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# NO REPLACEMENT. |
v4=${v0/%123/000} |
# Matches, but not at end. |
echo "v4 = $v4" |
# abc1234zip1234abc |
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# NO REPLACEMENT. |
exit 0
${!varprefix*}, ${!varprefix@}
Matches all previously declared variables beginning with varprefix.
xyz23=whatever
xyz24=
a=${!xyz*} |
# Expands to names of declared variables beginning with "xyz". |
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echo "a = $a" |
# a = |
xyz23 |
xyz24 |
a=${!xyz@} |
# Same as above. |
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echo "a = $a" |
# a = |
xyz23 |
xyz24 |
#Bash, version 2.04, adds this feature.
9.4.Typing variables: declare or typeset
The declare or typeset builtins (they are exact synonyms) permit restricting the properties of variables. This is a very weak form of the typing available in certain programming languages. The declare command is specific to version 2 or later of Bash. The typeset command also works in ksh scripts.
declare/typeset options
−r readonly
declare −r var1
(declare −r var1 works the same as readonly var1)
This is the rough equivalent of the C const type qualifier. An attempt to change the value of a readonly variable fails with an error message.
−i integer
declare −i number
# The script will treat subsequent occurrences of "number" as an integer.
number=3 |
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echo "number = $number" |
# number = 3 |
number=three
echo "number = $number" # number = 0
# Tries to evaluate "three" as an integer.
Note that certain arithmetic operations are permitted for declared integer variables without the need for expr or let.
−a array
declare −a indices
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The variable indices will be treated as an array.
−f functions
declare −f
A declare −f line with no arguments in a script causes a listing of all the functions previously defined in that script.
declare −f function_name
A declare −f function_name in a script lists just the function named.
−x export
declare −x var3
This declares a variable as available for exporting outside the environment of the script itself.
var=$value
declare −x var3=373
The declare command permits assigning a value to a variable in the same statement as setting its properties.
Example 9−17. Using declare to type variables
#!/bin/bash
func1 ()
{
echo This is a function.
}
declare −f |
# Lists the function above. |
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echo |
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declare −i var1 |
# var1 is an integer. |
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var1=2367 |
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echo "var1 declared as $var1" |
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var1=var1+1 |
# Integer declaration eliminates the need for 'let'. |
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echo "var1 incremented by 1 is $var1." |
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# Attempt to change variable declared as integer |
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echo "Attempting to change var1 to floating point value, 2367.1." |
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var1=2367.1 |
# Results in error message, with no change to variable. |
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echo "var1 is still $var1" |
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echo |
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declare −r var2=13.36 |
# 'declare' permits setting a variable property |
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#+ and simultaneously assigning it a value. |
echo "var2 declared as $var2" # Attempt to change readonly variable. |
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var2=13.37 |
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# Generates error message, and exit from script. |
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echo "var2 is still $var2" # This line will not execute.
exit 0 |
# Script will not exit here. |
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9.5. Indirect References to Variables
Assume that the value of a variable is the name of a second variable. Is it somehow possible to retrieve the value of this second variable from the first one? For example, if a=letter_of_alphabet and letter_of_alphabet=z, can a reference to a return z? This can indeed be done, and it is called an indirect reference. It uses the unusual eval var1=\$$var2 notation.
Example 9−18. Indirect References
#!/bin/bash
# Indirect variable referencing.
a=letter_of_alphabet letter_of_alphabet=z
echo
#Direct reference. echo "a = $a"
#Indirect reference. eval a=\$$a
echo "Now a = $a"
echo
# Now, let's try changing the second order reference.
t=table_cell_3 table_cell_3=24
echo "\"table_cell_3\" = $table_cell_3"
echo −n "dereferenced \"t\" = "; eval echo \$$t
#In this simple case,
#eval t=\$$t; echo "\"t\" = $t"
#also works (why?).
echo
t=table_cell_3 NEW_VAL=387 table_cell_3=$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, S.C., for clearing up the above behavior.)
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#Another method is the ${!t} notation, discussed in "Bash, version 2" section.
#See also example "ex78.sh".
exit 0
Example 9−19. Passing an indirect reference to awk
#!/bin/bash
#Another version of the "column totaler" script
#that adds up a specified column (of numbers) in the target file.
#This uses indirect references.
ARGS=2
E_WRONGARGS=65
if [ $# −ne "$ARGS" ] # Check for proper no. of command line args. then
echo "Usage: `basename $0` filename column−number" exit $E_WRONGARGS
fi
filename=$1 column_number=$2
#===== Same as original script, up to this point =====#
# A multi−line awk script is invoked by awk ' ..... '
#Begin awk script.
#−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
awk "
{ total += \$${column_number} # indirect reference
}
END {
print total
}
""$filename"
#−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
#End awk script.
#Indirect variable reference avoids the hassles
#of referencing a shell variable within the embedded awk script.
#Thanks, Stephane Chazelas.
exit 0
This method of indirect referencing is a bit tricky. If the second order variable changes its value, then the the first order variable must be properly dereferenced (as in the above example). Fortunately, the ${!variable} notation introduced with version 2 of Bash (see Example 35−2) makes
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