# Job Scripts¶

This page documents the basics of how to write job scripts for the HPC clusters. Cluster-specific details are kept in separate sub pages for each cluster:

## Job Script Basics¶

To run a job on the cluster involves creating a shell script called a job script. The job script is a plain-text file containing any number of commands, including your main computational task, i.e., it may copy or rename files, cd into the proper directory, etc., all before doing the “real” work. The lines in the script file are the commands to be executed, in the given order. Lines starting with a # are ignored as comments, except lines that start with #SBATCH, which are not executed, but contain special instructions to the queue system.

If you are not familiar with shell scripts, they are simply a set of commands that you could have typed at the command line. You can find more information about shell scripts here: Introduction to Bash shell scripts.

A job script consists of a couple of parts, in this order:

• The first line, which is always #!/bin/bash2

• Paremeters to the queue system

• Commands to set up the execution environment

• The actual commands you want to be run

Parameters to the queue system may be specified on the sbatch command line and/or in #SBATCH lines in the job script. There can be as many #SBATCH lines as you want, and you can combine several parameters on the same line. If a parameter is specified both on the command line and in the job script, the parameter specified on the command line takes precedence. The #SBATCH lines must precede any commands in the script.

Which parameters are allowed or required depends the job type and cluster, but two parameters must be present in (almost) any job:

• --account, which specifies the project the job will run in. Required by all jobs.

• --time, which specifies how long a job should be allowed to run. If it has not finished within that time, it will be cancelled.

The other parameters will be described in the sub pages for each cluster.

It is recommended to start the commands to set up the environment with

set -o errexit  # Exit the script on any error
set -o nounset  # Treat any unset variables as an error

module --quiet purge  # Reset the modules to the system default


and will most likely include one or more

module load SomeProgram/SomeVersion


to set up environment variables like \$PATH to get access to the specified programs. It is recommended to specify the explicit version in the module load command. We also recommend adding a

module list    # For easier debugging


after the module load commands. See Software Module Scheme for more information about software modules.

All in all, a generic job script might look like this:

#!/bin/bash

# Job name:
#SBATCH --job-name=YourJobname
#
# Project:
#SBATCH --account=nnXXXXk
#
# Wall time limit:
#SBATCH --time=DD-HH:MM:SS
#
# Other parameters:
#SBATCH ...

## Set up job environment:
set -o errexit  # Exit the script on any error
set -o nounset  # Treat any unset variables as an error

module --quiet purge  # Reset the modules to the system default
module list

## Do some work:
YourCommands


files/generic_job.sh

## Wall Time Limit¶

The wall time limit (--time) is required for all jobs.

The most used formats for the time specification is DD-HH:MM:SS and HH:MM:SS, where DD is days, HH hours, MM minutes and SS seconds. For instance:

• 3-12:00:00: 3.5 days

• 7:30:00: 7.5 hours

We recommend you to be as precise as you can when specifying the wall time limit as it will inflict on how fast your jobs will start to run: It is easier for a short job to get started between two larger, higher priority jobs (so-called backfilling). On the other hand, if the job has not finished before the wall time limit, it will be cancelled, so too long is better than too short due to lost work!

## Footnotes¶

2

Technically, any script language that uses # as a comment character can be used, but the recommended, and the only one supported by the Metacenter, is bash.