shell 的变量
#!/bin/bash
# ex9.sh
# Variables: assignment and substitution
a=375
hello=$a
# ^ ^
#-------------------------------------------------------------------------
# No space permitted on either side of = sign when initializing variables.
# What happens if there is a space?
# "VARIABLE =value"
# ^
#% Script tries to run "VARIABLE" command with one argument, "=value".
# "VARIABLE= value"
# ^
#% Script tries to run "value" command with
#+ the environmental variable "VARIABLE" set to "".
#-------------------------------------------------------------------------
echo hello # hello
# Not a variable reference, just the string "hello" ...
echo $hello # 375
# ^ This *is* a variable reference.
echo ${hello} # 375
# Likewise a variable reference, as above.
# Quoting . . .
echo "$hello" # 375
echo "${hello}" # 375
echo
hello="A B C D"
echo $hello # A B C D
echo "$hello" # A B C D
# As we see, echo $hello and echo "$hello" give different results.
# =======================================
# Quoting a variable preserves whitespace.
# =======================================
echo
echo '$hello' # $hello
# ^ ^
# Variable referencing disabled (escaped) by single quotes,
#+ which causes the "$" to be interpreted literally.
# Notice the effect of different types of quoting.
hello= # Setting it to a null value.
echo "\$hello (null value) = $hello" # $hello (null value) =
# Note that setting a variable to a null value is not the same as
#+ unsetting it, although the end result is the same (see below).
# --------------------------------------------------------------
# It is permissible to set multiple variables on the same line,
#+ if separated by white space.
# Caution, this may reduce legibility, and may not be portable.
var1=21 var2=22 var3=$V3
echo
echo "var1=$var1 var2=$var2 var3=$var3"
# May cause problems with legacy versions of "sh" . . .
# --------------------------------------------------------------
echo; echo
numbers="one two three"
# ^ ^
other_numbers="1 2 3"
# ^ ^
# If there is whitespace embedded within a variable,
#+ then quotes are necessary.
# other_numbers=1 2 3 # Gives an error message.
echo "numbers = $numbers"
echo "other_numbers = $other_numbers" # other_numbers = 1 2 3
# Escaping the whitespace also works.
mixed_bag=2\ ---\ Whatever
# ^ ^ Space after escape (\).
echo "$mixed_bag" # 2 --- Whatever
echo; echo
echo "uninitialized_variable = $uninitialized_variable"
# Uninitialized variable has null value (no value at all!).
uninitialized_variable= # Declaring, but not initializing it --
#+ same as setting it to a null value, as above.
echo "uninitialized_variable = $uninitialized_variable"
# It still has a null value.
uninitialized_variable=23 # Set it.
unset uninitialized_variable # Unset it.
echo "uninitialized_variable = $uninitialized_variable"
# uninitialized_variable =
# It still has a null value.
echo
exit 0
对比区别：
newer@ubuntu:~/script$ a=`ls -l`;echo $a
total 20 -rwxr-xr-x 1 root root 200 Nov 17 17:02 1.sh -rwxrwxr-x 1 newer newer 26 Nov 17 21:13 2.sh -rwxr-xr-x 1 root root 182 Nov 15 05:35 bianliang.sh -rwxr-xr-x 1 root root 112 Nov 14 17:12 case.sh -rwxr-xr-x 1 root root 2093 Nov 15 05:50 clean.sh
newer@ubuntu:~/script$ a=`echo hello`;echo "$a"
hello
newer@ubuntu:~/script$ a=`ls -l`;echo "$a"
total 20
-rwxr-xr-x 1 root root 200 Nov 17 17:02 1.sh
-rwxrwxr-x 1 newer newer 26 Nov 17 21:13 2.sh
-rwxr-xr-x 1 root root 182 Nov 15 05:35 bianliang.sh
-rwxr-xr-x 1 root root 112 Nov 14 17:12 case.sh
-rwxr-xr-x 1 root root 2093 Nov 15 05:50 clean.sh
除了反引号之外，还可以使用（）对变量进行赋值。
newer@ubuntu:~/script$ arch=$(uname -a);echo $arch
Linux ubuntu 4.2.0-27-generic #32~14.04.1-Ubuntu SMP Fri Jan 22 15:32:26 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
判断变量是否被初始化
if [ -z "$unassigned" ]
then
echo "\$unassigned is NULL."
fi # $unassigned is NULL.
