I would like to return exit code "0" from a failed command. Is there any easier way of doing this, rather than:
function a() {
ls aaaaa 2>&1;
}
if ! $(a); then
return 0
else
return 5
fi
Simply append return 0 to the function to force a function to always exit successful.
function a() {
ls aaaaa 2>&1
return 0
}
a
echo $? # prints 0
If you wish to do it inline for any reason you can append || true to the command:
ls aaaaa 2>&1 || true
echo $? # prints 0
If you wish to invert the exit status simple prepend the command with !
! ls aaaaa 2>&1
echo $? # prints 0
! ls /etc/resolv.conf 2>&1
echo $? # prints 1
Also if you state what you are trying to achieve overall we might be able to guide you to better answers.
It may be helpful for some people to try timeout command for commands that expect input (like SIGINT = keyboard interrupt) to be stopped like:
timeout 10 kubectl proxy &
This will execute kubectl proxy for 10 seconds (so you can perform the actions you need using the proxy) and then will gracefully terminate kubectl proxy
example:
timeout 3 kubectl proxy &
[1] 759
Starting to serve on 127.0.0.1:8001
echo $?
0
The help of timeout will also help on specific cases
timeout --help
Usage: timeout [OPTION] DURATION COMMAND [ARG]...
or: timeout [OPTION]
Start COMMAND, and kill it if still running after DURATION.
Mandatory arguments to long options are mandatory for short options too.
--preserve-status
exit with the same status as COMMAND, even when the
command times out
--foreground
when not running timeout directly from a shell prompt,
allow COMMAND to read from the TTY and get TTY signals;
in this mode, children of COMMAND will not be timed out
-k, --kill-after=DURATION
also send a KILL signal if COMMAND is still running
this long after the initial signal was sent
-s, --signal=SIGNAL
specify the signal to be sent on timeout;
SIGNAL may be a name like 'HUP' or a number;
see 'kill -l' for a list of signals
--help display this help and exit
--version output version information and exit
Related
Consider these two C programs:
#include <signal.h>
int main(void) {
raise(SIGTERM);
}
int main(void) {
return 143;
}
If I run either one, the value of $? in bash will be 143. The wait syscall lets you distinguish them, though:
wait4(-1, [{WIFSIGNALED(s) && WTERMSIG(s) == SIGTERM}], 0, NULL) = 11148
wait4(-1, [{WIFEXITED(s) && WEXITSTATUS(s) == 143}], 0, NULL) = 11214
And bash clearly uses this knowledge, since the first one results in Terminated being printed to the terminal (oddly, this happens even if I redirect both stdout and stderr elsewhere), and the second one doesn't. How can I differentiate these two cases from a bash script?
I believe getting the full exit codes from pure bash/shell is not possible.
The answers on Unix' StackExchange are very comprehensive.
What's common between all shells is that $? contains the lowest 8 bits of the exit code (the number passed to exit()) if the process terminated normally.
Where it differs is when the process is terminated by a signal. In all cases, and that's required by POSIX, the number will be greater than 128. POSIX doesn't specify what the value may be. In practice though, in all Bourne-like shells that I know, the lowest 7 bits of $? will contain the signal number. But, where n is the signal number,
in ash, zsh, pdksh, bash, the Bourne shell, $? is 128 + n. What that means is that in those shells, if you get a $? of 129, you don't know whether it's because the process exited with exit(129) or whether it was killed by the signal 1 (HUP on most systems). But the rationale is that shells, when they do exit themselves, by default return the exit status of the last exited command. By making sure $? is never greater than 255, that allows to have a consistent exit status:
$ bash -c 'sh -c "kill \$\$"; printf "%x\n" "$?"'
bash: line 1: 16720 Terminated sh -c "kill \$\$"
8f # 128 + 15
$ bash -c 'sh -c "kill \$\$"; exit'; printf '%x\n' "$?"
bash: line 1: 16726 Terminated sh -c "kill \$\$"
8f # here that 0x8f is from a exit(143) done by bash. Though it's
# not from a killed process, that does tell us that probably
# something was killed by a SIGTERM
For this reason, i believe, that you would need to run a command outside of bash to catch the exit code.
