What does if [ "${1}" == "pre" ]; do?


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# 1  
What does if [ "${1}" == "pre" ]; do?

I am trying to run a command after my computer returns from suspend.

Script came from here.

Running scripts before and after suspend with systemd << Just another Linux geek

(It is not currently not working.)

What does the if do?

Code:
#!/bin/sh
#
# Created 3/31/19 in /usr/lib/systemd/system-sleep/
if [ "${1}" == "pre" ]; then
  # Do the thing you want before suspend here, e.g.:
  echo "we are suspending at $(date)..." > /tmp/systemd_suspend_test
elif [ "${1}" == "post" ]; then
  # Do the thing you want after resume here, e.g.:
  echo "...and we are back from $(date)" >> /tmp/systemd_suspend_test
  gxmessage -fg red -font  'sans 30' -timeout 3  ' Computer has now resumed from suspend state.'
fi


Last edited by drew77; 04-01-2019 at 11:56 AM..
# 2  
Quote:
Originally Posted by drew77
Code:
if [ "${1}" == "pre" ]; then

"${1}" is the long form of "$1". It is the first positional parameter the script got. If you call a script like this:

Code:
$ ./somescript.sh first second third

then inside ./somescript.sh the variable "$1" would be set to "first", the variable "$2" would be set to "second" and "$3" would be set to "third". So this line is asking if the first parameter to the script equals "pre".

I hope this helps.

bakunin
# 3  
Also note that syntactically
Code:
if [ "${1}" == "pre" ]; then

should be:
Code:
if [ "${1}" = "pre" ]; then

Whatever happens to be the shell /bin/sh on your system may or may not be able to handle this correctly..

Another thing to verify is that gxmessage needs to be installed and in your default search path in order for the script to work, since the PATH variable is not set inside that script.
This User Gave Thanks to Scrutinizer For This Post:
# 4  
Thanks.

All my scripts are in /home/andy/bin.

Code:
echo $PATH
/home/andy/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin


Last edited by drew77; 04-01-2019 at 11:50 AM..
# 5  
man systemd-suspend.service

The documentation leaves much to be desired.

Code:
SYSTEMD-SUSPEND.SERVICE(8)                                        systemd-suspend.service                                       SYSTEMD-SUSPEND.SERVICE(8)

NAME
       systemd-suspend.service, systemd-hibernate.service, systemd-hybrid-sleep.service, systemd-suspend-then-hibernate.service, systemd-sleep - System
       sleep state logic

SYNOPSIS
       systemd-suspend.service

       systemd-hibernate.service

       systemd-hybrid-sleep.service

       systemd-suspend-then-hibernate.service

       /lib/systemd/system-sleep

DESCRIPTION
       systemd-suspend.service is a system service that is pulled in by suspend.target and is responsible for the actual system suspend. Similarly,
       systemd-hibernate.service is pulled in by hibernate.target to execute the actual hibernation. Finally, systemd-hybrid-sleep.service is pulled in by
       hybrid-sleep.target to execute hybrid hibernation with system suspend and pulled in by suspend-then-hibernate.target to execute system suspend with
       a timeout that will activate hibernate later.

       Immediately before entering system suspend and/or hibernation systemd-suspend.service (and the other mentioned units, respectively) will run all
       executables in /lib/systemd/system-sleep/ and pass two arguments to them. The first argument will be "pre", the second either "suspend",
       "hibernate", "hybrid-sleep", or "suspend-then-hibernate" depending on the chosen action. Immediately after leaving system suspend and/or
       hibernation the same executables are run, but the first argument is now "post". All executables in this directory are executed in parallel, and
       execution of the action is not continued until all executables have finished.

       Note that scripts or binaries dropped in /lib/systemd/system-sleep/ are intended for local use only and should be considered hacks. If applications
       want to react to system suspend/hibernation and resume, they should rather use the Inhibitor interface[1].

       Note that systemd-suspend.service, systemd-hibernate.service, and systemd-hybrid-sleep.service systemd-suspend-then-hibernate.service should never
       be executed directly. Instead, trigger system sleep states with a command such as "systemctl suspend" or similar.

       Internally, this service will echo a string like "mem" into /sys/power/state, to trigger the actual system suspend. What exactly is written where
       can be configured in the "[Sleep]" section of /etc/systemd/sleep.conf or a sleep.conf.d file. See systemd-sleep.conf(5).

OPTIONS
       systemd-sleep understands the following commands:

       -h, --help
           Print a short help text and exit.

       --version
           Print a short version string and exit.

       suspend, hibernate, hybrid-sleep, suspend-then-hibernate
           Suspend, hibernate, suspend then hibernate, or put the system to hybrid sleep.

SEE ALSO
       systemd-sleep.conf(5), systemd(1), systemctl(1), systemd.special(7), systemd-halt.service(8)

NOTES
        1. Inhibitor interface
           https://www.freedesktop.org/wiki/Software/systemd/inhibit

systemd 237                                                                                                                     SYSTEMD-SUSPEND.SERVICE(8)

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