How to check command status while redirect standard error output to a file? - bash

I have a bash script having the following command
rm ${thefile}
In order to ensure the command is execute successfully, I use $? variable to check on the status, but this variable doesn't show the exact error? To do this, I redirect the standard error output to a log file using following command:
rm ${file} >> ${LOG_FILE} 2>&1
With this command I can't use $? variable to check status on the rm command because the command behind the rm command is executed successfully, thus $? variable is kind of useless here.
May I know is there a solution that could combine both features where I'm able to check on the status of rm command and at mean time I'm allow to redirect the output?

With this command I can't use $? variable to check status on the rm command because the command behind the rm command is executed successfully, thus $? variable is kind of useless here.
That is simply not true. All of the redirections are part of a single command, and $? contains its exit status.
What you may be thinking of is cases where you have multiple commands arranged in a pipeline:
command-1 | command-2
When you do that, $? is set to the exit status of the last command in the pipeline (in this case command-2), and you need to use the PIPESTATUS array to get the exit status of other commands. (In this example ${PIPESTATUS[0]} is the exit status of command-1 and ${PIPESTATUS[1]} is equivalent to $?.)

What you probably need is the shell option pipefail in bash (from man bash):
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 sta‐
tus, or zero if all commands exit successfully. If the reserved word ! precedes
a pipeline, the exit status of that pipeline is the logical negation of the exit
status as described above. The shell waits for all commands in the pipeline to
terminate before returning a value.
> shopt -s -o pipefail
> true | false
> echo $?
1
> false | true
> echo $?
1
true | true
echo $?
0

Related

How to exit gitlab job when script fails [duplicate]

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

`grep` cause bash script stop [duplicate]

