How to read output of sed into a variable

You can use command substitution as:

new_filename=$(echo "$a" | sed 's/.txt/.log/')

or the less recommended backtick way:

new_filename=`echo "$a" | sed 's/.txt/.log/'`

You can use backticks to assign the output of a command to a variable:

logfile=`echo $a | sed 's/.txt/.log/'`

That's assuming you're using Bash.

Alternatively, for this particular problem Bash has pattern matching constructs itself:

stem=$(textfile%%.txt)
logfile=$(stem).log

or

logfile=$(textfile/%.txt/.log)

The % in the last example will ensure only the last .txt is replaced.

The simplest way is

logfile="${a/\.txt/\.log}"

If it should be allowed that the filename in $a has more than one occurrence of .txt in it, use the following solution. Its more safe. It only changes the last occurrence of .txt

logfile="${a%%\.txt}.log"

if you have Bash/ksh

$ var="abcd.txt"
$ echo ${var%.txt}.log
abcd.log
$ variable=${var%.txt}.log