When I'm looking at bash script code, I sometimes see | and sometimes see ||, but I don't know which is preferable.
I'm trying to do something like ..
set -e;
ret=0 && { which ansible || ret=$?; }
if [[ ${ret} -ne 0 ]]; then
# install ansible here
fi
Please advise which OR operator is preferred in this scenario.
| isn't an OR operator at all. You could use ||, though:
which ansible || {
true # put your code to install ansible here
}
This is equivalent to an if:
if ! which ansible; then
true # put your code to install ansible here
fi
By the way -- consider making a habit of using type (a shell builtin) rather than which (an external command). type is both faster and has a better understanding of shell behavior: If you have an ansible command that's provided by, say, a shell function invoking the real command, which won't know that it's there, but type will correctly detect it as available.
There is a big difference between using a single pipe (pipe output from one command to be used as input for the next command) and a process control OR (double pipe).
cat /etc/issue | less
This runs the cat command on the /etc/issue file, and instead of immediately sending the output to stdout it is piped to be the input for the less command. Yes, this isn't a great example, since you could instead simply do less /etc/issue - but at least you can see how it works
touch /etc/testing || echo Did not work
For this one, the touch command is run, or attempted to run. If it has a non-zero exit status, then the double pipe OR kicks in, and tries to execute the echo command. If the touch command worked, then whatever the other choice is (our echo command in this case) is never attempted...
Related
I am quite new to bash (barely any experience at all) and I need some help with a bash script.
I am using docker-compose to create multiple containers - for this example let's say 2 containers. The 2nd container will execute a bash command, but before that, I need to check that the 1st container is operational and fully configured. Instead of using a sleep command I want to create a bash script that will be located in the 2nd container and once executed do the following:
Execute a command and log the console output in a file
Read that file and check if a String is present. The command that I will execute in the previous step will take a few seconds (5 - 10) seconds to complete and I need to read the file after it has finished executing. I suppose i can add sleep to make sure the command is finished executing or is there a better way to do this?
If the string is not present I want to execute the same command again until I find the String I am looking for
Once I find the string I am looking for I want to exit the loop and execute a different command
I found out how to do this in Java, but if I need to do this in a bash script.
The docker-containers have alpine as an operating system, but I updated the Dockerfile to install bash.
I tried this solution, but it does not work.
#!/bin/bash
[command to be executed] > allout.txt 2>&1
until
tail -n 0 -F /path/to/file | \
while read LINE
do
if echo "$LINE" | grep -q $string
then
echo -e "$string found in the console output"
fi
done
do
echo "String is not present. Executing command again"
sleep 5
[command to be executed] > allout.txt 2>&1
done
echo -e "String is found"
In your docker-compose file make use of depends_on option.
depends_on will take care of startup and shutdown sequence of your multiple containers.
But it does not check whether a container is ready before moving to another container startup. To handle this scenario check this out.
As described in this link,
You can use tools such as wait-for-it, dockerize, or sh-compatible wait-for. These are small wrapper scripts which you can include in your application’s image to poll a given host and port until it’s accepting TCP connections.
OR
Alternatively, write your own wrapper script to perform a more application-specific health check.
In case you don't want to make use of above tools then check this out. Here they use a combination of HEALTHCHECK and service_healthy condition as shown here. For complete example check this.
Just:
while :; do
# 1. Execute a command and log the console output in a file
command > output.log
# TODO: handle errors, etc.
# 2. Read that file and check if a String is present.
if grep -q "searched_string" output.log; then
# Once I find the string I am looking for I want to exit the loop
break;
fi
# 3. If the string is not present I want to execute the same command again until I find the String I am looking for
# add ex. sleep 0.1 for the loop to delay a little bit, not to use 100% cpu
done
# ...and execute a different command
different_command
You can timeout a command with timeout.
Notes:
colon is a utility that returns a zero exit status, much like true, I prefer while : instead of while true, they mean the same.
The code presented should work in any posix shell.
Specifics:
I'm trying to build a bash script which needs to do a couple of things.
Firstly, it needs to run a third party script that I cannot manipulate. This script will build a project and then start a node server which outputs data to the terminal continually. This process needs to continue indefinitely so I can't have any exit codes.
Secondly, I need to wait for a specific line of output from the first script, namely 'Started your app.'.
Once that line has been output to the terminal, I need to launch a separate set of commands, either from another subscript or from an if or while block, which will change a few lines of code in the project that was built by the first script to resolve some dependencies for a later step.
