Commands tagged eval

Any changes to BASH shell made in .bashrc will be active in the current terminal window from the moment you execute this command, ie. aliases, prompt settings etc. No need to restart terminal.

(In BASH 'source' simile to 'eval' lets you generally execute any bunch of commands stacked in a text file).

If $INBACK is set, command will launch in foreground and inverse.

Very useful in script !

We could apply the inverse comportement like that :

eval command ${INBACK:+&}

It's quite easy to capture the output of a command and assign it in a shell's variable:

But, what if we want to perform the same task with just one program invocation? Here comes the power of eval! date(1) outputs a string like "day=29; month=07; year=11" (notice the semicolons I added on purpose at date's custom output) which is a legal shell line. This like is then parsed and executed by the shell once again with the help of eval. Just setting 3 variables!

Inspired by LinuxJournal's column "Dave Taylor's Work the Shell".

In order to create, let's say, 10 directories with a single process we can use the command:

something extremely boring to type! Why not use seq?

and use its output inside the curly braces?

The obvious solution

is, unfortunately, too naive and doesn't work. The answer is the order of the shell expansions (feature of Bourne Shell, actually), where brace expansion happens before command substitution (according to Bash's manual).

The solution is to put another level of substitution, using the powerful and mystic command eval.

I found the trick in a similar problem in the post at http://stackoverflow.com/questions/6549037/bash-brace-expansion-in-scripts-not-working-due-to-unwanted-escaping

Suppose you have 11 marbles, 4 of which are red, the rest being blue. The marbles are indistinguishable, apart from colour. How many different ways are there to arrange the marbles in a line? And how many ways are there to arrange them so that no two red marbles are adjacent?

There are simple mathematical solutions to these questions, but it's also possible to generate and count all possibilities directly on the command line, using little more than brace expansion, grep and wc!

The answer to the question posed above is that there are 330 ways of arranging the marbles in a line, 70 of which have no two red marbles adjacent. See the sample output.

To follow the call to marbles 11 4: after c=''; for i in $(seq $1); do c+='{b,r}'; done;, $c equals {b,r}{b,r}{b,r}{b,r}{b,r}{b,r}{b,r}{b,r}{b,r}{b,r}{b,r}

After x=$(eval echo $c), and brace expansion, $x equals bbbbbbbbbbb bbbbbbbbbbr ... rrrrrrrrrrb rrrrrrrrrrr, which is all 2^11 = 2048 strings of 11 b's and r's.

After p=''; for i in $(seq $2); do p+='b*r'; done;, $p equals b*rb*rb*rb*r

Next, after y=$(grep -wo "${p}b*"

Finally, grep -vc 'rr'

I used to do a lot of path manipulation to set up my development environment (PATH, LD_LIBRARY_PATH, etc), and one part of my environment wasn't always aware of what the rest of the environment needed in the path. Thus resetting the entire PATH variable wasn't an option; modifying it made sense.

The original version of the functions used sed, which turned out to be really slow when called many times from my bashrc, and it could take up to 10 seconds to login. Switching to parameter substitution sped things up significantly.

The commands here don't clean up the path when they are done (so e.g. the path gets cluttered with colons). But the code is easy to read for a one-liner.

The full function looks like this:

I tried using regexes to make this into a cleaner one-liner, but remove_path ended up being cryptic and not working as well:

Validate a file using xmllint. If there are parser errors, edit the file in vim at the line of the first error.

This one uses dictionary.com

The original was a little bit too complicated for me. This one does not use any variables.

Looks up a word on merriam-webster.com, does a screen scrape for the FIRST audio pronunciation and plays it.

USAGE: Put this one-liner into a shell script (e.g., ~/bin/pronounce) and run it from the command line giving it the word to say:

If the word isn't found in merriam-webster, no audio is played and the script returns an error value. However, M-W is a fairly complete dictionary (better than howjsay.com which won't let you hear how to pronounce naughty words).

ASSUMPTIONS: GNU's sed (which supports -r for extended regular expressions) and Linux's aplay. Aplay can be replaced by any program that can play .WAV files from stdin.

KNOWN BUGS: only the FIRST pronunciation is played, which is problematic if you wanted a particular form (plural, adjectival, etc) of the word. For example, if you run this:

you'll hear a voice saying "onomatopoeia".

Playing the correct form of the word is possible, but doing so might make the screen scraper even more fragile than it already is. (The slightest change to the format of m-w.com could break it).

The coolest way I've found to backup a wordpress mysql database using encryption, and using local variables created directly from the wp-config.php file so that you don't have to type them- which would allow someone sniffing your terminal or viewing your shell history to see your info.

I use a variation of this for my servers that have hundreds of wordpress installs and databases by using a find command for the wp-config.php file and passing that through xargs to my function.