I'm trying to save the exit code from running psql in a shell script. Assuming the psql command right before this snippet was bad, I'm expecting this snippet to return anything but 1, which it initially does. But when I assign it to exitcode and then echo it, the returned value is a 0...
$ echo $?
1
$ exitcode=$?
$ echo 'simply'
simply
$ #echo $?
0
$ #echo 'coding'
coding
$ echo $exitcode
0
I'm trying to get the variable exitcode to print or echo 1 like the first line does. How do I exactly do this?
When you print $? the second time, it's not the exit code of the previous command.
You need to assign $? to exitcode immediately before running any other command.
i.e.
$ psql
$ echo $?
1
$ exitcode=$?
should be
$ psql
$ exitcode=$?
in order to preserve the exit code of psql.
Related
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 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
Is it possible to propagate an exit code to the caller in case of a syntax error in a Bash script with an EXIT trap? For example, if I have:
#! /bin/bash
set -eu
trap "echo dying!!" EXIT
echo yeah
echo $UNBOUND_VARIABLE
echo boo
Then, running it gives an exit code 0 even if the script did not really end successfully:
$ bash test.sh
yeah
test.sh: line 8: UNBOUND_VARIABLE: unbound variable
dying!!
$ echo $?
0
But if I comment out the exit trap, the script returns 1. Alternatively, if I replace the line with the unbound variable with a command that returns nonzero (e.g. /bin/false), that exit value is propagated as I would like it to.
The shell exits with the result of the last executed command. In your trap case, that's echo, which usually returns with success.
To propagate your value, simply exit with it.
#!/bin/bash
set -eu
die() {
echo "Dying!!"
exit "$1"
}
trap 'die $?' EXIT
echo yeah
echo $unbound
echo boo
Also note that set -e is considered harmful -- it makes you think the script will exit if a command fails, which it won't always do.
This behavior is related to different Bash versions. The original script works as expected on Bash 4.2 but not on 3.2. Having the error-prone code in a separate script file and running it in a subshell works around problems in earlier Bash versions:
#!/bin/bash
$BASH sub.sh
RETVAL=$?
if [[ "$RETVAL" != "0" ]]; then
echo "Dying!! Exit code: $RETVAL"
fi
sub.sh:
set -eu
echo yeah
echo $UNBOUND_VARIABLE
echo boo
I realize the fact but I don't know why:
cat abc | echo $?
if abc does not exist, but above command still return 0. Anyone knows the theory about why?
The reason why it must be this way is that a pipeline is made of processes running simultaneously. cat's exit code can't possibly be passed to echo as an argument because arguments are set when the command begins running, and echo begins running before cat has finished.
echo doesn't take input from stdin, so echo on the right side of a pipe character is always a mistake.
UPDATE:
Since it is now clear that you are asking about a real problem, not just misunderstanding what you saw, I tried it myself. I get what I think is the correct result (1) from a majority of shells I tried (dash, zsh, pdksh, posh, and bash 4.2.37) but 0 from bash 4.1.10 and ksh (Version JM 93u+ 2012-02-29).
I assume the change in bash's behavior between versions is intentional, and the 4.1.x behavior is considered a bug. You'd probably find it in the changelog if you looked hard enough. Don't know about ksh.
csh and tcsh (with $status in place of $?) also say 0, but I bet nobody cares about that.
People with bigger shell collections are invited to test:
for sh in /bin/sh /bin/ksh /bin/bash /bin/zsh ...insert more shells here...; do
echo -n "$sh "
$sh -c 'false;true|echo $?'
done
It does not have anything to do with cat abc, but with the previous command you executed. So the code you get when doing cat abc | echo $? is telling if the previous command in your history was successful or not.
From man bash:
Special Parameters
? - Expands to the exit status of the most recently executed foreground pipeline.
So when you do:
cat abc | echo $?
The echo $? refers to the previous command you used, not cat abc.
Example
$ cat a
hello
$ echo $?
0
$ cat aldsjfaslkdfj
cat: aldsjfaslkdfj: No such file or directory
$ echo $?
1
So
$ cat a
$ cat a | echo $?
0
$ cat aldsjfaslkdfj
cat: aldsjfaslkdfj: No such file or directory
$ cat a | echo $?
1
echo $? will give output of previous command which you have executed before not output of piped command. So, you will always get echo $? as 0 even if command failed before pipe.
You pipe the output from 'cat abc' to 'echo $?' which is not what you want.
You want to echo the exit code of 'cat'
cat abc; echo $?
is what you want. Or simply write it in two lines if you can.
Pretty new to shell scripting. I am trying to do the following:
#!/bin/bash
unzip myfile.zip
#do stuff if unzip successful
I know that I can just chain the commands together in with && but there is quite a chunk, it would not be terribly maintainable.
You can use the exit status of the command explicitly in the test:
if ! unzip myfile.zip &> /dev/null; then
# handle error
fi
You can use $?. It returns:
- 0 if the command was successfully executed.
- !0 if the command was unsuccessful.
So you can do
#!/bin/bash
unzip myfile.zip
if [ "$?" -eq 0 ]; then
#do stuff on unzip successful
fi
Test
$ cat a
hello
$ echo $?
0
$ cat b
cat: b: No such file or directory
$ echo $?
1
The variable $? contains the exit status of the previous command. A successful exit status for (most) commands is (usually) 0, so just check for that...
#!/bin/bash
unzip myfile.zip
if [ $? == 0 ]
then
# Do something
fi
If you want the shell to check the result of the executed commands and stop interpretation when something returns non-zero value you can add set -e which means Exit immediately if a command exits with a non-zero status. I'm using this often in scripts.
#!/bin/sh
set -e
# here goes the rest