So, how can I capture the output of the first subscript and use that to run another set of commands when a particular line is output to the terminal, all while allowing the first script to run in the terminal, and without using timers and without creating a huge file from the output of subscript1 as it will run indefinitely?
Pseudo-code:
#!/usr/bin/env bash
# This script needs to stay running & will output to the terminal (at some point)
# a string that we need to wait/watch for to launch subscript2
sh subscript1
# This can't run until subscript1 has output a particular string to the terminal
# This could be another script, or an if or while block
sh subscript2
I have been beating my head against my desk for hours trying to get this to work. Any help would be appreciated!
I think this is a bad idea — much better to have subscript1 changed to be automation-friendly — but in theory you can write:
sh subscript1 \
| {
while IFS= read -r line ; do
printf '%s\n' "$line"
if [[ "$line" = 'Started your app.' ]] ; then
sh subscript2 &
break
fi
done
cat
}
I have code for setting the zsh as default shell:
if [ -t 1 ]; then
exec zsh
fi
What exactly does the command if [ -t 1 ] do here?
I have code for setting the zsh as default shell:
No, you haven't. That's not what your code does, though it produces a similar effect.
if [ -t 1 ]; then
exec zsh
fi
What exactly does the command if [ -t 1 ] do here?
The command [ -t 1 ] is executed. If it exits with status 0 (indicating success, which for the [ command means the condition evaluates to true) then the commands in the body of the if statement are executed.
It may be a bit surprising that [ is a command, rather than part of shell syntax, but that's the case. Your if could be rewritten equivalently to use the test command instead:
if test -t 1; then
# ...
The other key thing, then, is the -t 1 part. You can find out about that in the manual for the test or [ command, but to save you the trouble, it is a conditional expression that evaluates whether file descriptor 1 (the shell's standard output) is connected to a terminal. This is similar to, but not exactly the same thing as, evaluating whether the shell is an interactive one.
Overall, the code presented has the effect of replacing the current (presumably bash) shell with zsh if the standard output is connected to a terminal. This is indirect, and a bit tricky; it would probably be better to genuinely set your login shell to /bin/zsh (or wherever it is installed) via the chsh command.
if command; then other_command; fi runs command and then, if that command exits with a return code of 0 ("success"), runs other_command.
The command [...] is designed to take the place of the Boolean expressions that you find in traditional programming languages. It has a number of options for what goes between the brackets and exits with 0=success if those options evaluate to a true value.
The specific subcommand -t tests a file descriptor to see if it is attached to a terminal. The file descriptor 1 is where the script's output is going (aka "standard output" or "stdout" for short). So -t 1 is true and [ -t 1 ] returns success if and only if the output of the script is going to a terminal (instead of into a file or pipe or something).
In that case, the current shell is replaced (via exec) by a copy of zsh. Which will hopefully not run the same script, since zsh works the same way and will make the same decision and go into an infinite loop execing itself.
Say I start with the following statement, which echo-s a string into the ether:
$ echo "foo" 1>/dev/null
I then submit the following pipeline:
$ echo "foo" | cat -e - 1>/dev/null
I then leave the process out:
$ echo "foo" | 1>/dev/null
Why is this not returning an error message? The documentation on bash and piping doesn't seem to make direct mention of may be the cause. Is there an EOF sent before the first read from echo (or whatever the process is, which is running upstream of the pipe)?
A shell simple command is not required to have a command name. For a command without a command-name:
variable assignments apply to the current execution environment. The following will set two variables to argument values:
arg1=$1 arg3=$3
redirections occur in a subshell, but the subshell doesn't do anything other than initialize the redirect. The following will truncate or create the indicated file (if you have appropriate permissions):
>/file/to/empty
However, a command must have at least one word. A completely empty command is a syntax error (which is why it is occasionally necessary to use :).
Answer summarized from Posix XCU§2.9.1
I've only been writing actual .sh scripts since sometime this morning, and I'm a bit stuck. I'm trying to write a script to check to see if a process is running, and to start it if it isn't. (I plan to run this script once every 10 to 15 minutes with cron.)
