I have a weird situation where Bash doesn't seem to be assigning one of my variables correctly, or it might be to do with pipes. I'm honestly not sure. Here's the code:
test=0
if [ "$#" -ne 4 ]; then
echo "Error: You have passed an incorrect number of parameters" > client.pipe
test=1
if [ $test -eq 1 ]; then <-------THIS SOMETIMES DOESN'T EXECUTE
echo "end_result" > client.pipe
exit 1
What's going on here is this is a server script for a command: 'select'.
The select command takes in 4 parameters: an id (to tell it which pipe to output messages to), a database name, a table name, and then a string of column ids.
At the other end of the client pipe, the client script is listening for messages, and if it receives 'end_result' it stops listening. As you can see, no matter what happens 'end_result' should get sent back to the pipe and printed by the client script, but sometimes it doesn't happen, and I get stuck in an infinite while loop. Here's the client script so you can see where the 'listening' is happening.
while true; do
read message < client.pipe
if [ "$message" == "end_result" ]; then
echo $message
break
else
echo $message
fi
done
If I pass in the incorrect number of parameters to the first script, it prints out 'Error: you have passed an incorrect num of parameters' to the pipe, but then sometimes it doesn't assign 1 to test, and it doesn't then send 'end_result' to the pipe and exit. I can't really see what the issue is, and as I've said it works probably 7 times out of 10... I can get around the issue by having the parent script send 'end_result' to the client, but it's a bit of a hack.
I'd really appreciate it if anyone could see what the issue is, and I'm happy to provide more info about the code if required.
Many thanks, R
EDIT:
I'm almost certain the problem is to do with reading from the client pipe and that while loop, as though something were getting stuck in the pipe...
This was the solution, provided by #WilliamPursell:
while true; do
read message
if [ "$message" == "end_result" ]; then
echo $message
break
else
echo $message
fi
done < client.pipe
Related
I am trying to learn linux bash scripting. I have a script and I want to get the return value of this script and store it in a variable.
Any help is welcome.
Thank you in advance.
#!/bin/bash
HOST_NAME=$1
{ echo "105" ; sleep 5; } | telnet $HOST_NAME 9761;
To avoid confusion don't think/talk of it as a return value, think of it as what it is - an exit status.
In most programming languages you can capture the return value of a function by capturing whatever that function returns in a variable, e.g. with a C-like language :
int foo() {
printf("35\n");
return 7;
}
void main() {
int var;
var=foo();
}
the variable var in main() after calling foo() will hold the value 7 and 35 will have been printed to stdout. In shell however with similar-looking code:
foo() {
printf "35\n"
return 7
}
main() {
local var
var=$(foo)
}
var will have the value 35 and the unmentioned builtin variable $? which always holds the exit status of the last command run will have the value 7. If you wanted to duplicate the C behavior where 35 goes to stdout and var contains 7 then that'd be:
foo() {
printf "35\n"
return 7
}
main() {
local var
foo
var=$?
}
The fact that shell functions use the keyword return to report their exit status is confusing at first if you're used to other Algol-based languages like C but if they used exit then it'd terminate the whole process so they had to use something and it quickly becomes obvious what it really means.
So when taking about shell scripts and functions use the words "output" and "exit status", not "return" which some people in some contexts will assume means either of those 2 things, and that'll avoid all confusion.
Btw to avoid making things even more complicated I said above that $? is a variable but it's really the value of the "special parameter" ?. If you really want to understand the difference right now then see https://www.gnu.org/software/bash/manual/bash.html#Shell-Parameters for a discussion of shell parameters which includes "special parameters" like ? and #, "positional parameters" like 1 and 2, and "variables" like HOME and var as used in my script above.
The $? shell variable stores the return value, however with the Linux telnet client this may not be as useful as you think. The client will return 1 if the remote host closes the connection (or any remote or network error occurs) and 0 if the local client side closes the connection successfully. The problem being that many services are written so that they send data and then close the TCP connection themselves without waiting for the client:
$ telnet time-b.timefreq.bldrdoc.gov 13
Trying 132.163.96.2...
Connected to time-b-b.nist.gov.
Escape character is '^]'.
58600 19-04-27 13:56:16 50 0 0 736.0 UTC(NIST) *
Connection closed by foreign host.
$ echo $?
1
Even if the client sends a command to the server to quit over the TCP stream, this still results in the remote side closing the connection, with the same result:
$ telnet mail.tardis.ed.ac.uk 25
Trying 193.62.81.50...
Connected to isolus.tardis.ed.ac.uk.
Escape character is '^]'.