I'm studying the content of this preinst file that the script executes before that package is unpacked from its Debian archive (.deb) file.
The script has the following code:
#!/bin/bash
set -e
# Automatically added by dh_installinit
if [ "$1" = install ]; then
if [ -d /usr/share/MyApplicationName ]; then
echo "MyApplicationName is just installed"
return 1
fi
rm -Rf $HOME/.config/nautilus-actions/nautilus-actions.conf
rm -Rf $HOME/.local/share/file-manager/actions/*
fi
# End automatically added section
My first query is about the line:
set -e
I think that the rest of the script is pretty simple: It checks whether the Debian/Ubuntu package manager is executing an install operation. If it is, it checks whether my application has just been installed on the system. If it has, the script prints the message "MyApplicationName is just installed" and ends (return 1 mean that ends with an “error”, doesn’t it?).
If the user is asking the Debian/Ubuntu package system to install my package, the script also deletes two directories.
Is this right or am I missing something?
From help set :
-e Exit immediately if a command exits with a non-zero status.
But it's considered bad practice by some (bash FAQ and irc freenode #bash FAQ authors). It's recommended to use:
trap 'do_something' ERR
to run do_something function when errors occur.
See http://mywiki.wooledge.org/BashFAQ/105
set -e stops the execution of a script if a command or pipeline has an error - which is the opposite of the default shell behaviour, which is to ignore errors in scripts. Type help set in a terminal to see the documentation for this built-in command.
I found this post while trying to figure out what the exit status was for a script that was aborted due to a set -e. The answer didn't appear obvious to me; hence this answer. Basically, set -e aborts the execution of a command (e.g. a shell script) and returns the exit status code of the command that failed (i.e. the inner script, not the outer script).
For example, suppose I have the shell script outer-test.sh:
#!/bin/sh
set -e
./inner-test.sh
exit 62;
The code for inner-test.sh is:
#!/bin/sh
exit 26;
When I run outer-script.sh from the command line, my outer script terminates with the exit code of the inner script:
$ ./outer-test.sh
$ echo $?
26
As per bash - The Set Builtin manual, if -e/errexit is set, the shell exits immediately if a pipeline consisting of a single simple command, a list or a compound command returns a non-zero status.
By default, the exit status of a pipeline is the exit status of the last command in the pipeline, unless the pipefail option is enabled (it's disabled by default).
If so, the pipeline's return status of the last (rightmost) command to exit with a non-zero status, or zero if all commands exit successfully.
If you'd like to execute something on exit, try defining trap, for example:
trap onexit EXIT
where onexit is your function to do something on exit, like below which is printing the simple stack trace:
onexit(){ while caller $((n++)); do :; done; }
There is similar option -E/errtrace which would trap on ERR instead, e.g.:
trap onerr ERR
Examples
Zero status example:
$ true; echo $?
0
Non-zero status example:
$ false; echo $?
1
Negating status examples:
$ ! false; echo $?
0
$ false || true; echo $?
0
Test with pipefail being disabled:
$ bash -c 'set +o pipefail -e; true | true | true; echo success'; echo $?
success
0
$ bash -c 'set +o pipefail -e; false | false | true; echo success'; echo $?
success
0
$ bash -c 'set +o pipefail -e; true | true | false; echo success'; echo $?
1
Test with pipefail being enabled:
$ bash -c 'set -o pipefail -e; true | false | true; echo success'; echo $?
1
This is an old question, but none of the answers here discuss the use of set -e aka set -o errexit in Debian package handling scripts. The use of this option is mandatory in these scripts, per Debian policy; the intent is apparently to avoid any possibility of an unhandled error condition.
What this means in practice is that you have to understand under what conditions the commands you run could return an error, and handle each of those errors explicitly.
Common gotchas are e.g. diff (returns an error when there is a difference) and grep (returns an error when there is no match). You can avoid the errors with explicit handling:
diff this that ||
echo "$0: there was a difference" >&2
grep cat food ||
echo "$0: no cat in the food" >&2
(Notice also how we take care to include the current script's name in the message, and writing diagnostic messages to standard error instead of standard output.)
If no explicit handling is really necessary or useful, explicitly do nothing:
diff this that || true
grep cat food || :
(The use of the shell's : no-op command is slightly obscure, but fairly commonly seen.)
Just to reiterate,
something || other
is shorthand for
if something; then
: nothing
else
other
fi
i.e. we explicitly say other should be run if and only if something fails. The longhand if (and other shell flow control statements like while, until) is also a valid way to handle an error (indeed, if it weren't, shell scripts with set -e could never contain flow control statements!)
And also, just to be explicit, in the absence of a handler like this, set -e would cause the entire script to immediately fail with an error if diff found a difference, or if grep didn't find a match.
On the other hand, some commands don't produce an error exit status when you'd want them to. Commonly problematic commands are find (exit status does not reflect whether files were actually found) and sed (exit status won't reveal whether the script received any input or actually performed any commands successfully). A simple guard in some scenarios is to pipe to a command which does scream if there is no output:
find things | grep .
sed -e 's/o/me/' stuff | grep ^
It should be noted that the exit status of a pipeline is the exit status of the last command in that pipeline. So the above commands actually completely mask the status of find and sed, and only tell you whether grep finally succeeded.
(Bash, of course, has set -o pipefail; but Debian package scripts cannot use Bash features. The policy firmly dictates the use of POSIX sh for these scripts, though this was not always the case.)
In many situations, this is something to separately watch out for when coding defensively. Sometimes you have to e.g. go through a temporary file so you can see whether the command which produced that output finished successfully, even when idiom and convenience would otherwise direct you to use a shell pipeline.
I believe the intention is for the script in question to fail fast.
To test this yourself, simply type set -e at a bash prompt. Now, try running ls. You'll get a directory listing. Now, type lsd. That command is not recognized and will return an error code, and so your bash prompt will close (due to set -e).
Now, to understand this in the context of a 'script', use this simple script:
#!/bin/bash
# set -e
lsd
ls
If you run it as is, you'll get the directory listing from the ls on the last line. If you uncomment the set -e and run again, you won't see the directory listing as bash stops processing once it encounters the error from lsd.
set -e The set -e option instructs bash to immediately exit if any command [1] has a non-zero exit status. You wouldn't want to set this for your command-line shell, but in a script it's massively helpful. In all widely used general-purpose programming languages, an unhandled runtime error - whether that's a thrown exception in Java, or a segmentation fault in C, or a syntax error in Python - immediately halts execution of the program; subsequent lines are not executed.
By default, bash does not do this. This default behavior is exactly what you want if you are using bash on the command line
you don't want a typo to log you out! But in a script, you really want the opposite.
If one line in a script fails, but the last line succeeds, the whole script has a successful exit code. That makes it very easy to miss the error.
Again, what you want when using bash as your command-line shell and using it in scripts are at odds here. Being intolerant of errors is a lot better in scripts, and that's what set -e gives you.
copied from : https://gist.github.com/mohanpedala/1e2ff5661761d3abd0385e8223e16425
this may help you .
Script 1: without setting -e
#!/bin/bash
decho "hi"
echo "hello"
This will throw error in decho and program continuous to next line
Script 2: With setting -e
#!/bin/bash
set -e
decho "hi"
echo "hello"
# Up to decho "hi" shell will process and program exit, it will not proceed further
It stops execution of a script if a command fails.
A notable exception is an if statement. eg:
set -e
false
echo never executed
set -e
if false; then
echo never executed
fi
echo executed
false
echo never executed
cat a.sh
#! /bin/bash
#going forward report subshell or command exit value if errors
#set -e
(cat b.txt)
echo "hi"
./a.sh; echo $?
cat: b.txt: No such file or directory
hi
0
with set -e commented out we see that echo "hi" exit status being reported and hi is printed.
cat a.sh
#! /bin/bash
#going forward report subshell or command exit value if errors
set -e
(cat b.txt)
echo "hi"
./a.sh; echo $?
cat: b.txt: No such file or directory
1
Now we see b.txt error being reported instead and no hi printed.
So default behaviour of shell script is to ignore command errors and continue processing and report exit status of last command. If you want to exit on error and report its status we can use -e option.