Here's what I have so far:
#!/bin/bash
APPCHK=$(ps aux | grep -c "/usr/bin/rsync -rvz -e ssh /home/e-smith/files/ibays/drive-i/files/Warehouse\ Pics/organized_pics imgserv#192.168.0.140:~/webapps/pavlick_container/public/images
")
RUNSYNC=$(rsync -rvz -e ssh /home/e-smith/files/ibays/drive-i/files/Warehouse\ Pics/organized_pics imgserv#192.168.0.140:~/webapps/pavlick_container/public/images)
if [ $APPCHK < '2' ];
then
$RUNSYNC
fi
exit
Here's the error that I'm getting:
$ ./image_sync.sh
rsync: mkdir "/home/i/webapps/pavlick_container/public/images" failed: No such file or directory (2)
rsync error: error in file IO (code 11) at main.c(595) [Receiver=3.0.7]
rsync: connection unexpectedly closed (9 bytes received so far) [sender]
rsync error: error in rsync protocol data stream (code 12) at io.c(601) [sender=3.0.7]
./image_sync.sh: line 8: 2: No such file or directory
TRTWF is that
rsync -rvz -e ssh /home/e-smith/files/ibays/drive-i/files/Warehouse\ Pics/organized_pics imgserv#192.168.0.140:~/webapps/pavlick_container/public/images
runs just fine from a terminal window.
What am I doing wrong?
Your grep call is wrong on two counts. The pattern shouldn't include a newline. To look for an exact string, use grep -F 'substring' or grep -xF 'exact whole line'.
Finding if a process is running with ps | grep is highly brittle. On most unices (at least Solaris, Linux and *BSD), use pgrep: pgrep -f 'PATTERN' returns true if there's a running process whose command line matches PATTERN.
Every program returns a status code, either 0 to indicate success or a number between 1 and 255 to indicate failure. In the shell, any command is a valid boolean expression; the status code 0 is treated as true and anything else as false.
$(…) means run the command inside the parentheses and capture its output. So rsync is executed as soon as the shell hits the definition of the RUNSYNC variable. To store a block of shell code, use a function (example below, although you don't actually need a function here, you could just write the code directly).
Your test [ $APPCHK < 2 ] should be [ $APPCHK -lt 2 ]: < means input redirection. (In bash, you can also write [[ foo < bar ]], but that's string comparison, not numeric comparison.)
~/ at the beginning of the remote rsync path is optional. Also, -e ssh is the default unless your version of rsync is really old.
exit at the end of the script is useless, the script will exit anyway.
Here's a script taking the above into account:
#!/bin/bash
run_rsync () {
rsync -rvz '/home/e-smith/files/ibays/drive-i/files/Warehouse Pics/organized_pics' \
imgserv#192.168.0.140:webapps/pavlick_container/public/images
}
process_pattern='/usr/bin/rsync -rvz /home/e-smith/files/ibays/drive-i/files/Warehouse Pics/organized_pics imgserv#192\.168\.0\.140:webapps/pavlick_container/public/images'
if pgrep -xF "$process_pattern"; then
run_rsync
fi
Looks like with your rsync command that some directory along this path is wrong: ~/webapps/pavlick_container/public/images
Have you checked on the server 192.168.0.140 in imgserv's home directory to see if "pavlick_container/public" exists? That's my guess.
You have a number of problems. First you are running the commands instead of putting the commands in variables. There is also a much easier way.
RUNSYNC="rsync -rvz -e ssh /home/e-smith/files/ibays/drive-i/files/Warehouse\ Pics/organized_pics imgserv#192.168.0.140:~/webapps/pavlick_container/public/images"
if ! pgrep -f "rsync.*organized_pics"; then $RUNSYNC; fi
First of all, the way of checking if the program is running is mostly wrong. This may or may not work. You should rely on some special file you create when your script starts, that it is deleted when your script ends. This will tell you if the script is running, just checking if this file exists.
Then, try to either put a \ before the ~ or to remove the ~/ completely. If cron is run as other user, the tilde will be substituted in the client for the user directory. It works for the command line because maybe the home directory of your user in both machines match, but not in the user the cron is running. A guess at this point, but again, try to remove the ~/ and see if it works.
If your real code is missing a closing dlb-quote on the grep target, you're going to get weird results from the get-go.
Also, ps aux will not list a complete command line result like you show (at least on all the the pss I have used).
You need to make it ps auxwww. Often you will see people add | grep -v grep | (you'll see why at some point). This can be reduced to changing your static search target slightly like "/usr/bin/rsync" to "/usr/bin/[r]sync ".
Other users are also helping with their comments. Using a flag file as #DiegoSevilla mentions is marginally deprecated. use a mkdir /tmp/MyWatcher_flagDir for your flag. Directory creation is an atomic activity (where as file creations are not), and this will eliminate any errors you might encounter from having 2 copies of you monitor try to make a flag file at the same time. Only one process will succeed in making or removing a flag dir.
I hope this helps.