Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting | ||
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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 9-21. Indirect References
1 #!/bin/bash 2 # Indirect variable referencing. 3 4 a=letter_of_alphabet 5 letter_of_alphabet=z 6 7 echo 8 9 # Direct reference. 10 echo "a = $a" 11 12 # Indirect reference. 13 eval a=\$$a 14 echo "Now a = $a" 15 16 echo 17 18 19 # Now, let's try changing the second order reference. 20 21 t=table_cell_3 22 table_cell_3=24 23 echo "\"table_cell_3\" = $table_cell_3" 24 echo -n "dereferenced \"t\" = "; eval echo \$$t 25 # In this simple case, 26 # eval t=\$$t; echo "\"t\" = $t" 27 # also works (why?). 28 29 echo 30 31 t=table_cell_3 32 NEW_VAL=387 33 table_cell_3=$NEW_VAL 34 echo "Changing value of \"table_cell_3\" to $NEW_VAL." 35 echo "\"table_cell_3\" now $table_cell_3" 36 echo -n "dereferenced \"t\" now "; eval echo \$$t 37 # "eval" takes the two arguments "echo" and "\$$t" (set equal to $table_cell_3) 38 echo 39 40 # (Thanks, S.C., for clearing up the above behavior.) 41 42 43 # Another method is the ${!t} notation, discussed in "Bash, version 2" section. 44 # See also example "ex78.sh". 45 46 exit 0 |
Example 9-22. Passing an indirect reference to awk
1 #!/bin/bash 2 3 # Another version of the "column totaler" script 4 # that adds up a specified column (of numbers) in the target file. 5 # This uses indirect references. 6 7 ARGS=2 8 E_WRONGARGS=65 9 10 if [ $# -ne "$ARGS" ] # Check for proper no. of command line args. 11 then 12 echo "Usage: `basename $0` filename column-number" 13 exit $E_WRONGARGS 14 fi 15 16 filename=$1 17 column_number=$2 18 19 #===== Same as original script, up to this point =====# 20 21 22 # A multi-line awk script is invoked by awk ' ..... ' 23 24 25 # Begin awk script. 26 # ------------------------------------------------ 27 awk " 28 29 { total += \$${column_number} # indirect reference 30 } 31 END { 32 print total 33 } 34 35 " "$filename" 36 # ------------------------------------------------ 37 # End awk script. 38 39 # Indirect variable reference avoids the hassles 40 # of referencing a shell variable within the embedded awk script. 41 # Thanks, Stephane Chazelas. 42 43 44 exit 0 |
This method of indirect referencing is a bit tricky. If the second order variable changes its value, then 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 indirect referencing more intuitive. |