I'd love to return the exact value if wget command fails in efficient way without changing it.
Can exit #? output the returned value from wget?
Ex.
# If it succeeds, then wget returns zero instead of non zero
## 0 No problems occurred.
## 1 Generic error code.
## 2 Parse error—for instance, when parsing command-line options, the ‘.wgetrc’ or ‘.netrc’...
## 3 File I/O error.
## 4 Network failure.
## 5 SSL verification failure.
## 6 Username/password authentication failure.
## 7 Protocol errors.
## 8 Server issued an error response.
wget https://www.google.co.jp/images/branding/googlelogo/2x/googlelogo_color_120x44dp.png -o test.img
if [ $? -ne 0 ]
then
# exit 16 # failed ends1 <== This doesn't tell anything
exit #?
fi
wget https://www.google.co.jp/images/branding/googlelogo/2x/googlelogo_color_120x44dp.png -o test.img
# grab wget's exit code
exit_code=$?
# if exit code is not 0 (failed), then return it
test $exit_code -eq 0 || exit $exit_code
The -e option of Bash may do what you want:
Exit immediately if a pipeline (which may consist of a single simple command), a list, or a compound command (see SHELL GRAMMAR), exits with a non-zero status.
It's also important to know that
Bash's exit status is the exit status of the last command executed in the script.
My experiments with Bash 4.4 suggest that the exit status of the failing command is returned, even if a trap handler is invoked:
$ ( trap 'echo $?' ERR; set -e; ( exit 3 ) ; echo true ) ; echo $?
3
3
So you can write:
#!/bin/bash
url=https://www.google.co.jp/images/branding/googlelogo/2x/googlelogo_color_120x44dp.png
set -e
wget -o test.img "$url"
set +e # if you no longer want exit on fail
For just one command in your script, you might prefer an explicit test and exit like this:
wget -o test.img "$url" || exit $?
Further, exit with no argument is the same as exit $?, so that can be simplified to just
wget -o test.img "$url" || exit
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
#!/bin/bash
# exitlab
#
# example of exit status
# check for non-existent file
# exit status will be 2
# create file and check it
# exit status will be 0
#
ls xyzzy.345 > /dev/null 2>&1
status='echo $?'
echo "status is $status"
# create the file and check again
# status will not be 0
touch xyzzy.345
ls xyzzy.345 > /dev/null 2>&1
status='echo $?'
echo "status is $status"
#remove the file
rm xyzzy.345
edx.org has a Lab and this is the script. When I run it, the output is as follows:
status is echo $?
status is echo $?
I think the output is supposed to be either 0 or 2. I tried putting parentheses like status='(echo $?) but that results in status is echo $?. Then, I tried putting parentheses outside of the single quotes status=( 'echo $?' ) but this gave me the same output status is echo $?.
Any ideas?
You are looking for command substitution (status=$(echo $?)), although it isn't necessary. You can assign the value of $? directly to status:
status=$?
You need to use double-quotes here for the variable substitution to take place. Change
status='echo $?'
to
status="echo $?"
You might find this guide helpful: Bash Guide for Beginners
Below code is the part of my shell script.
But I am not able to understand why exit status(sshStatus) always coming 0?
I want ssh output as well as exit status.
Please help me to find the solution.
local output="$(ssh -q -o ConnectTimeout=10 \
-o BatchMode=yes \
-o StrictHostKeyChecking=no \
-o UserKnownHostsFile=/dev/null \
$user#$host "$command" 2>&1)"
local sshStatus=$?
command can be :
command="[ ! -d /home/upendra/dfs ]"
command="cat /home/upendra/a.txt"
command="sh /home/upendra/dfs/bin/start-datanode.sh"
Whenever i'm calling command like below directly on shell prompt:
ssh -q -o ConnectTimeout=10 \
-o BatchMode=yes \
-o StrictHostKeyChecking=no \
-o UserKnownHostsFile=/dev/null \
upendra#172.20.20.2 "[ ! -d /home/upendra/dfs ]" 2>&1
Then exit status(echo $?) is coming 1. This is correct because this directory not exists on host.
