script Q : how do I force bash to perform the math?

kevinyeandel · 02-20-2009, 01:32 AM

Hi

I want to know how old a file is so I've reduced the problem to give me two timestamps but:

echo `date +%s` - `stat -c %Z howoldisthisfile.txt`

gives

1235114433 - 1235093198

instead of

21235

I've tried () around various parts but just end up with syntax errors.

How do I persuade bash to give up the result without assigning to variables.

Many thanks
kevin

David the H. · 02-20-2009, 02:15 AM

The mathematical operation syntax for bash is $(( )). I also recommend $() for command nesting instead of using the ` backticks

Code:

echo "$(( $(date +%s) - $(stat -c %Z howoldisthisfile.txt) ))"

You do realize that on posix filesystems there is no "created time" flag, only a "last modified" one, I hope. It's impossible to know with certainty how old a file actually is, only how long it's been since it was last changed.

arckane · 02-20-2009, 02:22 AM

Expr way:

Code:

expr `date +%s` - `stat -c %Z howoldisthisfile.txt`

It's not pure bash as you need bc, but this'll work too:

Code:

echo "`date +%s`-`stat -c %Z howoldisthisfile.txt`" | bc

unSpawn · 02-20-2009, 02:24 AM

Code:

# quick
echo $[`date +%s` - `stat -c %Z howoldisthisfile.txt`]
# precise
echo `date +%s` - `stat -c %Z howoldisthisfile.txt`|bc -l
# Full M/C time
date --date="$(echo $[$(date +%s) - $(stat -c %Z .Xdefaults)] seconds ago)"

kevinyeandel · 02-20-2009, 02:35 AM

Quote:

Originally Posted by unSpawn

Code:

# quick
echo $[`date +%s` - `stat -c %Z howoldisthisfile.txt`]
# precise
echo `date +%s` - `stat -c %Z howoldisthisfile.txt`|bc -l
# Full M/C time
date --date="$(echo $[$(date +%s) - $(stat -c %Z .Xdefaults)] seconds ago)"

Thank you all for your help - quote a lot learned from your collective contributions!

Kevin