220 isolus.tardis.ed.ac.uk ESMTP Postfix (Debian/GNU)
QUIT
221 2.0.0 Bye
Connection closed by foreign host.
$ echo $?
1
So, you're going to get a 1 no matter what really. If you want the return value of a remote script, this is easier with ssh like this:
$ ssh ssh.tardis.ed.ac.uk "exit 5"
THE TARDIS PROJECT | pubpubpubpubpubpubpubpubpub | Authorised access only
$ echo $?
5
As far as I know the only time telnet would return zero (i.e. success) is if you escape and quite the client, like this:
$ telnet www.google.com 80
Trying 216.58.210.36...
Connected to www.google.com.
Escape character is '^]'.
^]
telnet> quit
Connection closed.
$ echo $?
0
Hope this helps.
It depends on what you mean by return value.
Processes (on UNIX-like systems) can return to a shell a single unsigned byte as an exit status, so that gives a value in the range 0-255. By convention zero means success and any other value indicates a failure.
(In lower-level languages, like C, you can get more than just this exit status, but that's not visible in bash).
The exit status of the last command run is stored in variable ?, so you can get its value from $?, however since many programs only return either 0 (it worked) or 1 (it didn't work), that's not much use.
Bash conditionals, like if and while test for success (exit code of 0) or failure (exit code of non-zero):
if some-command
then
echo "It worked"
else
echo "It didn't work"
fi
However ....
If you mean you want to get the output from the script, that's a different matter. You can capture it using:
var=$(some-command)
But wait, that only captures normal output, routed to a stream called stdout (file descriptor 1), it does not capture error messages which most programs write to a stream called stderr (file descriptor 2). To capture errors as well you need to redirect file descriptor 2 to file descriptor 1:
var=$(some-command 2>&1)
The output text is now in variable var.
The ? variable always stores the exit code of the previous command.
You can retrieve the value with $?.
Some context: http://tldp.org/LDP/abs/html/exit-status.html
Ok, this is going to seem like a basic question at first, but please hear me out. It's more complex than the title makes it seem!
Here is the goal of what I am trying to do. I would like to output to console similar to Linux boot.
Operating system is doing something... [ OK ]
Now this would seem to be obvious... Just use printf and set columns. Here is the first problem. The console needs to first print the action
Operating system is doing something...
Then it needs to actually do the work and then continue by outputting to the same line with the [ OK ].
This would again seem easy to do using printf. Simply do the work (in this case, call a function) and return a conditional check and then finish running the printf to output either [ OK ] or [ FAIL ]. This technically works, but I ran into LOTS of complications doing this. This is because the function must be called inside a subshell and I cant pass certain variables that I need. So printf is out.
How about just using echo -n? That should work right? Echo the first part, run the function, then continue echoing based on the return to the same line. The problem with this solution is I can no longer preserve the column formatting that I can with printf.
Operating system is doing something... [ OK ]
Operating system is doing something else... [ OK ]
Short example... [ OK ]
Any suggestions how I can fix any of these problems to get a working solution? Thanks
Here is another way I tried with printf. This gives the illusion of working, but the method is actually flawed because it does not give you the progress indication, ie the function runs first before it ever prints out the the function is running. The "hey im doing stuff" prints immediately with the "hey im done" message. As a result, its pointless.
VALIDATE $HOST; printf "%-50s %10s\n" " Validating and sanitizing input..." "$(if [ -z "$ERROR" ]; then echo "[$GREEN OK $RESET]"; else echo "[$RED FAIL $RESET] - $ERROR"; echo; exit; fi)"
There's no particular reason all the printf strings have to be printed together, unless you're worried some code you call is going to move the cursor.
Reordering your example:
printf "%-50s " " Validating and sanitizing input..."
VALIDATE $HOST
if [ -z "$ERROR" ]; then
printf "%10s\n" "[$GREEN OK $RESET]";
else
printf "%10s\n" "[$RED FAIL $RESET] - $ERROR"
echo
exit
fi
I have no idea what $ERROR contains or where it is supposed to display.
I have a script that requires at least one argument. If the arguments are incorrect I will obviously return a non-zero exit code.
I have decided to print the usage instructions when called with no arguments, but I'm uncertain of the exit code. Should it be
0 (success)
one of the values [1-125] (error)
An argument could be made either way. Is some option recommended?
It depends on whether you want the callers of your script to know programatically why it failed.
If, for example, it will only ever be run by a human, you could just return 1 and rely on the error text to inform them.
You could even return zero regardless if you only ever want a human to easily know (via the output) if it failed but I would consider this very bad form - Microsoft have made this mistake in the past with some of their commands not setting %errorlevel%, making it very difficult for a script to figure out whether it's worked or not.
