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I am going through shell scripting online lessons as my work requires me to learn shell scripting.

I came across "awk" and "nawk " commands and my learning hasn't yet reached up to it.

In a nutshell, I know that awk/nawk search for a particular pattern and perform an action in case a match has been found.

Even despite of that, I couldn't understand what the following line is meant for: 

eval $( cat ${MMORPHYDIR}/${PWFILE} | nawk ' /^#BEGIN '${ENV_NAME}'/,/^#END '${ENV_NAME}'/ { print }' | egrep "${USEFULL_PARAM}" )

Please help me to understand what this line does or is intended to do.

Unheilig
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itp dusra
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  • These days `nawk` is essentially the 'only' `awk`; the old `awk` for which it is, by comparison, 'new awk', is seldom encountered (but you might find `oawk` on a few systems such as Solaris or AIX; in that case, it isn't clear whether plain `awk` will be `nawk` or `oawk`). GNU Awk has facilities not specified in `nawk` (but it supports all the goodies that were in `nawk` but not in `oawk`). This doesn't affect your question very much. – Jonathan Leffler Jul 11 '16 at 03:06
  • The code exhibits [UUoC — Useless Use of `cat`](https://stackoverflow.com/questions/11710552/useless-use-of-cat/). – Jonathan Leffler Jul 11 '16 at 03:08

2 Answers2

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... awk '/start/,/end/'

prints the records between start and end patterns. {print} can be omitted since it's implied. The ^ in your script indicates beginning of a line.

Note that cat and eval are unnecessary, the same can be written as

$ awk '...' "${MMORPHYDIR}/${PWFILE}"

also grep can be included in the awk script as well.

karakfa
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    I agree the `cat` is unnecessary; the `grep` is probably unnecessary too since `awk` supports practically the same regexes. Are you sure the `eval` is not? It interprets the output of the `cat | awk | grep` ensemble as shell script. If the output includes variable settings, the `eval` sets those variables; your plain `awk` does not. – Jonathan Leffler Jul 11 '16 at 03:10
  • yes, you're right `eval` might be needed. Wasn't sure what comes out of the script. – karakfa Jul 11 '16 at 03:28
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awk is THE standard, general purpose tool for manipulating text on all UNIX-like systems. There are various flavors of awk, all with the same core functionality plus some differences. nawk is the very unfortunately named new awk because it was named that about 30 years ago as the successor to 1977s old awk (e.g. /bin/awk on Solaris, aka old, broken awk which must never be used) and is now actually best avoided as it doesn't even support the minimal awk functionality required by POSIX (e.g. character classes). Important lesson there: never use the word "new" in the name of any software entity!

The best awk to use these days is GNU awk, gawk, as it supports all POSIX functionality plus a ton of useful extensions, is generally available, is extremely well documented, and has a massive user base.

wrt:

eval $( cat ${MMORPHYDIR}/${PWFILE} | nawk ' /^#BEGIN '${ENV_NAME}'/,/^#END '${ENV_NAME}'/ { print }' | egrep "${USEFULL_PARAM}" )

That is a complete mess, doing literally about a dozen things that should never be done in shell or in awk. Trying to explain it would be like trying to explain someone mixing concrete with a screwdriver. Forget you ever saw it and move on.

To learn awk, read the book Effective Awk Programming, 4th Edition, by Arnold Robbins.

Ed Morton
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