How can I extract the 11th and 12th characters of a grep match in Bash? - bash

Question

I have a text file called temp.txt that consists of 3 serial numbers.
AB400-251429-0014
AA200-251429-0028
AD200-251430-0046
The 11th and 12th characters in the serial number correspond to the week. I want to extract this number for each unit and do something with it (but for this example just echo it). I have the following code:
while read line; do
week=` grep A[ABD][42]00 $line | cut -c11-12 `
echo $week
done < temp.txt
Looks like it's not working as cut is expecting a filename called the serial number in each case. Is there an alternative way to do this?

Answer 1

The problem is not with cut but with grep which expects a filename, but gets the line contents. Also, the expression doesn't match the IDs: they don't start with S followed by A, B, or D.
You can process lines in bash without starting a subshell:
while read line ; do
echo 11th and 12th characters are: "${line:10:2}".
done < temp.txt
Your original approach is still possible:
week=$( echo "$line" | grep 'S[ABD][42]00' | cut -c11-12 )
Note that for non matching lines, $week would be empty.

Answer 2

You can try also the:
grep -oP '.{10}\K..' filename
for your input prints
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The \K mean, A variable length look-behind. With other words, the grep would look for the pattern before \K but would not include it into the result.
More precise selection of the lines:
grep -oP '[ABD][42]00-.{4}\K..' # or more precise
grep -oP '^\w[ABD][42]00-.{4}\K..' # or even more
grep -oP '^[A-Z][ABD][42]00-.{4}\K..' # or
grep -oP '^[A-Z][ABD][42]00-\d{4}\K..' # or
prints like the above, but selects the interesting lines... :)

Answer 3

I would use this simple awk
awk '{print substr($0,11,2)}' text.file
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To get it into an array that you can use later:
results=($(awk '{print substr($0,11,2)}' text.file))
echo "${results[#]}"
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Answer 4

TL;DR
Looping with Bash is pretty inefficient, especially when reading a file a line at a time. You can get what you want faster and more effectively by using grep to cut only the interesting lines, or by using awk to avoid having to call cut in a separate pipelined process.
GNU Grep and Cut Solution
$ grep '[[:alpha:]][ABD][42]' temp.txt | cut -c11,12
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Awk Solutions
# As far as I know, this will work on most awks. If you find an exception,
# please post a constructive comment!
$ awk -v NF=1 -v FPAT=. '/[[:alpha:]][ABD][42]00/ { print $11 $12 }' temp.txt
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# A more elegant solution as noted by #rici that works with GNU awk,
# and possibly others.
$ gawk -v FS= '/[[:alpha:]][ABD][42]00/ { print $11 $12 }' temp.txt
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Store the Results in a Bash Array
Either way, you can store the results of your match in an Bash array to use later. For example:
$ results=(`grep '[[:alpha:]][ABD][42]00' temp.txt | cut -c11,12`)
$ echo "${results[#]}"
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