Utilizing the eval command in Linux to run variables as instructions

There are in all probability plenty of Linux customers who’ve by no means encountered the eval command. In reality, it’s probably not a “command”, however a bash built-in that’s meant to course of the worth of a variable as a command. For instance, in case you arrange a variable that features the command to show the present time in Sydney, Australia, it will in all probability appear to be this:

$ dt=”TZ=’Australia/Sydney’ date”

You might then run it like this:

$ eval $dt

Thu Jul  7 06:32:14 AM AEST 2022

Doing that may prevent the difficulty of memorizing the date command syntax and specifying a time zone, however let’s look a bit extra intently at eval to see what else it might probably do for you.

The bash man web page will inform you a bit concerning the eval command, although you would need to scroll down almost 5,000 strains to search out it. There are, in spite of everything, no man pages for built-ins. This is what it will inform you:

    The args are learn and concatenated collectively right into a  single  com-

    mand.   This command is then learn and executed by the shell, and

    its exit standing is returned as the worth of eval.  If there  are

    no args, or solely null arguments, eval returns 0.

After all, for something you do by typing instructions within the terminal window, eval is not more likely to prevent a lot time or effort. You might, in spite of everything, create an alias to do the identical be just right for you. Right here’s how that will look:

$ alias dt=”TZ=’Australia/Sydney’ date”

$ dt

Thu Jul  7 06:37:31 AM AEST 2022

Save that alias in your .bashrc file, and also you’ll have a simple time checking what time it’s in Sydney everytime you really feel the urge.

If you’re constructing a posh command in items, nevertheless—particularly in scripts that want to gather the mandatory information when they’re run—utilizing eval to run the command could make operating the command so much simpler.

For example, whereas engaged on a latest script, I wanted to search out phrases that have been 5 letters lengthy, had particular letters in identified positions and different particular letters in unsure positions. I additionally knew that a variety of letters weren’t included within the phrases that I wanted to search out. Step-by-step, the script constructed an extended string of grep instructions to look by the phrases file to search out solely the phrases that match my specs. Right here’s an instance of what such a command would possibly appear to be with all of its grep (match) and its grep -v (ignore) instructions and the match that it discovered:

$ grep ^ch..t$ /usr/share/dict/phrases | eval grep -v ‘q’ | grep -v ‘w’ | grep -v ‘r’ | grep -v ‘y’ | grep -v ‘u’ | grep -v ‘i’ | grep -v ‘o’ | grep -v ‘p’ | grep -v ‘s’ | grep -v ‘d’ | grep -v ‘f’ | grep -v ‘g’ | grep -v ‘j’ | grep -v ‘ok’ | grep -v ‘l’ | grep -v ‘z’ | grep -v ‘x’ | grep -v ‘v’ | grep -v ‘b’ | grep -v ‘n’ | grep -v ‘m’ | grep a | grep e | grep -v ..a.. | grep -v …e.

cheat

In reality, in case you run that command towards your phrases file, it is best to get the identical response. In case you flip it right into a variable as proven under, it would work the identical.

$ cmd=”grep ^ch..t$ /usr/share/dict/phrases | grep -v ‘q’ | grep -v ‘w’ | grep -v ‘r’ | grep -v ‘y’ | grep -v ‘u’ | grep -v ‘i’ | grep -v ‘o’ | grep -v ‘p’ | grep -v ‘s’ | grep -v ‘d’ | grep -v ‘f’ | grep -v ‘g’ | grep -v ‘j’ | grep -v ‘ok’ | grep -v ‘l’ | grep -v ‘z’ | grep -v ‘x’ | grep -v ‘v’ | grep -v ‘b’ | grep -v ‘n’ | grep -v ‘m’ | grep a | grep e | grep -v ..a.. | grep -v …e.”

$ eval $cmd

cheat

My script makes use of eval to run the $cmd variable as a command and passes the output to the column command which makes the output extra helpful when there are dozens or a whole lot of matching phrases. The end result within the instance proven was the one phrase “cheat” pulled from the almost half 1,000,000 strains within the phrases file. All I wanted to do was reply a number of questions on what I knew and didn’t know.

eval $cmd | column

Wrap-Up

The eval command can be utilized to run easy or very complicated instructions which are saved as variables. It evaluates the instructions and arguments within the variable after which executes them. It’s most helpful if you find yourself developing lengthy and complex instructions – particularly in scripts when the small print depend upon information that modifications or responses supplied when the scripts are run.

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