How do I capture the exit codes from $PIPESTATUS and use it to exit the script?

I am using $PIPESTATUS to print the exit code for each pipe command. I now know that pipes run in parallel but once the exit code is <>0, how do I get the script to exit instead of progressing to the next command? Thanks.
I can't put set -e at the top because once the error is detected, the script exits but $PIPESTATUS isn't displayed because that echo command is after any failed command in the pipeline.
set -o pipefail
true | false | true || { declare -p PIPESTATUS; exit; }
echo "whoops"
output:
declare -a PIPESTATUS=([0]="0" [1]="1" [2]="0")
From Bash Reference Manual:
pipefail
If set, the return value of a pipeline is the value of the
last (rightmost) command to exit with a non-zero status, or zero if
all commands in the pipeline exit successfully. This option is
disabled by default.

Abort bash script if git pull fails [duplicate]

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

Combining wget and zenity/yad

I'm trying to provide some kind of a GUI for wget download process by using zenity/yad. I have come up with this:
wget http://example.com/ 2>&1 | \
sed -u 's/^[a-zA-Z\-].*//; s/.* \{1,2\}\([0-9]\{1,3\}\)%.*/\1\n#Downloading... \1%/; s/^20[0-9][0-9].*/#Done./' | \
zenity --progress --percentage=0 --title=Download dialog --text=Starting... --auto-close --auto-kill
Now, suppose wget runs into an error. I need to inform the user that the download failed. Since the $? variable seems to have a value of 0 regardless of success or failure (perhaps because $? is storing zenity's exit status?), I can't tell if the download failed or succeeded.
How can I rectify the above described problem?
You can say:
set -o pipefail
Saying so would cause $? to report the exit code of the last command in the pipeline to exit with a non-zero status.
Quoting from The Set Builtin:
pipefail
If set, the return value of a pipeline is the value of the last (rightmost) command to exit with a non-zero status, or zero if all
commands in the pipeline exit successfully. This option is disabled by
default.
Additionally, the array PIPESTATUS would report the return code of all the commands in the pipeline. Saying:
echo "${PIPESTATUS[#]}"
would list all those. For your example, it'd display 3 numbers, e.g.
1 0 0
if wget failed.
Quoting from the manual:
PIPESTATUS
An array variable (see Arrays) containing a list of exit status values from the processes in the most-recently-executed foreground
pipeline (which may contain only a single command).

Resources