I'd like to achieve the test with a cleaner, less ugly one-liner:
#!/bin/bash
test -d "$1" || (echo "Argument 1: '$1' is not a directory" 1>&2 ; exit 1) || exit 1
# ... script continues if $1 is directory...
Basically I am after something which does not duplicate the exit, and preferably does not spawn a sub-shell (and as a result should also look less ugly), but still fits in one line.
Without a subshell and without duplicating exit:
test -d "$1" || { echo "Argument 1: '$1' is not a directory" 1>&2 ; exit 1; }
You may also want to refer to Grouping Commands:
{}
{ list; }
Placing a list of commands between curly braces causes the list to be executed in the current shell context. No subshell is created. The
semicolon (or newline) following list is required.
Related
The following script calls another program reading its output in a while loop (see Bash - How to pipe input to while loop and preserve variables after loop ends):
while read -r col0 col1; do
# [...]
done < <(other_program [args ...])
How can I check for the exit code of other_program to see if the loop was executed properly?
Note: ls -d / /nosuch is used as an example command below, because it fails (exit code 1) while still producing stdout output (/) (in addition to stderr output).
Bash v4.2+ solution:
ccarton's helpful answer works well in principle, but by default the while loop runs in a subshell, which means that any variables created or modified in the loop will not be visible to the current shell.
In Bash v4.2+, you can change this by turning the lastpipe option on, which makes the last segment of a pipeline run in the current shell;
as in ccarton's answer, the pipefail option must be set to have $? reflect the exit code of the first failing command in the pipeline:
shopt -s lastpipe # run the last segment of a pipeline in the current shell
shopt -so pipefail # reflect a pipeline's first failing command's exit code in $?
ls -d / /nosuch | while read -r line; do
result=$line
done
echo "result: [$result]; exit code: $?"
The above yields (stderr output omitted):
result: [/]; exit code: 1
As you can see, the $result variable, set in the while loop, is available, and the ls command's (nonzero) exit code is reflected in $?.
Bash v3+ solution:
ikkachu's helpful answer works well and shows advanced techniques, but it is a bit cumbersome.
Here is a simpler alternative:
while read -r line || { ec=$line && break; }; do # Note the `|| { ...; }` part.
result=$line
done < <(ls -d / /nosuch; printf $?) # Note the `; printf $?` part.
echo "result: [$result]; exit code: $ec"
By appending the value of $?, the ls command's exit code, to the output without a trailing \n (printf $?), read reads it in the last loop operation, but indicates failure (exit code 1), which would normally exit the loop.
We can detect this case with ||, and assign the exit code (that was still read into $line) to variable $ec and exit the loop then.
On the off chance that the command's output doesn't have a trailing \n, more work is needed:
while read -r line ||
{ [[ $line =~ ^(.*)/([0-9]+)$ ]] && ec=${BASH_REMATCH[2]} && line=${BASH_REMATCH[1]};
[[ -n $line ]]; }
do
result=$line
done < <(printf 'no trailing newline'; ls /nosuch; printf "/$?")
echo "result: [$result]; exit code: $ec"
The above yields (stderr output omitted):
result: [no trailing newline]; exit code: 1
At least one way would be to redirect the output of the background process through a named pipe. This would allow to pick up its PID and then get the exit status through waiting on the PID.
#!/bin/bash
mkfifo pipe || exit 1
(echo foo ; exit 19) > pipe &
pid=$!
while read x ; do echo "read: $x" ; done < pipe
wait $pid
echo "exit status of bg process: $?"
rm pipe
If you can use a direct pipe (i.e. don't mind the loop being run in a subshell), you could use Bash's PIPESTATUS, which contains the exit codes of all commands in the pipeline:
(echo foo ; exit 19) | while read x ; do
echo "read: $x" ; done;
echo "status: ${PIPESTATUS[0]}"
A simple way is to use the bash pipefail option to propagate the first error code from a pipeline.
set -o pipefail
other_program | while read x; do
echo "Read: $x"
done || echo "Error: $?"
Another way is to use coproc (requires 4.0+).
coproc other_program [args ...]
while read -r -u ${COPROC[0]} col0 col1; do
# [...]
done
wait $COPROC_PID || echo "Error exit status: $?"
coproc frees you from having to setup asynchronicity and stdin/stdout redirection that you'd otherwise need to do in an equivalent mkfifo.
I am trying to figure out how the while loop logic works in bash.
If I have 4 instructions; that are in a loop, they will be executed n times, following the conditions set in the loop.
