Advanced Bash−Scripting Guide

Chapter 32. Gotchas
Turandot: Gli enigmi sono tre, la morte una!
Caleph: No, no! Gli enigmi sono tre, una la vita!
Puccini
Assigning reserved words or characters to variable names.
case=value0 # Causes problems.
23skidoo=value1 # Also problems.
# Variable names starting with a digit are reserved by the shell.
# Try _23skidoo=value1. Starting variables with an underscore is o.k.
# However... using just the underscore will not work.
_=25
echo $_
# $_ is a special variable set to last arg of last command.
xyz((!*=value2
# Causes severe problems.
Using a hyphen or other reserved characters in a variable name.
var−1=23
# Use 'var_1' instead.
Using the same name for a variable and a function. This can make a script difficult to understand.
do_something ()
{
echo "This function does something with \"$1\"."
}
do_something=do_something
do_something do_something
# All this is legal, but highly confusing.
Using whitespace inappropriately. In contrast to other programming languages, Bash can be quite finicky
about whitespace.
var1 = 23 # 'var1=23' is correct.
# On line above, Bash attempts to execute command "var1"
# with the arguments "=" and "23".
let c = $a − $b
# 'let c=$a−$b' or 'let "c = $a − $b"' are correct.
if [ $a −le 5] # if [ $a −le 5 ] is correct.
# if [ "$a" −le 5 ] is even better.
# [[ $a −le 5 ]] also works.
Assuming uninitialized variables (variables before a value is assigned to them) are "zeroed out". An
uninitialized variable has a value of "null", not zero.
#!/bin/bash
Chapter 32. Gotchas 327