#!/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
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 $?
1
#!/bin/bash -e
should suffice.
I am just throwing in another one for reference since there was an additional question to Mark Edgars input and here is an additional example and touches on the topic overall:
[[ `cmd` ]] && echo success_else_silence
Which is the same as cmd || exit errcode as someone showed.
For example, I want to make sure a partition is unmounted if mounted:
[[ `mount | grep /dev/sda1` ]] && umount /dev/sda1
In a nodejs project I have a shortcut yarn lint that runs couple of linters in such way:
lint_1 && lint_2 && lint_3
If any of these find an error it return an error code, as a result yarn lint itself returns error code, as a result - build fails.
It works somewhat fine, catches all the errors though there is a small issue: If a linter fails with error code - rest of the linters wont be executed.
What I would like - execute all of them (so they all print all errors) and only then fail.
I know that I can create a bash script (that I will run in yarn lint), run each of the linters one by one collecting return codes and then check if any of codes is non-zero - exit 1 and it will fail the yarn lint. But I am wondering is there more elegant way to do it?
You could trap on ERR and set a flag. This would run each of the linters and exit with failure if any one of them fails:
#!/bin/bash
result=0
trap 'result=1' ERR
lint_1
lint_2
lint_3
exit "$result"
What I would like - execute all of them (so they all print all errors) and only then fail
Basically we have a list of exit codes to catch. If any of them is nonzero, we need to set a variable to have nonzero value. Expanding that to a list, would look like this:
result=0
if ! lint_1; then result=1; fi
if ! lint_2; then result=1; fi
if ! lint_3; then result=1; fi
exit "$result"
As a programmer, I see that we have a pattern here. So we can go with an array, but bash does not have 2d arrays. It would be a workaround with eval to get around quoted parameters. It is doable. You have to use eval, to double evaulate the array "pointer"/name, but works. Note that eval is evil.
cmds_1=(lint_1 "arg with spaces you pass to lint_1")
cmds_2=(lint_2)
cmds_3=(lint_3)
result=0
# compgen results list of variables starting with `cmds_`
# so naming is important
for i in $(compgen -v cmds_); do
# at first, `$i` is only expanded
# then the array is expanded `"${cmds_?[#]}"`
if ! eval "\"\${$i[#]}\""; then
result=1
fi
done
exit "$result"
We can also go with xargs. From manual EXIT STATUS is 123 if __any__ invocation of the command exited with status 1-125. If you know that your programs will exit between 1-125 exit status you can (usually xargs handles different exit statuses correctly anyway (returns 123), but let's stay conforming):
xargs -l1 -- bash -c '"$#"' -- <<EOF
lint_1 "arg with spaces you pass to lint_1"
lint_2
lint_3
EOF
result=$? # or just exit "$?"
exit "$result"
which looks strangely clean. On a side note, by passing just -P <number of jobs> to xargs you can execute all the command in parallel. You can accommodate for the 1-125 error range, by handling the error inside the bash script, ie.
xargs -l1 -- bash -c '"$#" || exit 1' -- <<EOF
lint_1 "arg with spaces you pass to lint_1"
lint_2
lint_3
EOF
result=$?
exit "$result"
And I have another idea. After each command we can output the return status on a dedicated file descriptor. Then from all return statuses filter zeros and check if there are any other statuses on the stream. If they are, we should exit with nonzero status. This feels like a work-done-around and is basically the same as the first code snipped, but the if ! ....; then result=1; fi is simplified to ; echo $? >&10.
tmp=$(mktemp)
(
lint_1 "arg with spaces you pass to lint_1"; echo $? >&10
lint_2; echo $? >&10
lint_3; echo $? >&10
) 10> >(
[ -z "$(grep -v 0)" ]
echo $? > "$tmp"
)
result="$(cat "$tmp"; rm "$tmp")"
exit "$result"
From the options presented, I would go with the other answer ;) or with the xargs second snipped.
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 want to redirect make's log and see what's doing on make. Here is the script
make |& tee make.log # bash syntax
# make 2>&1 | tee make.log # or, sh syntax
[ $? -ne 0 ] && echo "Error: stopped" && exit 1
echo "Done"
I found it won't execute the Error exit when make failed.
I guess it is caused by the pipe, but how to refine the build script?
Since you're already restricting yourself to using bash, not portable POSIX sh, you can just use bash's pipefail shell option; run set -o pipefail. The man page for bash says:
If pipefail is enabled, the pipeline's return status is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands exit successfully.
eg.
#!/bin/bash
set -o pipefail
if make |& tee make.log ; then
echo "Done"
else
echo "Error: stopped"
exit 1
fi
Suppose a shell script (/bin/sh or /bin/bash) contained several commands. How can I cleanly make the script terminate if any of the commands has a failing exit status? Obviously, one can use if blocks and/or callbacks, but is there a cleaner, more concise way? Using && is not really an option either, because the commands can be long, or the script could have non-trivial things like loops and conditionals.
With standard sh and bash, you can
set -e
It will
$ help set
...
-e Exit immediately if a command exits with a non-zero status.
It also works (from what I could gather) with zsh. It also should work for any Bourne shell descendant.
With csh/tcsh, you have to launch your script with #!/bin/csh -e
May be you could use:
$ <any_command> || exit 1
You can check $? to see what the most recent exit code is..
e.g
#!/bin/sh
# A Tidier approach
check_errs()
{
# Function. Parameter 1 is the return code
# Para. 2 is text to display on failure.
if [ "${1}" -ne "0" ]; then
echo "ERROR # ${1} : ${2}"
# as a bonus, make our script exit with the right error code.
exit ${1}
fi
}
### main script starts here ###
grep "^${1}:" /etc/passwd > /dev/null 2>&1
check_errs $? "User ${1} not found in /etc/passwd"
USERNAME=`grep "^${1}:" /etc/passwd|cut -d":" -f1`
check_errs $? "Cut returned an error"
echo "USERNAME: $USERNAME"
check_errs $? "echo returned an error - very strange!"