Block comments in bash

In C/C++, to comment out a block of code, I generally use

 #if 0

 ...stuff

 #endif 

 Or block comment (if there are no other comments)

 /* 

 ...stuff

 */ 

 But there is no such thing in bash, so the common hack is

 : '

 ...stuff

 '

 

i.e., use the : (no-op) operator and then the entire block to be commented goes into a single-quoted string! 

But even more simpler is:

 if false; then

 ... stuff

 fi

 why not!? :=)

Enable tracing on bash scripts

With shell scripts, it becomes difficult at times to debug which command took too long, or at which step has an issue. Even though I add generous amount of debug logs there are times where -x (execution trace) is invaluable. But I don't want to litter my regular logs with the -x output either!
So, after a bit of search found this nice snippet of bash:

trace_file() {
    exec 4>>"$1"
    BASH_XTRACEFD=4
    set -x
}

With this function defined, I can invoke it in any script, with the path to trace file:

trace_file /tmp/mytrace

Oh! But how would I know without 'running' it, if its ok?

Have you come across your teammates, checking in scripts without running it, with syntax errors, malformed XML documents, broken Makefiles .... well, there is some hope, you could help (by sharing this link! :-) )

Mostly, if its code in a compiled language (like C), developers at least compile the code (might not test!) before checking into version control. But, in case of XML documents, scripts etc, the code gets checked in, and it will not error out until some test case is completely run, or in the nightly regressions!

Its not really difficult to do some basic validation of scripts, Makefiles, or even XML documents before a check-in. Most languages/tools have an option to do basic checks - syntax checks, dry run (without actually executing the commands), bytecode generation etc. Here are some, which I often use:

Make

make -n

Does a dry-run, shows which targets would get built on an actual run.

Perl

perl -c

Does a compilation and says if syntax is OK or not (there are caveats with BEGIN blocks, but at least there is something!)

Python

pycompile

This is good, there is a separate utility to do the compilation! (just run pycompile on your scripts once before you proceed)

bash

bash -n

Again, syntax checks without actually executing anything.

XML

xmllint

The name says it all! Even though there are plenty of options, you could just run once [without even options] on your XML docs to see if its well-formed.

Lua

luac

Lua bytecode compiler, like pycompile. Generates luac.out which you can delete :-)

Simple asynchronous task spawn from shell

Yesterday was a hard day at work! I was meddling with some shell (bash) script, where I had to make a CLI work; a CLI command which would restart self (the whole set of services, including the ones who spawned the CLI).

That was somehow not working! before the script could restart the services, it would get terminated! I did a lot of reading, and learnt lot many bash tricks in a day that I haven't learnt in a long time! :)

Now, for this issue, what I needed was a way to tell someone "Please restart me" because "I'm not able to restart myself!" - ah! asynchronous task/job.

I already had a daemon to execute a task, but since it was also part of the processes that get killed on restart - I could not use it. I have a simple shell script, and I needed no additional complexity, what could I do ? ... a little more digging on stackexchange led me to a simple solution.

Schedule a job to be executed at a later time (or immediately) using at!

So, my solution boiled down to a very simple change, instead of restart, I had to say

restart | at now 

wow!