More bash snippets
Here are some more bash useful bash snippets:
Arithmetic, like incrementing a counter:
i=1
j=$((i+1))
echo $j
Looping over things:
# Run a command several times with specific values
for i in 1 2 3; do echo $i; done
# Loop over a specifc range of values using 'seq'
for i in $(seq 1 3); do echo $i; done
# Run a command on all txt files in current directory
for f in `ls *.txt`; do head $j; done
Redirecting output into files:
# Redirect standard output
date 1> out.log
# Redirect error output
blah 2> error.log
# Redirect both into different files
for cmd in date blah; do $cmd; done 1> out.log 2> error.log
# Redirect any kind of output into same file
for cmd in date blah; do $cmd; done &> all.log
Here’s a snippet I use for running a long running command, for example over a range of objects (accessible via some kind of ID)
# Open a screen session
screen -S BigJob
# Launch the job loop
for id in 1 2 3 4 5; do echo $id; date; do_something_with $id; done &> bigJob.log
# Detach again by holding CTRL-A and pressing D
The jobs are running safely in a screen environment, i.e. you don’t have to worry that you accidentaly kill the process by getting disconnected from the SSH session. And you can check the progress live via
tail -f bigJob.log
As the snippets prints the $id you can see which object/id is currently
processed; and using date in the snippet allows you to work out how long
each job took.