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Commands by xakon from sorted by
Terminal - Commands by xakon - 6 results
eval $(date +"day=%d; month=%m; year=%y")
2011-07-29 12:47:26
User: xakon
Functions: date eval
Tags: bash eval
4

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".

eval "mkdir test{$(seq -s, 1 10)}"
2011-07-23 17:09:01
User: xakon
Functions: eval
Tags: bash seq eval
0

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

mkdir test{1,2,3,4,5,6,7,8,9,10}

something extremely boring to type! Why not use seq?

seq -s, 1 10

and use its output inside the curly braces?

The obvious solution

mkdir test{$(seq -s, 1 10)}

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

echo -n "String to MD5" | md5sum | cut -f1 -d' '
2011-03-05 03:53:09
User: xakon
Functions: cut echo md5sum
-2

Nothing special about hashing here, only the use of cut, I think, could result at fewer keystrokes.

\cp something toSomeWhereElse
2010-09-25 03:07:04
User: xakon
0

If the problem is an aliased synonym for a command, you can still execute the original command by pre-pending it with a reverse-slash '\'. This works at least in Bash, but I guess the aliasing system refers definitely to Bash (and not only).

git archive HEAD --format=zip > archive.zip
echo 'c84fa6b830e38ee8a551df61172d53d7 myfile' | md5sum -c