question on awk and sed

[Charles Li_2]

is it possible to use $variable inside sed and awk:
svar="xyz"
sed -e 's/$tvar/x/'

avar=5
awk '{ print $'$avar' }'

Thanks,Charles

[Steven E. Protter · Accepted solution]

For sure yes with sed.

You may need to use the structure ${tvar} first.

Why don't you just try it.

SEP

[Tom Danzig]

In awk the answer is yes.

For example:
BEGIN{
X=50
}
{
printf("%d\n",X)
}

Will print 50.

To pass variables from the shell into awk, use:

awk -v var=value

[Charles Li_2]

I would have, but I did not know the syntax. Should it be {},[],(),"",'' or ``.

[Tom Danzig]

As Steven says, use curly braces arount the variable name to isolate it. i.e ${varname}

[Ramkumar Devanathan]

Charles,

1. Enclose the regex within double quotes, and not single quotes.
sed -e "s/$tvar/x/" ...

2. Works fine for me.
awk '{ print $'$avar' }'
this works too -
awk "{ print \$$avar ;}" ...

- ramd.

[Charles Li_2]

Tom,
I want to do the following with awk:

for loop
K=awk '{ print $$loop }'
## giving K a different value each time the loop is executed.
endloop

is this possible?

[Charles Li_2]

Works great. Thanks to all.

[Tom Danzig]

Charls,

It should be. Remember not to preceed variables in the awk portion with a $. The shell uses the $ but not awk.

[curt larson_1]

well I guess that depends on what you mean by "inside sed and awk". sed doesn't have support variables. But, it seems what your trying to do is shell expansion of a variable before it is sent to sed.

abc="abc";sed -e 's/$abc/def/'

the shell doesn't do variable substitution with single quotes. So, the above wouldn't do what your wanting. double quotes do, so either of these will do what your wanting to do.

abc="abc";sed -e "s/$abc/def/"
abc="abc";sed -e 's/'$abc'/def/'
once again sed isn't using the variable, the shell is expanding the variable before sed sees it.
abc="abc";sed -e "s/$abc/def/" gets expanded by the shell to
sed -e "s/abc/def/" before the sed command is ever called. So, your making use of a variable in the shell but sed never knows about it.

the same can be done with awk. But awk has methods to used variables and can be passed variables. unless the awk script is very short, i'd use parameter passing with awk because of all the possible misunderstanding that could be caused by which quotes match up and which variables are expanded by the shell and which are used with awk itself.

your awk example could be done as
avar=5
print "1 2 3 4 5" |
awk "{ print \$$avar; }"