Consider the following bash code:
#!/usr/bin/env bash
set -euo pipefail
Test() {
grep 'XXX' data
echo 'test message'
}
data is an empty file existing in the same directory as the above bash script.
If Test function is invoked with the following code, it produces nothing to stdout.
Test
The reason is that grep command return exit status 1 since data does not contains XXX. So set -euo pipefail causes the bash script to exit immediately.
But if it is invoked with:
output=$(eval 'Test')
echo "output: $output"
It produes the following message to stdout:
output: test message
It seems that set -euo pipefail does not have an effect on eval. So grep command does not cause the bash script to exit.
As long as bash is invoked as bash, you're mostly correct, except that it's the $(...), not the eval, that suppresses -e. Beware though if it may be called as sh (or bash --posix): this is item #40 on the list of changes between POSIX and non-POSIX mode:
Enabling POSIX mode has the effect of setting the
'inherit_errexit' option, so subshells spawned to execute command
substitutions inherit the value of the '-e' option from the parent
shell. When the 'inherit_errexit' option is not enabled, Bash
clears the '-e' option in such subshells.
Related
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 $?
cat: b.txt: No such file or directory
1
Now we see b.txt error being reported instead and no hi printed.
So default behaviour of shell script is to ignore command errors and continue processing and report exit status of last command. If you want to exit on error and report its status we can use -e option.
I'm new to Bash and I had a hard time to figure out why when I was using set -o errexit and a command fail, the script was not exiting.
It seems because I declared a local variable!
Please tell me how I can use local variable and set -o errexit at the same time.
Example:
#!/bin/bash
set -o errexit
set -o nounset
set -o pipefail
function test {
local output=$(ls --badoption)
echo "error code is $?"
echo "output=$output"
}
test
Result:
./test.sh
ls: illegal option -- -
usage: ls [-ABCFGHLOPRSTUWabcdefghiklmnopqrstuwx1] [file ...]
error code is 0
output=
But:
#!/bin/bash
set -o errexit
set -o nounset
set -o pipefail
function test {
output=$(ls --badoption)
echo "error code is $?"
echo "output=$output"
}
test
Result:
ls: illegal option -- -
usage: ls [-ABCFGHLOPRSTUWabcdefghiklmnopqrstuwx1] [file ...]
It's because set -o errexit (or running bash with -e) only affects simple commands (as defined in the bash man page). cmd in local output=$(cmd) isn't considered a simple command, so -e doesn't have any affect (and $? doesn't have any relation to how cmd exited). A simple workaround is to split the line by replacing:
local output=$(ls --badoption)
with:
local output
output=$(ls --badoption)
In my testing that does everything you want, it will exit immediately, and if you replace the ls --badoption with just ls, it works, and setting output before calling test then echoing it afterwords shows output really is local to test.
The documentation of local says:
The return status is zero unless local is used outside a function, an invalid name is supplied, or name is a readonly variable.
So the return status of the local command is not affected by whether the command executed in the command substitution succeeded.
As blm indicated in his answer, you can get the effect you want by separating the declaration and assignment of the local variable. The return status of an ordinary assignment that uses command substitution is the return status of the subshell that executes the command.
I'm a bit confused here. My goal here is to have the bash script exit with a non-zero exit code when any of the commands within the script fails. Using the -e flag, I assumed this would be the case, even when using subshells. Below is a simplified example:
#!/bin/bash -e
(false)
echo $?
echo "Line reached!"
Here is the output when ran:
[$]>Tests/Exec/continuous-integration.sh
1
Line reached!
Bash version: 3.2.25 on CentOS
It appears as though this is related to your version of bash. On machines that I have access to, bash version 3.1.17 and 3.2.39 exhibit this behaviour, bash 4.1.5 does not.
Although a bit ugly, a solution that works in both versions could be something like this:
#!/bin/bash -e
(false) || exit $?
echo $?
echo "Line reached!"
There are some notes in the bash source changelog which related to bugs with the set -e option.
I have seen this behavior in bash version 3.2.51 on both SuSE 11.3 and Mac OS prior to El Capitan. Bash 3.2.57 on El Capitan has the "correct" behavior, ie like bash 4.
However, the workaround proposed above, adding "|| exit $?" after the subshell's closing paren, defeats the intent of the -e flag no matter what version of bash. From man bash:
-e Exit immediately if a simple command (see SHELL GRAMMAR above) exits
with a non-zero status. The shell does not exit if the command that fails is
part of the command list immediately following a while or until keyword, part
of the test in an if statement, part of a && or || list, ...