With some abstraction, a similar question has been asked regarding unbuffer which is a small script written in tcl. To be more precise, unbuffer uses the library libexpect with a tcl/tk wrapper.
From the source of unbuffer I extracted the relevant code to derive a workaround:
#!/bin/bash
expectStat() {
expect <(cat << EOT
set stty_init "-opost"
set timeout -1
eval [list spawn -noecho ] $#
expect
send_user "[wait]\n"
EOT
)
}
expectStat sleep 5 &
wait
which returns approximately the following line if sleep exits normally:
18383 exp4 0 0
If sleep is killed before it's exiting itself, the above script will approximately return:
18383 exp4 0 0 CHILDKILLED SIGTERM {software termination signal}
If a script is terminated with exit 143, the script will approximately return:
18383 exp4 0 143
The meaning of these strings can be extracted from the manual for expect. The integrated function wait is returning the above return lines.
The first two values are the pid, and expect's name for the process.
The fourth is the exit status. If a singal occurs more information is printed. The sixth value is the signal send to the process on its termination.
wait
normally returns a list of four integers. The first integer is the pid of the process that was waited upon. The second integer is the corresponding spawn id. The third integer is -1 if an operating system error occurred, or 0 otherwise. If the third integer was 0, the fourth integer is the status returned by the spawned process. If the third integer was -1, the fourth integer is the value of errno set by the operating system. The global variable errorCode is also set.
Additional elements may appear at the end of the return value from wait. An optional fifth element identifies a class of information. Currently, the only possible value for this element is CHILDKILLED in which case the next two values are the C-style signal name and a short textual description.
This means the fourth value and if present the sixth value are the values you are looking for. Store the whole line and extract the signal and exit code, for example with the following code:
RET=$(expectStat script.sh 1>&1)
# Filter status
EXITVALUE="$(echo "$RET" | cut -d' ' -f4)"
SIGNAL=$(echo "$RET" | cut -d' ' -f6)
#echo "Exit value: $EXITVALUE, Signal: $SIGNAL"
if [ -n "$SIGNAL" ]; then
echo "Likely killed by signal"
else
echo "$EXITVALUE"
fi
Conclusively, this workaround is very inelegant. Maybe, there is another tool which brings its own c-based tools to get the occurrence of a signal.
wait is a syscall and also a bash builtin.
To differentiate the two cases from bash run the program in the background and use the builtin wait to report the outcome.
Following are examples of both a non-zero exit code and an uncaught signal. These examples use the exit and kill bash builtins in a child bash shell, instead of a child bash shell you would run your program.
$ bash -c 'kill -s SIGTERM $$' & wait
[1] 36068
[1]+ Terminated: 15 bash -c 'kill -s SIGTERM $$'
$ bash -c 'exit 143' & wait
[1] 36079
[1]+ Exit 143 bash -c 'exit 143'
$
As to why you see Terminated printed to the terminal even when you redirect stdout and stderr the reason is that is printed by bash, not by the program.
Update:
By explicitly using the wait builtin you can now redirect its stderr (with the exit status of the program) to a separate file.
The following examples show the three types of termination: normal exit 0, non-zero exit, and uncaught signal. The results reported by wait are stored in files tagged with the PID of the corresponding program.
$ bash -c 'exit 0' & wait 2> exit_status_pid_$!
[1] 40279
$ bash -c 'exit 143' & wait 2> exit_status_pid_$!
[1] 40291
$ bash -c 'kill -s SIGTERM $$' & wait 2> exit_status_pid_$!