Even if you decide not to indicate why it failed, you should at least indicate that it failed.
If you want a program to easily figure out why it failed, return a different code for each discernible error type.
Any Unix command when it returns control to its parent process returns a code, number between 0 and 255.
Success is traditionally represented with exit 0; failure is normally indicated with a non-zero exit-code. This value can indicate different reasons for failure.
So what you have also is another case of unsuccessful completion of your program which should be treated as returning a proper error code and not 0.
Also note that traditional Unix system calls don't signal an error code for empty arguments, they have to be caught explicitly. There are ones for argument list too long (E2BIG) and invalid arguments (EINVAL).
This is what I decided on using, based mostly on Inians answer and the reserved exit codes presented here.
I decided on somewhat reusing the exit codes defined in /usr/include/asm-generic/errno-base.h1. There was nothing suitable for "too little arguments", so I picked the first one from the 64-113 range.
# Error codes
E2BIG=7
EINVAL=22
E2SMALL=64
NAME="my-script"
USAGE=$(cat << END
the help text
RETURN CODES
$NAME has the following return codes:
0 success
$E2BIG Argument list too long
$EINVAL Invalid argument
$E2SMALL Argument list too short
END
)
# This script requires exactly one argument. If less are provided it prints
# the USAGE instructions
if [ ${#} -lt 1 ] ; then
printf '%s\n' "${USAGE}"
exit $E2SMALL
fi
if [ "${1}" = "-h" ] || [ "${1}" = "--help" ] ; then
printf '%s\n' "${USAGE}"
exit 0
fi
if [ ${#} -gt 1 ] ; then
printf '%s\n' "${USAGE}"
exit $E2BIG
fi
It's a bit verbose, but at least there's a properly defined error code if the script is called improperly.
1 This may be debatable, but since the codes were already defined...
During the process of writing a script, I will use the command's output in varying ways, and to different degrees - in order to troubleshoot the task at hand.. For example, in this snippet, which reads an Application's icon resource and returns whether or not it has the typical .icns extension...
icns=`defaults read /$application/Contents/Info CFBundleIconFile`
if ! [[ $icns =~ ^(.*)(.icns)$ ]]; then
echo -e $icns "is NOT OK YOU IDIOT! **** You need to add .icns to "$icns"."
else
echo -e $icns "\t Homey, it's cool. That shits got its .icns, proper."
fi
Inevitably, as each bug is squashed, and the stdout starts relating more to the actual function vs. the debugging process, this feedback is usually either commented out, silenced, or deleted - for obvious reasons.
However, if one wanted to provide a simple option - either hardcoded, or passed as a parameter, to optionally show some, all, or none of "this kind" of message at runtime - what is the best way to provide that simple functionality? I am looking to basically duplicate the functionality of set -x but instead of a line-by rundown, it would only print the notifications that I had architected specificically.
It seems excessive to replace each and every echo with an if that checks for a debug=1|0, yet I've been unable to find a concise explanation of how to implement a getopts/getopt scheme (never can remember which one is the built-in), etc. in my own scripts. This little expression seemed promising, but there is very little documentation re: 2>$1 out there (although I'm sure this is key to this puzzle)
[ $DBG ] && DEBUG="" || DEBUG='</dev/null'
check_errs() {
# Parameter 1 is the return code Para. 2 is text to display on failure.
if [ "${1}" -ne "0" ]; then
echo "ERROR # ${1} : ${2}"
else
echo "SUCESSS "
fi }
Any concise and reusable tricks to this trade would be welcomed, and if I'm totally missing the boat, or if it was a snake, and it would be biting me - I apologize.
One easy trick is to simply replace your "logging" echo comamnd by a variable, i.e.
TRACE=:
if test "$1" = "-v"; then
TRACE=echo
shift
fi
$TRACE "You passed the -v option"
You can have any number of these for different types of messages if you wish so.
you may check a common open source trace library with support for bash.
http://sourceforge.net/projects/utalm/
https://github.com/ArnoCan/utalm
WKR
Arno-Can Uestuensoez
I'm writing shell scripts where quite regularly some stuff is written
to a file, after which an application is executed that reads that file. I find that through our company the network latency differs vastly, so a simple sleep 2 for example will not be robust enough.
