bash programming: testing for empty string

[Phil Betts]

bootleg86 wrote on Wednesday, March 05, 2008 9:28 AM::

> Hi,
> 
> I'm trying to find the time of a file by doing this.
> 
> filetime=`ls -l --time-style=+%a:%H:%M /tmp/1.txt | awk '{print $6}'`
> 
> if [ -z "$filetime"]; then
>  echo "File does not exist"
> else
>  echo "Time file: $filetime"
> fi
> 
> However, when the file does not exist and filetime returns an empty
> string, it does not evaluate [ -z "$filetime" ] to true
> I have also tried the reverse which is
> 
> if [ -n "$filetime"]; then
>  echo "Time file: $filetime"
> else
>  echo "File does not exist"
> fi
> 
> but I still get the same results.
> 
> What does $filetime evaluate to when the command exits with an error?

WJFFM

However it's not good style to trigger a foreseeable error and rely
on the error handling of a command to do what you want.  Apart from 
anything else, doing it your way will be slower due to the number of 
unnecessary processes invoked when the file doesn't exist.

Instead, first test if the file exists, and only if so print its time:

if [ -e /tmp/1.txt ];then
  filetime=`ls -l --time-style=+%a:%H:%M /tmp/1.txt | awk '{print $6}'`
  echo "Time file: $filetime"
else
   echo "File does not exist"
fi

Also, if you don't actually need $filetime after you've printed it, 
save yourself a bit more time by:

  echo -n "Time file: "
  ls -l --time-style=+%a:%H:%M /tmp/1.txt | awk '{print $6}'


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