when piping command, is nice applied to all chain?

Question

Consider the following case

nice -10 mysqldump -u crontab -p'readonly' --databases "$1" | gzip > $hour_folder"/"$1"_h"$hour".sql.gz"

I aask you if nice is applied only to mysqldumnp or is applied to gzip, too?

Answer 1

No, nice (as opposed to the ! or time shell keywords for instance¹) is a standalone command, so it could not apply to other commands that are piped to it or that it is piped to.

The change of niceness is only applied to the process that nice is executed in that will later execute the command passed as arguments (and all the processes that that command may itself spawn as niceness is inherited upon fork).

If you want to increase the niceness of both commands, you'd need two separate calls to nice:

nice cmd1 | nice cmd2

Or have nice start a shell that starts the pipeline:

nice sh -c 'cmd1 | cmd2'

Or you could increase the niceness of the current shell or subshell process with renice prior to starting the pipeline (beware some renice implementations set the niceness while others increase it by the specified amount).

With zsh:

zmodload zsh/system
(renice 10 $sysparam[pid]; cmd1 | cmd2)

With bash:

(renice 10 "$BASHPID"; cmd1 | cmd2)

In zsh, commands started asynchronously have their niceness increased by 5 (unless the bgnice option is disabled), so you could also do:

(cmd1 | cmd2 &) &

To increase the niceness of both commands by 10.

$ ps -o nice,args | cat
 NI COMMAND
  0 /bin/zsh
  0 ps -o nice,args
  0 cat
$ nice ps -o nice,args | cat
 NI COMMAND
  0 /bin/zsh
 10 ps -o nice,args
  0 cat
$ nice ps -o nice,args | nice cat
 NI COMMAND
  0 /bin/zsh
 10 ps -o nice,args
 10 cat
$ ps -o nice,args | cat &
 NI COMMAND
  0 /bin/zsh
  5 ps -o nice,args
  5 cat
$ (ps -o nice,args | cat &) &
 NI COMMAND
  0 /bin/zsh
 10 ps -o nice,args
 10 cat

Also beware that in nice mysqldump..., a lot of the CPU time used by that operation will be spent by the mysqld process handling the requests by mysqldump, and nice only sets the niceness of the process that is executing mysqldump, that has no effect on the niceness of the MySQL server processes themselves.

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^{! and time in bash and some other Korn-like shells are keywords in the shell language, not commands. They apply to the following pipeline. Beware though that if there's a -p word following time or if time is quoted in any way, then the time standalone command is run instead and therefore only applies to the command passed as its arguments.}