[1] 40303
$ for f in exit_status_pid*; do echo $f: $(cat $f); done
exit_status_pid_40279: [1]+ Done bash -c 'exit 0'
exit_status_pid_40291: [1]+ Exit 143 bash -c 'exit 143'
exit_status_pid_40303: [1]+ Terminated: 15 bash -c 'kill -s SIGTERM $$'
$
This is straying farther from bash but bcc offers exitsnoop. Using the
examples from the description, on Debian Sid:
root#vsid:~# apt install bpfcc-tools linux-headers-amd64
root#vsid:~# exitsnoop-bpfcc
PCOMM PID PPID TID AGE(s) EXIT_CODE
example1 1041 948 1041 0.00 signal 15 (TERM)
example2 1042 948 1042 0.00 code 143
^C
See the install guide for other distributions.
Strace can capture most of the signals, but might not work for syscalls (e.g. kill -9 ), therefore, as mentioned in this article:
Auditd is a daemon process or service that does as the name implies and produces audit logs of System level activities. It is installed from the usual repository as the audit package and then is configured in /etc/audit/auditd.conf and the rules are in /etc/audit/audit.rules.
The article provides examples for Audit output, which can help determining if it's helpful for you:
The usual output will look like this:
time->Wed Jun 3 16:34:08 2015
type=SYSCALL msg=audit(1433363648.091:6342): arch=c000003e syscall=62 success=no exit=-3 a0=1e06 a1=0 a2=1e06 a3=fffffffffffffff0 items=0 ppid=10044 pid=10140 auid=500 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=2 comm=4174746163682041504920696E6974 exe="/opt/ibm/WebSphere/AppServer/java/jre/bin/java" subj=unconfined_u:unconfined_r:unconfined_java_t:s0-s0:c0.c1023 key="kill_signals"
There is also mention of System Tap, and a redirection to a guide .
I have a Bash shell script that invokes a number of commands.
I would like to have the shell script automatically exit with a return value of 1 if any of the commands return a non-zero value.
Is this possible without explicitly checking the result of each command?
For example,
dosomething1
if [[ $? -ne 0 ]]; then
exit 1
fi
dosomething2
if [[ $? -ne 0 ]]; then
exit 1
fi
Add this to the beginning of the script:
set -e
This will cause the shell to exit immediately if a simple command exits with a nonzero exit value. A simple command is any command not part of an if, while, or until test, or part of an && or || list.
See the bash manual on the "set" internal command for more details.
It's really annoying to have a script stubbornly continue when something fails in the middle and breaks assumptions for the rest of the script. I personally start almost all portable shell scripts with set -e.
If I'm working with bash specifically, I'll start with
set -Eeuo pipefail
This covers more error handling in a similar fashion. I consider these as sane defaults for new bash programs. Refer to the bash manual for more information on what these options do.
To add to the accepted answer:
Bear in mind that set -e sometimes is not enough, specially if you have pipes.
For example, suppose you have this script
#!/bin/bash
set -e
./configure > configure.log
make
... which works as expected: an error in configure aborts the execution.
Tomorrow you make a seemingly trivial change:
#!/bin/bash
set -e
./configure | tee configure.log
make
... and now it does not work. This is explained here, and a workaround (Bash only) is provided:
#!/bin/bash
set -e
set -o pipefail
./configure | tee configure.log
make
The if statements in your example are unnecessary. Just do it like this:
dosomething1 || exit 1
If you take Ville Laurikari's advice and use set -e then for some commands you may need to use this:
dosomething || true
The || true will make the command pipeline have a true return value even if the command fails so the the -e option will not kill the script.
If you have cleanup you need to do on exit, you can also use 'trap' with the pseudo-signal ERR. This works the same way as trapping INT or any other signal; bash throws ERR if any command exits with a nonzero value:
# Create the trap with
# trap COMMAND SIGNAME [SIGNAME2 SIGNAME3...]
trap "rm -f /tmp/$MYTMPFILE; exit 1" ERR INT TERM
command1
command2
command3
# Partially turn off the trap.
trap - ERR
# Now a control-C will still cause cleanup, but
# a nonzero exit code won't:
ps aux | grep blahblahblah
Or, especially if you're using "set -e", you could trap EXIT; your trap will then be executed when the script exits for any reason, including a normal end, interrupts, an exit caused by the -e option, etc.