Although; is the loop aware of the return code for each of these commands?
Ideally you could write a shell script with a while loop where you check for exit code of each command, and do it again if any of the command return an error.
Is this logic possible in bash or the loop is not aware of the return code of each command, and see the block between the do-done, as one?
EDIT:
example:
a=2
b="test"
while [ "$?" != 0 ]
do
ls $b
cd $b
grep "2.txt" | mv 2.txt $a.txt
ls $b
done
This simplified example is basically checking for a dir, cd into it, find a file with a specific name and rename it.
When I run this loop, if any of these commands return code is not 0, the loop should start all over, until all the commands return 0 and then the loop exit.
Is this actually true or the whole block of commands is handled as single entity, so if the last command return 0 it will exit?
The while command only checks the exit status of the condition statement immediately following it. It doesn't check the status of the statements inside the loop.
You can use an if statement inside the loop to break out of the loop.
while something
do
command1
if ! command2
then break
fi
command3
done
This will loop as long as something is successful, but stop if command2 fails.
Perhaps: use an infinite loop, and join all the commands with && with break at the end
while true
do
ls $b &&
cd $b &&
grep "2.txt" | mv 2.txt $a.txt &&
ls $b &&
break
done
Note while ...; do ...; done not while ...; do ...; loop
Imagine the code:
ls || echo "Unable to execute ls (returned non zero)"
What if I needed to execute more commands like:
ls || echo "this is echo 1" <some operator> echo "this is echo 2" <some operator> exit 1
in C (assumming I have a function ls) I could do (even if it looks insane):
ls() || (command1() && command2());
but I doubt I can use parentheses like this in bash.
I know I can create a Bash function that would contain these commands, but what if I needed to combine this with exit 1 (exit in a function would exit that function, not whole script)?
You can group multiple commands within { }. Saying:
some_command || { command1; command2; }
would execute command1 and command2 if some_command exited with a non-zero return code.
{}
{ list; }
Placing a list of commands between curly braces causes the list to be executed in the current shell context. No subshell is created. The
semicolon (or newline) following list is required.
In fact, you can use parentheses. They just tell bash "run the commands in a subshell". You can also use curlies:
ls || { echo 1 ; echo 2 ; }
Note that ; or a newline before the closing curlie is not optional.
I want to write code like this:
command="some command"
safeRunCommand $command
safeRunCommand() {
cmnd=$1
$($cmnd)
if [ $? != 0 ]; then
printf "Error when executing command: '$command'"
exit $ERROR_CODE
fi
}
But this code does not work the way I want. Where did I make the mistake?
Below is the fixed code:
#!/bin/ksh
safeRunCommand() {
typeset cmnd="$*"
typeset ret_code
echo cmnd=$cmnd
eval $cmnd
ret_code=$?
if [ $ret_code != 0 ]; then
printf "Error: [%d] when executing command: '$cmnd'" $ret_code
exit $ret_code
fi
}
command="ls -l | grep p"
safeRunCommand "$command"
Now if you look into this code, the few things that I changed are:
use of typeset is not necessary, but it is a good practice. It makes cmnd and ret_code local to safeRunCommand
use of ret_code is not necessary, but it is a good practice to store the return code in some variable (and store it ASAP), so that you can use it later like I did in printf "Error: [%d] when executing command: '$command'" $ret_code
pass the command with quotes surrounding the command like safeRunCommand "$command". If you don’t then cmnd will get only the value ls and not ls -l. And it is even more important if your command contains pipes.
you can use typeset cmnd="$*" instead of typeset cmnd="$1" if you want to keep the spaces. You can try with both depending upon how complex is your command argument.
'eval' is used to evaluate so that a command containing pipes can work fine
Note: Do remember some commands give 1 as the return code even though there isn't any error like grep. If grep found something it will return 0, else 1.
I had tested with KornShell and Bash. And it worked fine. Let me know if you face issues running this.
Try
safeRunCommand() {
"$#"
if [ $? != 0 ]; then
printf "Error when executing command: '$1'"
exit $ERROR_CODE
fi
}
It should be $cmd instead of $($cmd). It works fine with that on my box.
Your script works only for one-word commands, like ls. It will not work for "ls cpp". For this to work, replace cmd="$1"; $cmd with "$#". And, do not run your script as command="some cmd"; safeRun command. Run it as safeRun some cmd.
Also, when you have to debug your Bash scripts, execute with '-x' flag. [bash -x s.sh].
There are several things wrong with your script.
