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Top Forums Shell Programming and Scripting awk search/replace specific field, using variables for regexp & subsitution then overwrite file Post 302704397 by cyphex on Friday 21st of September 2012 02:03:00 PM
Old 09-21-2012
awk search/replace specific field, using variables for regexp & subsitution then overwrite file

Hello,
I'm trying the solve the following problem.
I have a file which I intend to use as a csv called master.csv
The columns are separated by commas.
I want to change the text on a specific row in either column 3,4,5 or 6 from xxx to yyy depending upon if column 1 matches a specified pattern.
The column to change (i.e. 3/4/5/6) is dependent upon the value of the variable in a nested loop
The awk statement is located within a nested loop of a parent loop. It will use the variable value from the parent loop for column 1 and the variable value from the nested loop for column 3/4/5/6.
Below is an example of the code:
Code:
 
#Update the CSV file
cat $USERDUMP |awk -F , '{print $1}' |sort -u |\
 while read username
 do
     for LOOP_LIST in $SERVER_LIST
     do
     grep $username $DATADUMP.$LOOP_LIST >> /dev/null 2>&1
     RET_CODE=$?
     if [[ $RET_CODE -eq 0  ]]
     then
          echo    "User $username exists on $LOOP_LIST"
          awk -F "," -v user=$username -v server=$LOOP_LIST '/$user/ {gsub(/$server/,"Yes"); print}' master.csv
     else
          echo "User $username does not have account on $LOOP_LIST"
     fi
     done
 done

I know there are a few problems with my awk statement:
firstly, When passing the variables to awk nothing is produced by the print command.
I have tried the above on the command line using fixed values and I get the output I require:
Code:
$ awk -F "," '/userA/ {gsub(/ServerA/,"Yes"); print }' master.csv
userA,comment,Yes,ServerB,ServerC,ServerD

However, this creates another problem as I want to edit the master csv file. I can't redirect the awk stdout to a temp file then move back because I'd lose all the other entries. The only workarounds I can see for this problem are:
a) using awk to print the entire list with the changes applied, then I can use temp file. I'm not sure this is even possible as it defys the point of using awk pattern matching to print specific columns/rows.
b) run sed -i within the awk command (i've been googling the syntax to this but can't find it).
Apologies that was long winded. If you want me to clarify anything please let me know.
In essence I want to know:
1) how to pass multiple variables to awk command and use them in an regular expression & subsitution
2) how to edit the master csv file
Any help/advice would be much appreciated
 

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A2P(1)							 Perl Programmers Reference Guide						    A2P(1)

NAME
a2p - Awk to Perl translator SYNOPSIS
a2p [options] filename DESCRIPTION
A2p takes an awk script specified on the command line (or from standard input) and produces a comparable perl script on the standard out- put. Options Options include: -D<number> sets debugging flags. -F<character> tells a2p that this awk script is always invoked with this -F switch. -n<fieldlist> specifies the names of the input fields if input does not have to be split into an array. If you were translating an awk script that processes the password file, you might say: a2p -7 -nlogin.password.uid.gid.gcos.shell.home Any delimiter can be used to separate the field names. -<number> causes a2p to assume that input will always have that many fields. -o tells a2p to use old awk behavior. The only current differences are: o Old awk always has a line loop, even if there are no line actions, whereas new awk does not. o In old awk, sprintf is extremely greedy about its arguments. For example, given the statement print sprintf(some_args), extra_args; old awk considers extra_args to be arguments to "sprintf"; new awk considers them arguments to "print". "Considerations" A2p cannot do as good a job translating as a human would, but it usually does pretty well. There are some areas where you may want to examine the perl script produced and tweak it some. Here are some of them, in no particular order. There is an awk idiom of putting int() around a string expression to force numeric interpretation, even though the argument is always inte- ger anyway. This is generally unneeded in perl, but a2p can't tell if the argument is always going to be integer, so it leaves it in. You may wish to remove it. Perl differentiates numeric comparison from string comparison. Awk has one operator for both that decides at run time which comparison to do. A2p does not try to do a complete job of awk emulation at this point. Instead it guesses which one you want. It's almost always right, but it can be spoofed. All such guesses are marked with the comment ""#???"". You should go through and check them. You might want to run at least once with the -w switch to perl, which will warn you if you use == where you should have used eq. Perl does not attempt to emulate the behavior of awk in which nonexistent array elements spring into existence simply by being referenced. If somehow you are relying on this mechanism to create null entries for a subsequent for...in, they won't be there in perl. If a2p makes a split line that assigns to a list of variables that looks like (Fld1, Fld2, Fld3...) you may want to rerun a2p using the -n option mentioned above. This will let you name the fields throughout the script. If it splits to an array instead, the script is probably referring to the number of fields somewhere. The exit statement in awk doesn't necessarily exit; it goes to the END block if there is one. Awk scripts that do contortions within the END block to bypass the block under such circumstances can be simplified by removing the conditional in the END block and just exiting directly from the perl script. Perl has two kinds of array, numerically-indexed and associative. Perl associative arrays are called "hashes". Awk arrays are usually translated to hashes, but if you happen to know that the index is always going to be numeric you could change the {...} to [...]. Itera- tion over a hash is done using the keys() function, but iteration over an array is NOT. You might need to modify any loop that iterates over such an array. Awk starts by assuming OFMT has the value %.6g. Perl starts by assuming its equivalent, $#, to have the value %.20g. You'll want to set $# explicitly if you use the default value of OFMT. Near the top of the line loop will be the split operation that is implicit in the awk script. There are times when you can move this down past some conditionals that test the entire record so that the split is not done as often. For aesthetic reasons you may wish to change the array base $[ from 1 back to perl's default of 0, but remember to change all array sub- scripts AND all substr() and index() operations to match. Cute comments that say "# Here is a workaround because awk is dumb" are passed through unmodified. Awk scripts are often embedded in a shell script that pipes stuff into and out of awk. Often the shell script wrapper can be incorporated into the perl script, since perl can start up pipes into and out of itself, and can do other things that awk can't do by itself. Scripts that refer to the special variables RSTART and RLENGTH can often be simplified by referring to the variables $`, $& and $', as long as they are within the scope of the pattern match that sets them. The produced perl script may have subroutines defined to deal with awk's semantics regarding getline and print. Since a2p usually picks correctness over efficiency. it is almost always possible to rewrite such code to be more efficient by discarding the semantic sugar. For efficiency, you may wish to remove the keyword from any return statement that is the last statement executed in a subroutine. A2p catches the most common case, but doesn't analyze embedded blocks for subtler cases. ARGV[0] translates to $ARGV0, but ARGV[n] translates to $ARGV[$n]. A loop that tries to iterate over ARGV[0] won't find it. ENVIRONMENT
A2p uses no environment variables. AUTHOR
Larry Wall <larry@wall.org> FILES
SEE ALSO
perl The perl compiler/interpreter s2p sed to perl translator DIAGNOSTICS
BUGS
It would be possible to emulate awk's behavior in selecting string versus numeric operations at run time by inspection of the operands, but it would be gross and inefficient. Besides, a2p almost always guesses right. Storage for the awk syntax tree is currently static, and can run out. perl v5.8.0 2002-06-01 A2P(1)
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