Shell – ksh – subtract 5 minutes from current time

kshscriptingshell-scripttimestamps

I need to get the date and time of the command date minus 5 mins.

date:
29 Aug 2018 21:56:01

result:
29 Aug 2018 21:51:02

I tried to search on the Internet/forums but most of it involved using the -d option and what I have doesn't support the -d option.

This one works on Bash but I need something that works on ksh.

$ printf "%(%d %h %Y %H:%M:%S)T\n" $(( $(printf "%(%s)T") - 5 * 60 ))

Best Answer

You could use perl:

echo $(( $(printf "%(%s)T") - 5 * 60 )) | perl -lne 'print scalar localtime $_'

Or in pure perl:

perl -le 'print scalar localtime(time()-5*60)'

To format use strftime():

perl -MPOSIX=strftime -le 'print strftime("%d %h %Y %H:%M:%S", localtime(time()-5*60))'

Related Solutions

Bash vs ksh pipes

Your question is a bit rambling. I'll answer what seems to be the central part, on the difference between ksh and bash that you observe.

You have encountered what is probably the #1 incompatibility between ksh and bash when it comes to scripts. ATT ksh (both ksh88 and ksh93) and zsh execute the last (rightmost) command in a pipeline in the parent shell, whereas other shells (Bourne, ash, bash, pdksh, mksh) execute all the commands including the last one in a subshell.

Here is a simple test program:

msg="a subshell"
true | msg="the parent shell"
echo "This shell runs the last command of a pipeline in $msg"

In ATT ksh and zsh, the second assignment to msg is executed in the parent shell so the effect is visible after the pipeline. In other shells, this assignment is executed in a subshell so the first assignment remains in place in the parent.

A workaround is to execute the rest of the script in the pipeline. This is a common idiom for reading data and doing some processing afterward:

output_some_stuff | {
  var=
  while IFS= read -r line; do
    var=$(process "$line")
  done
  use "$var"
}

You appear to have run into a ksh bug. I recommend upgrading to a non-buggy version. If that isn't possible, try Stephane Chazelas's workaround. While you can try running your scripts in bash, it is not (and does not pretend to be) a drop-in replacement for ksh; there are plenty of ksh features that bash doesn't have (and vice versa). Bash and ksh are only compatible in their POSIX core and some other central features (in particular arrays, [[ … ]], and local variables in functions declared by typeset).

You could also try zsh, which when invoked as ksh behaves in a way that's a bit closer to ksh than bash is. You may nonetheless run into incompatibilities.

Shell – Checking if a command is a built-in in ksh

If your concern is about aliases, just do:

[[ $(unalias -- "$cmd"; type -- "$cmd") = *builtin ]]

($(...) create a subshell environment, so unalias is only in effect there).

If you're also concerned about functions, also run command unset -f -- "$cmd" before type.

Best Answer

Related Solutions

Bash vs ksh pipes

Shell – Checking if a command is a built-in in ksh

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