I am need of shell script which have multiple commands like
Command -1 mv
command 2- cp
command -3 - sed
command -4 echo ,append etc
rc=$?
if =0 success
else
exist30
but even though move command failed script retuning return code as 0 and script showing success message.
Do i need to main RC for all the command or can i better handle return code for each command to make sure every command run successfully
If you have to deal with commands which may fail you can do the following:
if mv ...; then
echo mv success
else
echo mv failure
rc=1
fi
exit 1
For having your script to crash with the first failed command make sure to add the line set -e below your shebang (#!/bin/sh).
Keep in mind that the error handling above is compatible with set -e, the script will not stop.
set -eu is considered best practice for shell scripts, -u will result in a crash of your script if a variable used is unset.
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 wrote a little helper to start an Android emulator which looks like this:
# START ANDROID EMULATION
android-sim () {
if [[ -z "$1" ]]; then
echo "No Android device supplied as argument!"
return 1
fi
if [[ -d "$ANDROID_HOME" ]]; then
cd "$ANDROID_HOME/emulator" &>/dev/null
./emulator "#$1" &>/dev/null &
cd - &>/dev/null
else
echo "The variable ANDROID_HOME is not set to a correct directory!"
fi
}
I tried to suppress all the generated output with &>/dev/null. For the emulator itself I also tried to put the execution in the background.
But when I run the function I still get the following output:
# before exeuction
[09:11:48] sandrowinkler:~ $ android-sim pixel-29
[1] 23217
# after execution, I closed the emulator via GUI
[09:11:55] sandrowinkler:~ $
[1]+ Done ./emulator "#$1" &> /dev/null
(wd: /usr/local/share/android-sdk/emulator)
(wd now: ~)
According to this superuser post and this unix post I used the right way to silence the output. So what am I doing wrong here?
PS: I would also appreciate tips to improve my Bash code.
If the monitoring of the background job is not needed the double fork may be a solution.
( ./emulator "#$1" &>/dev/null & )
when the parent exit before the child process the child process is attached to process 1 so no more monitored by current shell.
also looking at the script, cd - could be avoided changing directory in the sub shell:
(
cd "$ANDROID_HOME/emulator"
./emulator "#$1" &
) &>/dev/null
because current shell will not be affected by subshell change directory
You've done it correctly. That output is not from your script, but from the shell, displaying information about the background process (the PID, the done message and the working directory when in ends).
If you redirect the input externally, like this:
andoroid-sim foo > log.txt
and after execution you open log.txt, you'll see what actually your program outputed (and those messages won't be there, even if they appear in the console).
I'm writting a script that will execute commands on different servers automatically and I am trying to log all it's content and output. I am having difficulties with redirections and command status output. Meaning was the command successfull and what is its output?
I have tried many directions such redirecting the command output to a function or file. I have tried a simple if statement. Both worked for there respected function. But when I am trying to combine both of them the script always return the command to be successfull. And to some level it is.
#!/bin/bash
function Executing(){
if [ "$1" != "" ]; then
if [ $DEBUG = "true" ]; then
echo "$1" | Debug i s
if $1 2>&1 | Debug o o;then
echo "$1" | Debug s r
else
echo "$1" | Debug e r
fi
else
eval $1
fi
fi
}
Executing "apt-get update"
Also note that the system requires sudo to execute apt-get update. Thus the script should report & log an error. Right now tho, whenever I call the Executing function, the function returns a successful execution. I am pretty sure it does because of the added pipe | debug o o that captures the output and formats it. Which I'll later redirect to a log file.
So I tested if apt-get update;then echo yes;else echo no; fi which worked in my terminal. But as soon as I pipe the output to the function debug, the if statement returns true.
Try with set -o pipefail.
Check the manual: The Set Builtin
Usually, bash returns the exit code of the last command of a pipe group, so your Debug command was the one that was checked. With that setting, bash returns the exit status of the last (rightmost) command to exit with a non-zero status. If your command fails, that is the status that get propagated.
I've figured out how to use aliases from within a bash script. They do get executed, but I don't get the result back in the variable "Status". All following types of launching an alias get the result back when used interactively, but "Status" stay empty in the script no matter if I use "shopt -s expand_aliases" or not:
$ Status=`start_ETAP-TestCluster 2>/dev/null`
$ echo $Status
{"JobIds":["8c54392c-363c-4e28-827e-a1384da7fd37"]}
$ Status=`${BASH_ALIASES[start_NetIntel-Cluster]} 2>/dev/null`
$ echo $Status
{"JobIds":["dd05f5cb-a490-4fe3-b4fd-be61f62bbb88"]}
$ eval Status=`${BASH_ALIASES[start_NetIntel-Cluster]} 2>/dev/null`
$ echo $Status
{JobIds:[25a246ab-e5f5-4d65-80ea-4ff5e15cc02c]}
$ eval $(echo Status=`${BASH_ALIASES[start_NetIntel-Cluster]} 2>/dev/null`)
$ echo $Status
{JobIds:[4ce2f4e5-0599-47f5-b7f4-cd2ec1de9909]}
Has anyone some hints and ideas how to get the result in the variable "Status" within a bash script ?
The command between back quotes, or betwen $( and ) is expanded by the shell to the ouput of command. The exit status can be retrieved with $? special variable.
output=$(command ) # output=`command ` #old syntax
status=$?
Otherwise standard output and error can be redirected to files.
command 1> output_file 2> error_file
status=$?
Also if only the exit status success is wanted the status can be checked with logical operators (exit status ==0 means success and logical value true)
command 1> output_file 2> error_file || {
echo failed
exit 1
}
Thank you for your help and hints. The main problem was that expanding the alias (like start_ETAP-Cluster) did not work properly. Thus using following in the script did help:
shopt -s expand_aliases
source ~/.bash_aliases
Using the $Status in my code was misleading, I changed it to $JobID, which reflects better what I wanted. I'm using web hooks in the aliases, which return a JSON Object with the job id. Furthermore assigning $Status=$? works also. Now the code does its work from within the script:
$ JobID=$( ${BASH_ALIASES[start_ETAP-Cluster]} 2>/dev/null )
$ echo $JobID
{"JobIds":["8c54392c-363c-4e28-827e-a1384da7fd37"]}
Next step will be to move the web hook calls from the aliases to bash functions, will give me more flexibility then aliases.