A subshell followed by "|| exit $?" apparently counts as a command list; and the bash -e flag will not apply to ANY command inside the subshell. Try it:
$ set -e
$ ( echo before the error; false; echo after the error, status $?; ) || echo after the subshell, status $?
before the error
after the error, status 1
$
Because the subshell is followed by ||, that "echo after the error" is run, even with set -e. Not only that, the subshell exits 0, because that "echo" ran. So "|| exit $?" would not even run the "exit". Probably not what we wanted!
So far as I know, the following formula is compatible with bash versions whether they honor bash -e after subshell, or not. It even behaves correctly if the -e flag happens to be reset:
Add the following line immediately after the closing parenthesis of every subshell in the bash script:
case $?/$- in ( 0/* ) ;; ( */*e* ) exit $? ;; esac # honor bash -e flag when subshell returns
The -e option is for current shell, for sub-shell, functions, etc we use -E
form man bash
-E If set, any trap on ERR is inherited by shell functions, command substitutions, and commands executed in a subshell environment.
And for advanced users, a kind of strict mode we have which is:
set -Eeuo pipefail
-E explained above
-e exit immediately
-u exit for unbound variable found
-o set option
pipefail exit if we had failure on a pipe
I am using this:
$ uname -a
CYGWIN_NT-6.1 bassoon 1.7.15(0.260/5/3) 2012-05-09 10:25 i686 Cygwin
$ bash --version
GNU bash, version 4.1.10(4)-release (i686-pc-cygwin)
$ cat myexpr.sh
#!/bin/sh
echo "In myexpr, Before expr"
ac_optarg=`expr x--with-gnu-as : 'x[^=]*=\(.*\)'`
echo "ac_optarg=$ac_optarg"
echo "In myexpr, After expr"
$ cat myexpr2.sh
#!/bin/sh
set -e
echo "In myexpr, Before expr"
ac_optarg=`expr x--with-gnu-as : 'x[^=]*=\(.*\)'`
echo "ac_optarg=$ac_optarg"
echo "In myexpr, After expr"
The only difference between the two scripts is that myexpr2.sh uses "set -e"
$ echo $$
2880
$ ./myexpr.sh
In myexpr, Before expr
ac_optarg=
In myexpr, After expr
$ ./myexpr2.sh
In myexpr, Before expr
Expected behavior, so far.
If I do this in the parent shell (PID 2880, above):
$ set -e
$ ./myexpr.sh
The parent shell exits! That is pID 2880 above where I did the "set -e"
This is not the behavior on Linux or cygwin 1.5.12. Is this a bug in cygwin or BASH on cygwin?
This is not a bug, it's a feature of the Bash environment. This happens when you don't have the Bash shell environment variable execfail set, and/or the Shell environment variable errexit.
execfail - (is a BASHOPTS)
If set, a non-interactive shell will not exit if it cannot execute
the file specified as an argument to the exec builtin command.
An interactive shell does not exit if exec fails.
errexit - (is a SHELLOPTS)
Exit immediately if a pipeline (see Pipelines), which may consist of a
single simple command (see Simple Commands), a subshell command enclosed
in parentheses (see Command Grouping), or one of the commands executed as
part of a command list enclosed by braces (see Command Grouping) returns a
non-zero status. The shell does not exit if the command that fails is part
of the command list immediately following a while or until keyword, part
of the test in an if statement, part of any command executed in a && or ||
list except the command following the final && or ||, any command in a
pipeline but the last, or if the command’s return status is being inverted
with !. A trap on ERR, if set, is executed before the shell exits.
This option applies to the shell environment and each subshell environment
separately (see Command Execution Environment), and may cause subshells to
exit before executing all the commands in the subshell.
Different Linux versions have different defaults for these.
You can check which are enabled with:
echo "SHELLOPTS=$SHELLOPTS"
echo "BASHOPTS=$BASHOPTS"
and you can see all of them using:
set -o && echo -e "\n" && shopt -p
So, you need to enable yours with:
shopt -s execfail
If that doesn't work, you may also have to unset (off) the errexit of $SHELLOPTS with:
set -o errexit
For further info, see: The GNU Bash Manual!
PS. "set" is using reverse logic so if you wanna use the 'e' flag you have to use a "+": set +e
This question already has answers here:
Aborting a shell script if any command returns a non-zero value
(10 answers)
Closed 3 years ago.
I've been writing some shell script and I would find it useful if there was the ability to halt the execution of said shell script if any of the commands failed. See below for an example:
#!/bin/bash
cd some_dir
./configure --some-flags
make
make install
So in this case, if the script can't change to the indicated directory, then it would certainly not want to do a ./configure afterwards if it fails.
Now I'm well aware that I could have an if check for each command (which I think is a hopeless solution), but is there a global setting to make the script exit if one of the commands fails?
