR=6
➜ echo $MYVAR
6

All values held in variables are strings, but if they are numeric, the shell will treat them as numbers when appropriate:

➜ NUMBER="10"
➜ expr $NUMBER + 5
15

When you refer to a variable’s value in a shell script, it’s a good idea to surround it with double quotes to prevent certain runtime errors. An undefined variable, or a variable with spaces in its value, will evaluate to something unexpected if not surrounded by quotes, causing your script to malfunction:

➜ FILENAME="My Document"            Space in the name
➜ ls $FILENAME                      Try to list it
ls: My: No such file or directory   Oops! ls saw 2 arguments
ls: Document: No such file or directory
➜ ls -l "$FILENAME"                 List it properly
My Document                         ls saw only 1 argument

If a variable name is evaluated adjacent to another string, surround it with curly braces to prevent unexpected behavior:

➜ NAME="apple"
➜ echo "The plural of $NAME is $NAMEs"
The plural of apple is             Oops! No variable “NAMEs”
➜ echo  "The plural of $NAME is ${NAME}s"
The plural of apple is apples      What we wanted