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Multiple variable assignments from command output in BASH

Terminal - Multiple variable assignments from command output in BASH
eval $(date +"day=%d; month=%m; year=%y")
2011-07-29 12:47:26
User: xakon
Functions: date eval
4
Multiple variable assignments from command output in BASH

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

day=$(date +%d) month=$(date +%m)

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".

Alternatives

There is 1 alternative - vote for the best!

Terminal - Alternatives
read day month year <<< $(date +'%d %m %y')
2011-07-29 15:05:19
User: putnamhill
Functions: date read
Tags: bash read
47

This version uses read instead of eval.

read day month year < <(date +'%d %m %y')
2011-07-30 06:06:29
User: frans
Functions: date read
Tags: bash read
10

No command substitution but subshell redirection

Know a better way?

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What others think

Very nice!

Comment by flatcap 142 weeks and 2 days ago

Using 'eval' in any language makes me nervous. Still, even a curmudgeon like me can see how useful it is to treat instructions as data and vice-versa.

Comment by Mozai 142 weeks and 1 day ago

Not strangely, I have the same feelings about 'eval' in almost every language. Yet, I totally agree with you as I re-discover once again its power and the usefulness of going around the wall when it stands in front of you while trying to solve a problem!

And by the way, I should not forget the power of 'eval' in many Lisp-like languages!

Comment by xakon 142 weeks and 1 day ago

I like how eval works with the stat's -s option.

Comment by putnamhill 141 weeks and 4 days ago

How do you mean? Can you please provide an example of your point? stat(1) doesn't have a '-s' switch in my system!

Comment by xakon 141 weeks and 4 days ago

I see what you mean. I don't have that option on my linux systems either. Maybe it's just a BSD thing. This is how it works on my mac:

stat -s some.file

st_dev=234881026 st_ino=3657562 st_mode=0100644 st_nlink=1 st_uid=501 st_gid=20 st_rdev=0 st_size=478 st_atime=1312497217 st_mtime=1309535724 st_ctime=1309535724 st_birthtime=1309535658 st_blksize=4096 st_blocks=8 st_flags=0

When you eval the output of stat -s (if you have that option), you get a bunch of variables set.

Comment by putnamhill 141 weeks and 2 days ago

Your point of view

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