The $? variable is rarely needed. The pseudo-idiom command; if [ $? -eq 0 ]; then X; fi should always be written as if command; then X; fi.
The cases where $? is required is when it needs to be checked against multiple values:
command
case $? in
(0) X;;
(1) Y;;
(2) Z;;
esac
or when $? needs to be reused or otherwise manipulated:
if command; then
echo "command successful" >&2
else
ret=$?
echo "command failed with exit code $ret" >&2
exit $ret
fi
Run it with -e or set -e at the top.
Also look at set -u.
On error, the below script will print a RED error message and exit.
Put this at the top of your bash script:
# BASH error handling:
# exit on command failure
set -e
# keep track of the last executed command
trap 'LAST_COMMAND=$CURRENT_COMMAND; CURRENT_COMMAND=$BASH_COMMAND' DEBUG
# on error: print the failed command
trap 'ERROR_CODE=$?; FAILED_COMMAND=$LAST_COMMAND; tput setaf 1; echo "ERROR: command \"$FAILED_COMMAND\" failed with exit code $ERROR_CODE"; put sgr0;' ERR INT TERM
An expression like
dosomething1 && dosomething2 && dosomething3
will stop processing when one of the commands returns with a non-zero value. For example, the following command will never print "done":
cat nosuchfile && echo "done"
echo $?
1
#!/bin/bash -e
should suffice.
I am just throwing in another one for reference since there was an additional question to Mark Edgars input and here is an additional example and touches on the topic overall:
[[ `cmd` ]] && echo success_else_silence
Which is the same as cmd || exit errcode as someone showed.
For example, I want to make sure a partition is unmounted if mounted:
[[ `mount | grep /dev/sda1` ]] && umount /dev/sda1
I have a short script like this:
#!/bin/bash
<some_process> | tee -a /tmp/some.log &
wait $(pidof <some_process_name>)
echo $?
The result is always 0, irrespective of the exit status of some_process.
I know PIPESTATUS can be used here, but why does tee break wait?
Well, this is something that, for some peculiar reason, the docs don't mention. The code, however, does:
int wait_for (pid) { /*...*/
/* If this child is part of a job, then we are really waiting for the
job to finish. Otherwise, we are waiting for the child to finish. [...] */
if (job == NO_JOB)
job = find_job (pid, 0, NULL);
So it's actually waiting for the whole job, which, as we know, normally yields the exit status of the last command in the chain.
To make matters worse, $PIPESTATUS can only be used with the last foreground command.
You can, however, utilize $PIPESTATUS in a subshell job, like this:
(<some_process> | tee -a /tmp/some.log; exit ${PIPESTATUS[0]}) &
# somewhere down the line:
wait %%<some_process>
The trick here is to use $PIPESTATUS but also wait -n:
Check this example:
#!/bin/bash
# start background task
(sleep 5 | false | tee -a /tmp/some.log ; exit ${PIPESTATUS[1]}) &
# foreground code comes here
echo "foo"
# wait for background task to finish
wait -n
echo $?
The reason you always get an exit status of 0 is that the exit status returned is that of the last command in the pipeline, which is tee. By using a pipe, you eliminate the normal detection of exit status of <some_command>.
From the bash man page:
The return status of a pipeline is the exit status of the last command, unless the pipefail option is enabled. If pipefail is enabled, the pipeline's return status is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands exit successfully.
So... The following might be what you want:
#!/usr/bin/env bash
set -o pipefail
<some_command> | tee -a /tmp/some.log &
wait %1
echo $?
Is there any way to a piped commands to replicate its previous command exit status?
For example:
#/bin/bash
(...)
function customizedLog() {
# do something with the piped command output
exit <returned value from the last piped comand/script (script.sh)>
}
script.sh | customizedLog
echo ${?} # here I wanna show the script exit value
(...)
