Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
echo
exit 0
An uninitialized variable has a "null" value − no assigned value at all (not zero!).
Using a variable before assigning a value to it will usually cause problems.
It is nevertheless possible to perform arithmetic operations on an uninitialized
variable.
echo "$uninitialized"
# (blank line)
let "uninitialized += 5"
# Add 5 to it.
echo "$uninitialized" # 5
# Conclusion:
# An uninitialized variable has no value, however
#+ it acts as if it were 0 in an arithmetic operation.
# This is undocumented (and probably non−portable) behavior.
See also Example 11−19.
4.2. Variable Assignment
=
the assignment operator (no space before & after)
Do not confuse this with = and −eq, which test, rather than assign!
Note that = can be either an assignment or a test operator, depending on context.
Example 4−2. Plain Variable Assignment
#!/bin/bash
# Naked variables
echo
# When is a variable "naked", i.e., lacking the '$' in front?
# When it is being assigned, rather than referenced.
# Assignment
a=879
echo "The value of \"a\" is $a."
# Assignment using 'let'
let a=16+5
echo "The value of \"a\" is now $a."
echo
# In a 'for' loop (really, a type of disguised assignment)
echo −n "Values of \"a\" in the loop are: "
for a in 7 8 9 11
Chapter 4. Introduction to Variables and Parameters 25