Tuesday, September 13, 2011

awk - Passing arguments or shell variables to awk



  In one of our earlier articles, we saw how to read a file in awk. At times, we might have some requirements wherein we need to pass some arguments to the awk program or to access a shell variable or an environment variable inside awk. Let us see in this article how to pass and access arguments in awk:

Let us take a sample file with contents, and a variable "x":
$ cat file1
24
12
34
45
$ echo $x
3
Now, say we want to add every value with the shell variable x.


1.awk provides a "-v" option to pass arguments. Using this, we can pass the shell variable to it.
$ awk -v val=$x '{print $0+val}' file1
27
15
37
48
  As seen above, the shell variable $x is assigned to the awk variable "val". This variable "val" can directly be accessed in awk.

2. awk provides another way of passing argument to awk without using -v. Just before specifying the file name to awk, provide the shell variable assignments to awk variables as shown below:
$ awk  '{print $0,val}' OFS=, val=$x file1
24,3
12,3
34,3
45,3
3. How to access environment variables in awk? Unlike shell variables, awk provides a way to access the environment variables without passing it as above. awk has a special variable ENVIRON which does the needful.
$ echo $x
3
$ export x
$ awk  '{print $0,ENVIRON["x"]}' OFS=,  file1
24,3
12,3
34,3
45,3

Quoting file content:

  Some times we might have a requirement wherein we have to quote the file contents. Assume, you have a file which contains the list of database tables. And for your requirement, you need to quote the file contents:
$ cat file
CUSTOMER
BILL
ACCOUNT

4. Pass a variable to awk which contains the double quote. Print the quote, line, quote.
$ awk -v q="'" '{print q $0 q}' file
'CUSTOMER'
'BILL'
'ACCOUNT'
5. Similarly, to double quote the contents, pass the variable within single quotes:
$ awk '{print q $0 q}' q='"' file
"CUSTOMER"
"BILL"
"ACCOUNT"
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