I know I could simply check the return using ${PIPESTATUS[0]}, but I really want to do this like the customizedLog function wasn't there.
Any thoughts?
In bash:
set -o pipefail
This will return the last non-zero exit status in a pipeline, or zero if all commands in the pipeline succeed.
set -o pipefail
script.sh | customizedLog
echo ${?}
Just make sure customizedLog succeeds (return 0), and you should pick up the exit status of script.sh. Test with false | customizedLog and true | customizedLog.
script.sh | customizedLog
The above will run in two separate processes (or 3, actually -- customizedLog will run in a bash fork as you can verify with something like ps -T --forest). As far as I know, with the UNIX process model, the only process that has access to a process's return information is its parent so there's no way customized log will be able to retrieve it.
So no, unless the previous command is run from a wrapper command that passes the exit status through the pipe (e.g., as the last line):
( command ; echo $? ) | piped_command_that_is_aware_of_such_an_arrangement
Greetings all. I'm setting up a cron job to execute a bash script, and I'm worried that the next one may start before the previous one ends. A little googling reveals that a popular way to address this is the flock command, used in the following manner:
flock -n lockfile myscript.sh
if [ $? -eq 1 ]; then
echo "Previous script is still running! Can't execute!"
fi
This works great. However, what do I do if I want to check the exit code of myscript.sh? Whatever exit code it returns will be overwritten by flock's, so I have no way of knowing if it executed successfully or not.
It looks like you can use the alternate form of flock, flock <fd>, where <fd> is a file descriptor. If you put this into a subshell, and redirect that file descriptor to your lock file, then flock will wait until it can write to that file (or error out if it can't open it immediately and you've passed -n). You can then do everything in your subshell, including testing the return value of scripts you run:
(
if flock -n 200
then
myscript.sh
echo $?
fi
) 200>lockfile
According to the flock man page, flock has a -E or --exit-conflict-code flag you can use to set what the exit code of flock should be in the case a conflict occurs:
-E, --conflict-exit-code number
The exit status used when the -n option is in use, and the conflicting lock exists, or the -w option is in use, and the timeout is reached. The default value is 1. The number has to be in the range of 0 to 255.
The man page also states:
EXIT STATUS
The command uses sysexits.h exit status values for everything, except when using either of the options -n or -w which report a failure to acquire the lock with a exit status given by the -E option, or 1 by default. The exit status given by -E has to be in the range of 0 to 255.
When using the command variant, and executing the child worked, then the exit status is that of the child command.
So, in the case of the -n or -w flags while using the "command" variant, you can see both exit statuses.
Example:
$ flock --exclusive /tmp/flock.lock bash -c 'exit 42'; echo $?
42
$ flock --exclusive /tmp/flock.lock flock --exclusive --nonblock --conflict-exit-code 100 /tmp/flock.lock bash -c 'exit 42'; echo $?
100
In the first example, we see that we get back the exit status of the process we're running with flock. In the second example, we are creating contention for the lock. In that case, flock itself returns the status code we tell it (100). If you do not specify a value with the --conflict-exit-code flag, it will return 1 instead. However, I prefer setting less common values to prevent confusion from other processess/scripts which also might return a value of 1.
#!/bin/bash
if ! pgrep myscript.sh; then
flock -n lockfile myscript.sh
fi
If I understand you right, you want to make sure 'myscript.sh' is not running before cron attempts to run your command again. Assuming that's right, we check to see if pgrep failed to find myscript.sh in the processes list and if so we run the flock command again.
Perhaps something like this would work for you.
#!/bin/bash
RETVAL=0
lockfailed()
{
echo "cannot flock"
exit 1
}
(
flock -w 2 42 || lockfailed
false
RETVAL=$?
echo "original retval $RETVAL"
exit $RETVAL
) 42>|/tmp/flocker
RETVAL=$?
echo "returned $RETVAL"
exit $RETVAL