第 28 章. 间接引用
我们已经看到引用变量，$var，获取它的值。但是，值的 value 呢？例如$$var?
实际的表示法是\$$var，通常前面会加上 eval (有时是 echo)。这被称为间接引用。
示例 28-1. 间接变量引用
#!/bin/bash
# ind-ref.sh: Indirect variable referencing.
# Accessing the contents of the contents of a variable.
# First, let's fool around a little.
var=23
echo "\$var = $var" # $var = 23
# So far, everything as expected. But ...
echo "\$\$var = $$var" # $$var = 4570var
# Not useful ...
# \$\$ expanded to PID of the script
# -- refer to the entry on the $$ variable --
#+ and "var" is echoed as plain text.
# (Thank you, Jakob Bohm, for pointing this out.)
echo "\\\$\$var = \$$var" # \$$var = $23
# As expected. The first $ is escaped and pasted on to
#+ the value of var ($var = 23 ).
# Meaningful, but still not useful.
# Now, let's start over and do it the right way.
# ============================================== #
a=letter_of_alphabet # Variable "a" holds the name of another variable.
letter_of_alphabet=z
echo
# Direct reference.
echo "a = $a" # a = letter_of_alphabet
# Indirect reference.
eval a=\$$a
# ^^^ Forcing an eval(uation), and ...
# ^ Escaping the first $ ...
# ------------------------------------------------------------------------
# The 'eval' forces an update of $a, sets it to the updated value of \$$a.
# So, we see why 'eval' so often shows up in indirect reference notation.
# ------------------------------------------------------------------------
echo "Now a = $a" # Now a = z
echo
# Now, let's try changing the second-order reference.
t=table_cell_3
table_cell_3=24
echo "\"table_cell_3\" = $table_cell_3" # "table_cell_3" = 24
echo -n "dereferenced \"t\" = "; eval echo \$$t # dereferenced "t" = 24
# In this simple case, the following also works (why?).
# eval t=\$$t; echo "\"t\" = $t"
echo
t=table_cell_3
NEW_VAL=387
table_cell_3=$NEW_VAL
echo "Changing value of \"table_cell_3\" to $NEW_VAL."
echo "\"table_cell_3\" now $table_cell_3"
echo -n "dereferenced \"t\" now "; eval echo \$$t
# "eval" takes the two arguments "echo" and "\$$t" (set equal to $table_cell_3)
echo
# (Thanks, Stephane Chazelas, for clearing up the above behavior.)
# A more straightforward method is the ${!t} notation, discussed in the
#+ "Bash, version 2" section.
# See also ex78.sh.
exit 0 |
变量的间接引用有什么实际用途呢？它赋予了 Bash 一点点 C 语言中指针的功能，例如在 查表中。而且，它还有其他一些非常有趣的应用……
Nils Radtke 展示了如何构建“动态”变量名并评估它们的内容。当source 配置文件时，这可能很有用。
#!/bin/bash
# ---------------------------------------------
# This could be "sourced" from a separate file.
isdnMyProviderRemoteNet=172.16.0.100
isdnYourProviderRemoteNet=10.0.0.10
isdnOnlineService="MyProvider"
# ---------------------------------------------
remoteNet=$(eval "echo \$$(echo isdn${isdnOnlineService}RemoteNet)")
remoteNet=$(eval "echo \$$(echo isdnMyProviderRemoteNet)")
remoteNet=$(eval "echo \$isdnMyProviderRemoteNet")
remoteNet=$(eval "echo $isdnMyProviderRemoteNet")
echo "$remoteNet" # 172.16.0.100
# ================================================================
# And, it gets even better.
# Consider the following snippet given a variable named getSparc,
#+ but no such variable getIa64:
chkMirrorArchs () {
arch="$1";
if [ "$(eval "echo \${$(echo get$(echo -ne $arch |
sed 's/^\(.\).*/\1/g' | tr 'a-z' 'A-Z'; echo $arch |
sed 's/^.\(.*\)/\1/g')):-false}")" = true ]
then
return 0;
else
return 1;
fi;
}
getSparc="true"
unset getIa64
chkMirrorArchs sparc
echo $? # 0
# True
chkMirrorArchs Ia64
echo $? # 1
# False
# Notes:
# -----
# Even the to-be-substituted variable name part is built explicitly.
# The parameters to the chkMirrorArchs calls are all lower case.
# The variable name is composed of two parts: "get" and "Sparc" . . . |
示例 28-2. 将间接引用传递给 awk
#!/bin/bash
# Another version of the "column totaler" script
#+ that adds up a specified column (of numbers) in the target file.
# This one uses indirect references.
ARGS=2
E_WRONGARGS=85
if [ $# -ne "$ARGS" ] # Check for proper number of command-line args.
then
echo "Usage: `basename $0` filename column-number"
exit $E_WRONGARGS
fi
filename=$1 # Name of file to operate on.
column_number=$2 # Which column to total up.
#===== Same as original script, up to this point =====#
# A multi-line awk script is invoked by
# awk "
# ...
# ...
# ...
# "
# Begin awk script.
# -------------------------------------------------
awk "
{ total += \$${column_number} # Indirect reference
}
END {
print total
}
" "$filename"
# Note that awk doesn't need an eval preceding \$$.
# -------------------------------------------------
# End awk script.
# Indirect variable reference avoids the hassles
#+ of referencing a shell variable within the embedded awk script.
# Thanks, Stephane Chazelas.
exit $? |
 | 这种间接引用的方法有点棘手。如果二阶变量改变了它的值，那么一阶变量必须被正确地解引用（如上面的例子中所示）。幸运的是，${!variable}Bash 版本 2 引入的 ${!variable} 符号 (参见 示例 37-2 和 示例 A-22) 使得间接引用更加直观。 |