I got solution on this page :bash shell - ssh remote script capture output and exit code? it is due to "local output". – Upendra
You are always getting exot status as 0 as your command is excecuted successfully
The exit status you obtain from the local machine is the exit status of the last command in ssh session
for example
$ ssh localhost
$ exit 5
$ echo $? #on local system
5
Consider a case without any command
$ ssh localhost
$ ls
#will list commands and exit succussfully
ctrl+d
$ echo $? #on local system
0
Here the exit status of ls command is 0 which is printed.
Every command returns an exit status (sometimes referred to as a return status or exit code). A successful command returns a 0, while an unsuccessful command returns a non-zero value that usually can be interpreted as an error code. Well-behaved UNIX commands, programs, and utilities return a 0 exit code upon successful completion, though there are some exceptions.
Likewise, functions within a script and the script itself return an exit status. The last command executed in the function or script determines the exit status. Within a script, an exit nnn command may be used to deliver an nnn exit status to the shell (nnn must be an integer in the 0 - 255 range).
#!/bin/bash
echo hello
echo $? # Exit status 0 returned because command executed successfully.
lskdf # Unrecognized command. echo $? # Non-zero exit status
returned -- command failed to execute.
echo
exit 113 # Will return 113 to shell.
# To verify this, type "echo $?" after script terminates.
Your code return exit code 0 which means your shell script execute successfully.
Greetings all. I'm setting up a cron job to execute a bash script, and I'm worried that the next one may start before the previous one ends. A little googling reveals that a popular way to address this is the flock command, used in the following manner:
flock -n lockfile myscript.sh
if [ $? -eq 1 ]; then
echo "Previous script is still running! Can't execute!"
fi
This works great. However, what do I do if I want to check the exit code of myscript.sh? Whatever exit code it returns will be overwritten by flock's, so I have no way of knowing if it executed successfully or not.
It looks like you can use the alternate form of flock, flock <fd>, where <fd> is a file descriptor. If you put this into a subshell, and redirect that file descriptor to your lock file, then flock will wait until it can write to that file (or error out if it can't open it immediately and you've passed -n). You can then do everything in your subshell, including testing the return value of scripts you run:
(
if flock -n 200
then
myscript.sh
echo $?
fi
) 200>lockfile
According to the flock man page, flock has a -E or --exit-conflict-code flag you can use to set what the exit code of flock should be in the case a conflict occurs:
-E, --conflict-exit-code number
The exit status used when the -n option is in use, and the conflicting lock exists, or the -w option is in use, and the timeout is reached. The default value is 1. The number has to be in the range of 0 to 255.
The man page also states:
EXIT STATUS
The command uses sysexits.h exit status values for everything, except when using either of the options -n or -w which report a failure to acquire the lock with a exit status given by the -E option, or 1 by default. The exit status given by -E has to be in the range of 0 to 255.
When using the command variant, and executing the child worked, then the exit status is that of the child command.
So, in the case of the -n or -w flags while using the "command" variant, you can see both exit statuses.
Example:
$ flock --exclusive /tmp/flock.lock bash -c 'exit 42'; echo $?
42
$ flock --exclusive /tmp/flock.lock flock --exclusive --nonblock --conflict-exit-code 100 /tmp/flock.lock bash -c 'exit 42'; echo $?
100
In the first example, we see that we get back the exit status of the process we're running with flock. In the second example, we are creating contention for the lock. In that case, flock itself returns the status code we tell it (100). If you do not specify a value with the --conflict-exit-code flag, it will return 1 instead. However, I prefer setting less common values to prevent confusion from other processess/scripts which also might return a value of 1.
#!/bin/bash
if ! pgrep myscript.sh; then
flock -n lockfile myscript.sh
fi
If I understand you right, you want to make sure 'myscript.sh' is not running before cron attempts to run your command again. Assuming that's right, we check to see if pgrep failed to find myscript.sh in the processes list and if so we run the flock command again.
Perhaps something like this would work for you.
#!/bin/bash
RETVAL=0
lockfailed()
{
echo "cannot flock"
exit 1
}
(
flock -w 2 42 || lockfailed
false
RETVAL=$?
echo "original retval $RETVAL"
exit $RETVAL
) 42>|/tmp/flocker
RETVAL=$?
echo "returned $RETVAL"
exit $RETVAL