Functions (subroutines) should be declared before attempting to call them. You probably want to return() but not exit() from your subroutine to allow the calling block to test the success or failure of a particular command. That aside, you don't capture 'ERROR_CODE' so that is always zero (undefined).
It's good practice to surround your variable references with curly braces, too. Your code might look like:
#!/bin/sh
command="/bin/date -u" #...Example Only
safeRunCommand() {
cmnd="$#" #...insure whitespace passed and preserved
$cmnd
ERROR_CODE=$? #...so we have it for the command we want
if [ ${ERROR_CODE} != 0 ]; then
printf "Error when executing command: '${command}'\n"
exit ${ERROR_CODE} #...consider 'return()' here
fi
}
safeRunCommand $command
command="cp"
safeRunCommand $command
The normal idea would be to run the command and then use $? to get the exit code. However, sometimes you have multiple cases in which you need to get the exit code. For example, you might need to hide its output, but still return the exit code, or print both the exit code and the output.
ec() { [[ "$1" == "-h" ]] && { shift && eval $* > /dev/null 2>&1; ec=$?; echo $ec; } || eval $*; ec=$?; }
This will give you the option to suppress the output of the command you want the exit code for. When the output is suppressed for the command, the exit code will directly be returned by the function.
I personally like to put this function in my .bashrc file.
Below I demonstrate a few ways in which you can use this:
# In this example, the output for the command will be
# normally displayed, and the exit code will be stored
# in the variable $ec.
$ ec echo test
test
$ echo $ec
0
# In this example, the exit code is output
# and the output of the command passed
# to the `ec` function is suppressed.
$ echo "Exit Code: $(ec -h echo test)"
Exit Code: 0
# In this example, the output of the command
# passed to the `ec` function is suppressed
# and the exit code is stored in `$ec`
$ ec -h echo test
$ echo $ec
0
Solution to your code using this function
#!/bin/bash
if [[ "$(ec -h 'ls -l | grep p')" != "0" ]]; then
echo "Error when executing command: 'grep p' [$ec]"
exit $ec;
fi
You should also note that the exit code you will be seeing will be for the grep command that's being run, as it is the last command being executed. Not the ls.
Bash: I want to run a command and pipe the results through some filter, but if the command fails, I want to return the command's error value, not the boring return value of the filter:
E.g.:
if !(cool_command | output_filter); then handle_the_error; fi
Or:
set -e
cool_command | output_filter
In either case it's the return value of cool_command that I care about -- for the 'if' condition in the first case, or to exit the script in the second case.
Is there some clean idiom for doing this?
Use the PIPESTATUS builtin variable.
From man bash:
PIPESTATUS
An array variable (see Arrays
below) containing a list of exit
status values from the processes in
the most-recently-executed foreground
pipeline (which may contain only a
single command).
If you didn't need to display the error output of the command, you could do something like
if ! echo | mysql $dbcreds mysql; then
error "Could not connect to MySQL. Did you forget to add '--db-user=' or '--db-password='?"
die "Check your credentials or ensure server is running with /etc/init.d/mysqld status"
fi
In the example, error and die are defined functions. elsewhere in the script. $dbcreds is also defined, though this is built from command line options. If there is no error generated by the command, nothing is returned. If an error occurs, text will be returned by this particular command.
Correct me if I'm wrong, but I get the impression you're really looking to do something a little more convoluted than
[ `id -u` -eq '0' ] || die "Must be run as root!"
where you actually grab the user ID prior to the if statement, and then perform the test. Doing it this way, you could then display the result if you choose. This would be
UID=`id -u`
if [ $UID -eq '0' ]; then
echo "User is root"
else
echo "User is not root"
exit 1 ##set an exit code higher than 0 if you're exiting because of an error
fi
The following script uses a fifo to filter the output in a separate process. This has the following advantages over the other answers. First, it is not bash specific. In particular it does not rely on PIPESTATUS. Second, output is not stalled until the command has completed.
$ cat >test_filter.sh <<EOF
#!/bin/sh
cmd()
{
echo $1
echo $2 >&2
return $3
}
filter()
{
while read line
do
echo "... $line"
done
}
tmpdir=$(mktemp -d)
fifo="$tmpdir"/out
mkfifo "$fifo"
filter <"$fifo" &
pid=$!
cmd a b 10 >"$fifo" 2>&1
ret=$?
wait $pid
echo exit code: $ret
rm -f "$fifo"
rmdir "$tmpdir"
EOF
$ sh ./test_filter.sh
... a
... b
exit code: 10