The script runs two scripts depending on the output of the first script, i want to choose to run the script or not. For e.g
Mainscript.sh
./script1.sh....
is there a way to check output of script1.sh in mainscript.sh?
Because if output that there is connection between two system then i need to run the second script. Can i do it with python or i can do with bash only?
A common arrangement is to write your scripts so that programmatic operation is convenient. In concrete terms, that means that your script should return a zero exit code on success, nonzero otherwise. Then what you ask for is a simple matter of
script1 && script2
or in more complex cases
if script1; then
echo "$0: countdown to takeoff" >&2
script2
else
echo "$0: trouble occurred" >&2
script1 --reverse --undo --apologize
script3 || make clean
echo "$0: reverted everything" >&2
fi
You can access the last command exit code in $?. Also you can save its output.
#!/bin/sh
OUTPUT=`./script1.sh`
echo $?
echo $OUTPUT
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 was compiling a program using makefile and wanted to display a message to the user if there is any error in the program.
I searched for this and found that the exit status is zero if make is successful else it is non-zero.
How to use these values of exit status to accomplish the above task or is there any other way of doing so ?
You have to create a bash script and call make command in that, you can check result of each command(or which command you want to check if successful) and print a message using echo if it fails as
#!/bin/bash
cd <YOUR_DIRECTORY_CONTAINING_MAKEFILE>
make
if [ $? -ne "0" ]; then
echo "Make failed"
exit 1
fi
echo "make Successful"
$? holds result of last command.
Now, to make your program and see result, you have to run this bash script
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
We are creating a bash script for a build server. We want to ensure that when we execute a bash command inside the script, it returns with a signal of 0. If it does not, we want execution to stop. Our solution so far is to do:
#some command
if [ $? -ne 0 ] ; then
#handle error
fi
after every command that could cause this problem. This makes the code quite long and doesn't seem very elegant. We could use a bash function, perhaps. Although working with $? can be a bit tricky, and we would still have to call the function after every command. Is there a better way? I've looked at the trap command but it seems to only do signal handling with the bash script I am writing, not any commands I call.
The robust, canonical way of doing this is:
#!/bin/bash
die() {
local ret=$?
echo >&2 "$*"
exit "$ret"
}
./configure || die "Project can't be configured"
make || die "Project doesn't build"
make install || die "Installation failed"
The fragile, convenient way of doing this is set -e:
#!/bin/bash
set -e # Script will exit in many (but not all) cases if a command fails
./configure
make
make install
or equivalently (with a custom error message):
#!/bin/bash
# Will be called for many (but not all) commands that fail
trap 'ret=$?; echo >&2 "Failure on line $LINENO, exiting."; exit $ret' ERR
./configure
make
make install
For the latter two, the script will not exit for any command that is part of a conditional statement or &&/||, so while:
backup() {
set -e
scp backup.tar.gz user#host:/backup
rm backup.tar.gz
}
backup
will correctly avoid executing rm if the transfer fails, later inserting a feature like this:
if backup
then
mail -s "Backup completed successfully" user#example.com
fi
will make the backup stop exiting on failure and accidentally delete backups.
How can I have my shell script echo to me that the script that it calls has failed?
#!/bin/sh
test="/Applications/test.sh"
sh $test
exit 0
exit 1
#!/bin/sh
if sh /Applications/test.sh; then
echo "Well done $USER"
exit 0
else
echo "script failed with code [$?]" >&2
exit 1
fi
The /Applications/test.sh script should be well coded to exit with conventional status. 0 if it's ok and > 0 if it fails.
Like you can see, no need to test the special variable $?, we use boolean expression directly.
I usually take the following approach:
#!/usr/bin/env bash
test="/Applications/test.sh"
sh "${test}"
exit_status=$?
if [[ ${exit_status} ]] ; then
echo "Error: ${test} failed with status ${exit_status}." >&2
else
echo "Success!"
fi
In terms of best practice, you should not. If a script fails, it should emit an error message before it terminates so that its parent doesn't have to. The main reason for this is that the process knows why it is failing, while the parent can only guess. In other words, you should just write:
#!/bin/sh
test="/Applications/test.sh"
sh $test
Although really, it would be more typical to just write:
#!/bin/sh
/Applications/test.sh
test.sh will emit the necessary error message, and your script will return the same value as did test.sh. Also, in its current form your script will always be successful, even if test.sh actually failed because exit 0; exit 1 is pretty pointless: the exit 1 will never be called.