How Would I Tell a Bash Script to Start Over from the Top

How would I tell a bash script to start over from the top?

Put it in a while loop. I'd also suggest you add a "sleep" so that you're not racing your machine's CPU as fast as it will go:

while true; do
##########################################################################
## CHECK TIME
##########################################################################
time=$(date +%k%M)

if [[ "$time" -ge 1800 ]] && [[ "$time" -le 2200 ]]; then
echo "Not a good time to transcode video!" && exit 0
else
echo "Excellent time to transcode video!" && echo "Lets get started!"
fi
##########################################################################
## CHECK TIME
##########################################################################
for i in {1..5}; do
echo $i
sleep 1
done
done

How do I write a bash script to restart a process if it dies?

Avoid PID-files, crons, or anything else that tries to evaluate processes that aren't their children.

There is a very good reason why in UNIX, you can ONLY wait on your children. Any method (ps parsing, pgrep, storing a PID, ...) that tries to work around that is flawed and has gaping holes in it. Just say no.

Instead you need the process that monitors your process to be the process' parent. What does this mean? It means only the process that starts your process can reliably wait for it to end. In bash, this is absolutely trivial.

until myserver; do
echo "Server 'myserver' crashed with exit code $?. Respawning.." >&2
sleep 1
done

The above piece of bash code runs myserver in an until loop. The first line starts myserver and waits for it to end. When it ends, until checks its exit status. If the exit status is 0, it means it ended gracefully (which means you asked it to shut down somehow, and it did so successfully). In that case we don't want to restart it (we just asked it to shut down!). If the exit status is not 0, until will run the loop body, which emits an error message on STDERR and restarts the loop (back to line 1) after 1 second.

Why do we wait a second? Because if something's wrong with the startup sequence of myserver and it crashes immediately, you'll have a very intensive loop of constant restarting and crashing on your hands. The sleep 1 takes away the strain from that.

Now all you need to do is start this bash script (asynchronously, probably), and it will monitor myserver and restart it as necessary. If you want to start the monitor on boot (making the server "survive" reboots), you can schedule it in your user's cron(1) with an @reboot rule. Open your cron rules with crontab:

crontab -e

Then add a rule to start your monitor script:

@reboot /usr/local/bin/myservermonitor

Alternatively; look at inittab(5) and /etc/inittab. You can add a line in there to have myserver start at a certain init level and be respawned automatically.


Edit.

Let me add some information on why not to use PID files. While they are very popular; they are also very flawed and there's no reason why you wouldn't just do it the correct way.

Consider this:

  1. PID recycling (killing the wrong process):

    • /etc/init.d/foo start: start foo, write foo's PID to /var/run/foo.pid
    • A while later: foo dies somehow.
    • A while later: any random process that starts (call it bar) takes a random PID, imagine it taking foo's old PID.
    • You notice foo's gone: /etc/init.d/foo/restart reads /var/run/foo.pid, checks to see if it's still alive, finds bar, thinks it's foo, kills it, starts a new foo.
  2. PID files go stale. You need over-complicated (or should I say, non-trivial) logic to check whether the PID file is stale, and any such logic is again vulnerable to 1..

  3. What if you don't even have write access or are in a read-only environment?

  4. It's pointless overcomplication; see how simple my example above is. No need to complicate that, at all.

See also: Are PID-files still flawed when doing it 'right'?

By the way; even worse than PID files is parsing ps! Don't ever do this.

  1. ps is very unportable. While you find it on almost every UNIX system; its arguments vary greatly if you want non-standard output. And standard output is ONLY for human consumption, not for scripted parsing!
  2. Parsing ps leads to a LOT of false positives. Take the ps aux | grep PID example, and now imagine someone starting a process with a number somewhere as argument that happens to be the same as the PID you stared your daemon with! Imagine two people starting an X session and you grepping for X to kill yours. It's just all kinds of bad.

If you don't want to manage the process yourself; there are some perfectly good systems out there that will act as monitor for your processes. Look into runit, for example.

How can I loop over the output of a shell command?

Never for loop over the results of a shell command if you want to process it line by line unless you are changing the value of the internal field separator $IFS to \n. This is because the lines will get subject of word splitting which leads to the actual results you are seeing. Meaning if you for example have a file like this:

foo bar
hello world

The following for loop

for i in $(cat file); do
echo "$i"
done

gives you:

foo
bar
hello
world

Even if you use IFS='\n' the lines might still get subject of Filename expansion


I recommend to use while + read instead because read reads line by line.

Furthermore I would use pgrep if you are searching for pids belonging to a certain binary. However, since python might appear as different binaries, like python2.7 or python3.4 I suggest to pass -f to pgrep which makes it search the whole command line rather than just searching for binaries called python. But this will also find processes which have been started like cat foo.py. You have been warned! At the end you can refine the regex passed to pgrep like you wish.

Example:

pgrep -f python | while read -r pid ; do
echo "$pid"
done

or if you also want the process name:

pgrep -af python | while read -r line ; do
echo "$line"
done

If you want the process name and the pid in separate variables:

pgrep -af python | while read -r pid cmd ; do
echo "pid: $pid, cmd: $cmd"
done

You see, read offers a flexible and stable way to process the output of a command line-by-line.


Btw, if you prefer your ps .. | grep command line over pgrep use the following loop:

ps -ewo pid,etime,cmd | grep python | grep -v grep | grep -v sh \
| while read -r pid etime cmd ; do
echo "$pid $cmd $etime"
done

Note how I changed the order of etime and cmd. Thus to be able to read cmd, which can contain whitespace, into a single variable. This works because read will break down the line into variables, as many times as you specified variables. The remaining part of the line - possibly including whitespace - will get assigned to the last variable which has been specified in the command line.

Why do you need to put #!/bin/bash at the beginning of a script file?

It's a convention so the *nix shell knows what kind of interpreter to run.

For example, older flavors of ATT defaulted to sh (the Bourne shell), while older versions of BSD defaulted to csh (the C shell).

Even today (where most systems run bash, the "Bourne Again Shell"), scripts can be in bash, python, perl, ruby, PHP, etc, etc. For example, you might see #!/bin/perl or #!/bin/perl5.

PS:
The exclamation mark (!) is affectionately called "bang". The shell comment symbol (#) is sometimes called "hash".

PPS:
Remember - under *nix, associating a suffix with a file type is merely a convention, not a "rule". An executable can be a binary program, any one of a million script types and other things as well. Hence the need for #!/bin/bash.

Bash script looping after end of file

zerobandwidth's answer is correct, but as an alternative answer, it is in fact quite easy to do what you initially wanted to do:

while true; do
# Your account creation code goes here

echo "Create another account (y/n)?"
read another
if [ "$another" != y ]; then
break
fi
done

How to wait in bash for several subprocesses to finish, and return exit code !=0 when any subprocess ends with code !=0?

wait also (optionally) takes the PID of the process to wait for, and with $! you get the PID of the last command launched in the background.
Modify the loop to store the PID of each spawned sub-process into an array, and then loop again waiting on each PID.

# run processes and store pids in array
for i in $n_procs; do
./procs[${i}] &
pids[${i}]=$!
done

# wait for all pids
for pid in ${pids[*]}; do
wait $pid
done


Related Topics



Leave a reply



Submit