CODEWITHSUNDEEP
awk
user1603289
user1603289

Reputation: 123

awk: setting environment variables directly from within an awk script

first post here, but been a lurker for ages. i have googled for ages, but cant find what i want (many abigious topic subjects which dont request what the topic suggests it does ...). not new to awk or scripting, just a little rusty :)

i'm trying to write an awk script which will set shell env values as it runs - for another bash script to pick up and use later on. i cannot simply use stdout from awk to report this value i want setting (i.e. "export whatever=awk cmd here"), as thats already directed to a 'results file' which the awkscript is creating (plus i have more than one variable to export in the final code anyway).

As an example test script, to demo my issue:

echo $MYSCRIPT_RESULT          # returns nothing, not currently set
echo | awk -f scriptfile.awk   # do whatever, setting MYSCRIPT_RESULT as we go
echo $MYSCRIPT_RESULT          # desired: returns the env value set in scriptfile.awk

within scriptfile.awk, i have tried (without sucess)

1/) building and executing an adhoc string directly:

{
  cmdline="export MYSCRIPT_RESULT=1"
  cmdline
}

2/) using the system function:

{
  cmdline="export MYSCRIPT_RESULT=1"
  system(cmdline)
}

... but these do not work. I suspect that these 2 commands are creating a subshell within the shell awk is executing from, and doing what i ask (proven by touching files as a test), but once the "cmd"/system calls have completed, the subshell dies, unfortunatley taking whatever i have set with it - so my env setting changes dont stick from "the caller of awk"'s perspective.

so my question is, how do you actually set env variables within awk directly, so that a calling process can access these env values after awk execution has completed? is it actually possible?

other than the adhoc/system ways above, which i have proven fail for me, i cannot see how this could be done (other than writing these values to a 'random' file somewhere to be picked up and read by the calling script, which imo is a little dirty anyway), hence, help!

all ideas/suggestions/comments welcomed!

Upvotes: 12

Views: 20071

Answers (4)

Vencato
Vencato

Reputation: 11

I found a good answer. Encapsulate averything in a subshell! The comand declare works as below:

#Creates 3 variables
declare var1=1 var2=2 var3=3

ex1:

#Exactly the same as above
$(awk 'BEGIN{var="declare "}{var=var"var1=1 var2=2 var3=3"}END{print var}')

I found some really interesting uses for this technique. In the next exemple I have several partitions with labels. I create variables using the labels as variable names and the device name as variable values.

ex2:

#Partition data
lsblk -o NAME,LABEL

NAME   LABEL
sda
├─sda1
├─sda2
├─sda5 System
├─sda6 Data
└─sda7 Arch

#Creates a subshell to execute the text
$(\
#Pipe lsblk to awk
lsblk -o NAME,LABEL | awk \
#Initiate the variable with the text for the declare command
'BEGIN{txt="declare "}'\
#Filters devices with labels Arch or Data
'/Data|Arch/'\
#Concatenate txt with itself plus text for the variables(name and value)
#substr eliminates the special caracters before the device name
'{txt=txt$2"="substr($1,3)" "}'\
#AWK prints the text and the subshell execute as a command
'END{print txt}'\
)

The end result of this is 2 variables: Data with value sda6 and Arch with value sda7.

The same exemple in a single line.

$(lsblk -o NAME,LABEL | awk 'BEGIN{txt="declare "}/Data|Arch/{txt=txt$2"="substr($1,3)" "}END{print txt}')

Upvotes: 0

tripleee
tripleee

Reputation: 189749

You cannot change the environment of your parent process. If

MYSCRIPT_RESULT=$(awk stuff)

is unacceptable, what you are asking cannot be done.

Upvotes: 8

sbts
sbts

Reputation: 101

You can also use something like is described at Set variable in current shell from awk

 unset var
 var=99
 declare $( echo "foobar" | awk '/foo/ {tmp="17"} END {print "var="tmp}' )
 echo "var=$var"
var=

The awk END clause is essential otherwise if there are no matches to the pattern declare dumps the current environment to stdout and doesn't change the content of your variable.

Multiple values can be set by separating them with spaces.

 declare a=1 b=2
 echo -e "a=$a\nb=$b"

NOTE: declare is bash only, for other shells, use eval with the same syntax.

Upvotes: 4

William Pursell
William Pursell

Reputation: 212504

You can do this, but it's a bit of a kludge. Since awk does not allow redirection to a file descriptor, you can use a fifo or a regular file:

$ mkfifo fifo
$ echo MYSCRIPT_RESULT=1 | awk '{ print > "fifo" }' &
$ IFS== read var value < fifo
$ eval export $var=$value

It's not really necessary to split the var and value; you could just as easily have awk print the "export" and just eval the output directly.

Upvotes: 3

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