Use the set -e builtin:
#!/bin/bash
set -e
# Any subsequent(*) commands which fail will cause the shell script to exit immediately
Alternatively, you can pass -e on the command line:
bash -e my_script.sh
You can also disable this behavior with set +e.
You may also want to employ all or some of the the -e -u -x and -o pipefail options like so:
set -euxo pipefail
-e exits on error, -u errors on undefined variables, -x prints commands before execution, and -o (for option) pipefail exits on command pipe failures. Some gotchas and workarounds are documented well here.
(*) Note:
The shell does not exit if the command that fails is part of the
command list immediately following a while or until keyword,
part of the test following the if or elif reserved words, part
of any command executed in a && or || list except the command
following the final && or ||, any command in a pipeline but
the last, or if the command's return value is being inverted with
!
(from man bash)
To exit the script as soon as one of the commands failed, add this at the beginning:
set -e
This causes the script to exit immediately when some command that is not part of some test (like in a if [ ... ] condition or a && construct) exits with a non-zero exit code.
Use it in conjunction with pipefail.
set -e
set -o pipefail
-e (errexit): Abort the script at the first error, when a command exits with non-zero status (except in until or while loops, if-tests, and list constructs)
-o pipefail: Causes a pipeline to return the exit status of the last command in the pipe that returned a non-zero return value.
Chapter 33. Options
Here is how to do it:
#!/bin/sh
abort()
{
echo >&2 '
***************
*** ABORTED ***
***************
'
echo "An error occurred. Exiting..." >&2
exit 1
}
trap 'abort' 0
set -e
# Add your script below....
# If an error occurs, the abort() function will be called.
#----------------------------------------------------------
# ===> Your script goes here
# Done!
trap : 0
echo >&2 '
************
*** DONE ***
************
'
An alternative to the accepted answer that fits in the first line:
#!/bin/bash -e
cd some_dir
./configure --some-flags
make
make install
One idiom is:
cd some_dir && ./configure --some-flags && make && make install
I realize that can get long, but for larger scripts you could break it into logical functions.
I think that what you are looking for is the trap command:
trap command signal [signal ...]
For more information, see this page.
Another option is to use the set -e command at the top of your script - it will make the script exit if any program / command returns a non true value.
One point missed in the existing answers is show how to inherit the error traps. The bash shell provides one such option for that using set
-E
If set, any trap on ERR is inherited by shell functions, command substitutions, and commands executed in a subshell environment. The ERR trap is normally not inherited in such cases.
Adam Rosenfield's answer recommendation to use set -e is right in certain cases but it has its own potential pitfalls. See GreyCat's BashFAQ - 105 - Why doesn't set -e (or set -o errexit, or trap ERR) do what I expected?
According to the manual, set -e exits
if a simple commandexits with a non-zero status. The shell does not exit if the command that fails is part of the command list immediately following a while or until keyword, part of the test in a if statement, part of an && or || list except the command following the final && or ||, any command in a pipeline but the last, or if the command's return value is being inverted via !".
which means, set -e does not work under the following simple cases (detailed explanations can be found on the wiki)
Using the arithmetic operator let or $((..)) ( bash 4.1 onwards) to increment a variable value as
#!/usr/bin/env bash
set -e
i=0
let i++ # or ((i++)) on bash 4.1 or later
echo "i is $i"
If the offending command is not part of the last command executed via && or ||. For e.g. the below trap wouldn't fire when its expected to
#!/usr/bin/env bash
set -e
test -d nosuchdir && echo no dir
echo survived
When used incorrectly in an if statement as, the exit code of the if statement is the exit code of the last executed command. In the example below the last executed command was echo which wouldn't fire the trap, even though the test -d failed
#!/usr/bin/env bash
set -e
f() { if test -d nosuchdir; then echo no dir; fi; }
f
echo survived
When used with command-substitution, they are ignored, unless inherit_errexit is set with bash 4.4
#!/usr/bin/env bash
set -e
foo=$(expr 1-1; true)
echo survived
when you use commands that look like assignments but aren't, such as export, declare, typeset or local. Here the function call to f will not exit as local has swept the error code that was set previously.
set -e
f() { local var=$(somecommand that fails); }
g() { local var; var=$(somecommand that fails); }
When used in a pipeline, and the offending command is not part of the last command. For e.g. the below command would still go through. One options is to enable pipefail by returning the exit code of the first failed process:
set -e
somecommand that fails | cat -
echo survived
The ideal recommendation is to not use set -e and implement an own version of error checking instead. More information on implementing custom error handling on one of my answers to Raise error in a Bash script