I tried to write a (configurable) timeout loop like this:
waitLoop()
{
local timeout=$1
local test="$2"
if ! $test
then
local counter=0
while ! $test && [ $counter -lt $timeout ]
do
sleep 1
((counter++))
done
if ! $test
then
exit 1
fi
fi
}
This works for test="[ -e $somefilename ]". However, testing existence is not enough, I sometimes need to test whether a certain string was written to the file. I tried
test="grep -sq \"^sometext$\" $somefilename", but this did not work. Can someone tell me why?
Are there other, less verbose options to perform such a test?
You can set your test variable this way:
test=$(grep -sq "^sometext$" $somefilename)
The reason your grep isn't working is that quotes are really hard to pass in arguments. You'll need to use eval:
if ! eval $test
I'd say the way to check for a string in a text file is grep.
What's your exact problem with it?
Also you might adjust your NFS mount parameters, to get rid of the root problem. A sync might also help. See NFS docs.
If you're wanting to use waitLoop in an "if", you might want to change the "exit" to a "return", so the rest of the script can handle the error situation (there's not even a message to the user about what failed before the script dies otherwise).
The other issue is using "$test" to hold a command means you don't get shell expansion when actually executing, just evaluating. So if you say test="grep \"foo\" \"bar baz\"", rather than looking for the three letter string foo in the file with the seven character name bar baz, it'll look for the five char string "foo" in the nine char file "bar baz".
So you can either decide you don't need the shell magic, and set test='grep -sq ^sometext$ somefilename', or you can get the shell to handle the quoting explicitly with something like:
if /bin/sh -c "$test"
then
...
Try using the file modification time to detect when it is written without opening it. Something like
old_mtime=`stat --format="%Z" file`
# Write to file.
new_mtime=$old_mtime
while [[ "$old_mtime" -eq "$new_mtime" ]]; do
sleep 2;
new_mtime=`stat --format="%Z" file`
done
This won't work, however, if multiple processes try to access the file at the same time.
I just had the exact same problem. I used a similar approach to the timeout wait that you include in your OP; however, I also included a file-size check. I reset my timeout timer if the file had increased in size since last it was checked. The files I'm writing can be a few gig, so they take a while to write across NFS.
This may be overkill for your particular case, but I also had my writing process calculate a hash of the file after it was done writing. I used md5, but something like crc32 would work, too. This hash was broadcast from the writer to the (multiple) readers, and the reader waits until a) the file size stops increasing and b) the (freshly computed) hash of the file matches the hash sent by the writer.
We have a similar issue, but for different reasons. We are reading s file, which is sent to an SFTP server. The machine running the script is not the SFTP server.
What I have done is set it up in cron (although a loop with a sleep would work too) to do a cksum of the file. When the old cksum matches the current cksum (the file has not changed for the determined amount of time) we know that the writes are complete, and transfer the file.
Just to be extra safe, we never overwrite a local file before making a backup, and only transfer at all when the remote file has two cksums in a row that match, and that cksum does not match the local file.
If you need code examples, I am sure I can dig them up.
The shell was splitting your predicate into words. Grab it all with $# as in the code below:
#! /bin/bash
waitFor()
{
local tries=$1
shift
local predicate="$#"
while [ $tries -ge 1 ]; do
(( tries-- ))
if $predicate >/dev/null 2>&1; then
return
else
[ $tries -gt 0 ] && sleep 1
fi
done
exit 1
}
pred='[ -e /etc/passwd ]'
waitFor 5 $pred
echo "$pred satisfied"
rm -f /tmp/baz
(sleep 2; echo blahblah >>/tmp/baz) &
(sleep 4; echo hasfoo >>/tmp/baz) &
pred='grep ^hasfoo /tmp/baz'
waitFor 5 $pred
echo "$pred satisfied"
Output:
$ ./waitngo
[ -e /etc/passwd ] satisfied
grep ^hasfoo /tmp/baz satisfied
Too bad the typescript isn't as interesting as watching it in real time.
Ok...this is a bit whacky...
If you have control over the file: you might be able to create a 'named pipe' here.
So (depending on how the writing program works) you can monitor the file in an synchronized fashion.
At its simplest:
Create the named pipe:
mkfifo file.txt
Set up the sync'd receiver:
while :
do
process.sh < file.txt
end
Create a test sender:
echo "Hello There" > file.txt
The 'process.sh' is where your logic goes : this will block until the sender has written its output. In theory the writer program won't need modifiying....
WARNING: if the receiver is not running for some reason, you may end up blocking the sender!
Not sure it fits your requirement here, but might be worth looking into.
Or to avoid synchronized, try 'lsof' ?
http://en.wikipedia.org/wiki/Lsof
Assuming that you only want to read from the file when nothing else is writing to it (ie, the writing process has finished) - you could check whether nothing else has file handle to it ?