In the fish shell the syntax for conditionals is hard to find. Does anyone have a link to an explanation of how to write an if, with ands and ors?
In particular I would like to write
if not $status
do a command
end
To do a command when the previous command returned a non-success. How do I do that?
See http://fishshell.com/docs/current/commands.html#if and http://fishshell.com/docs/current/tutorial.html#tut_conditionals.
Fish's if structure looks like this:
if COMMAND
# do something if it succeeded
else
# do something if it failed ($status != 0)
end
Then there are also the not, and and or commands, that you can use like
if not COMMAND1; or COMMAND2
# and so on
If you really want to test a variable (e.g. $status), you need to use test as the command, like
if test $status -eq 0
Do keep in mind that $status changes after every command, so if you need to use the status of an earlier command (common in prompts) you need to do what Joachim Pileborg said, save it to another variable.
Also, test has some issues with quotes (because it's one of the few parts of fish to adhere to POSIX) - if in test $foo -eq 0 $foo is undefined, test will error out, and if it is undefined in test -n $foo, the test will be true (because POSIX demands that test with one argument be true).
As a sidenote, in fish versions before 2.3.0, you need to add begin and end around a condition with and or or because it was interpreted weirdly.
So you'd have to do
if begin COMMAND; or COMMAND2; end
# do something for status = 0
The shortest short form would be
the_previous_command; or do_a_command
# ..................^^^^^
Assuming you get your $status from "the_previous_command"
I use the status variable to display it on the prompt if it's non-zero. For that I use the following function:
function __pileon_status_prompt
set -l __status $status
if test $__status != 0
printf '[%s%s%s]' (set_color red) $__status (set_color normal)
end
end
As you can see I set a local variable to the value of $status and the check that variable in the condition.
Related
Using only the features of the POSIX shell, is there a "simple command" that does nothing and does not change the value of $? People usually describe : as a no-op command for shell, but this always sets $? to zero, so it's not what I want.
This is needed by a program that generates shell scripts. In several places it needs to emit an if-then-else block
if CONDITION
then
TRUE-COMMANDS
else
FALSE-COMMANDS
fi
but, because of further complications that I'd rather not try to explain right now, it cannot reliably tell whether TRUE-COMMANDS and FALSE-COMMANDS are empty. An empty then- or else-clause will be a shell syntax error.
: can be put at the beginning of the then-clause to take care of TRUE-COMMANDS being empty, but that won't work for FALSE-COMMANDS because it clobbers $? and the FALSE-COMMANDS might want to look at the value $? was set to by the condition. For the same reason, : can't be put after FALSE-COMMANDS—the code after the if-then-else might want to see $? from the last operation within the if-then-else.
Bonus points if you can avoid:
Forking: (exit $?) does the job, but there are so many of these conditional blocks in a generated script that it produces a measurable slowdown.
Functions: given nop () { return $? } then nop does the job, but due to more complications that I'd rather not get into, it's not practical to have nop defined sufficiently early for all of the places that would need it.
The easiest would be to make use of a simple assignment. Instead of using :, do _rc=$?.
if condition; then
[ list-true ] # optional
_rc=$?
else
[ list-false ] # optional
_rc=$?
fi
( exit $_rc ) # optional
list-post-if
Using this variable _rc, you have stored the exit status of the last executed command, whether this is condition or the last command in list-true or list-false.
The arguments in favour of this method is the low overhead of an assignment.
The argument against is the need to at least rewrite list-post-if to make use of _rc instead of $?.
If the latter is not possible, or too tedious, you might concider to add a (exit $_rc) just after the conditional statement. This, however, requires a sub-shell, but it is only one.
Don't let anyone tell you there are only functions and sub-shells.
Can you create or distribute another tiny file? If so, you can create a file with just
return $?
and then source it as an "empty" command keeping the exit status:
$ echo 'return $?' > keepstatus
$ ls foobar
ls: fooobar: No such file or directory
$ . ./keepstatus
$ echo $?
2
$ sleep 100
^C
$ . ./keepstatus
$ echo $?
130
Doesn't fork, doesn't use functions, no extra variable, keeps the status and is as POSIXly as they come.
There's even a fourth way I can see, when I sacrifice the bonus points for forking and assuming, since you are in autoconf m4 territory, that finding and using a host compiler is a breeze.
cat > keepstatus.c <<EOF
#include <stdlib.h>
int main(int argc, char **argv) {
return argv[1] ? atoi(argv[1]) : 0;
}
EOF
$CC -o keepstatus keepstatus.c
Then use /path/to/keepstatus $?.
It’s certainly not possible in a POSIX-compliant shell to write a command that does not touch $?. By reading through IEEE Std 1003.1-2017 vol. 3 chapter 2 ‘Shell Command Language’, we learn that:
§2.8.2 says that ‘[e]ach command has an exit status’ (so no command has no exit status).
§2.5.2 ‘Special Parameters’ says that $? ‘[e]xpands to the decimal exit status of the most recent pipeline (see Section 2.9.2)’.
§2.9.2 ‘Pipelines’ says that a pipeline is a sequence of one or more commands, optionally preceded (as a whole) by !, with commands joined by |. About exit status, it says
If the pipeline does not begin with the ! reserved word, the exit status shall be the exit status of the last command specified in the pipeline. Otherwise, the exit status shall be the logical NOT of the exit status of the last command.
§2.9 ‘Shell Commands’ defines a ‘command’ and says that ‘[u]nless otherwise stated, the exit status of a command shall be that of the last simple command executed by the command’. A ‘command’ may be any of the following:
A simple command (§2.9.1), which is simply an external program, a shell built-in or a variable assignment (without a command name). The former will of course return the exit status of the command executed. About the latter, the specification says:
If there is no command name, but the command contained a command substitution, the command shall complete with the exit status of the last command substitution performed. Otherwise, the command shall complete with a zero exit status.
A pipeline, described by §2.9.2 mentioned above.
A compound-list (§2.9.3), which is a sequence of one or more {sequences of one or more {sequences of one or more pipelines (see above), joined by && or ||}, each terminated by ; or & (with a final ; optional)}, joined by newline characters. A compound-list returns the exit status of the last pipeline executed synchronously; asynchronously-executed pipelines (those terminated by &) set exit status to zero. None of the sequences may be empty, which guarantees at least one pipeline will be executed.
A compound command (§2.9.4), which is either:
A subshell or a braced compound-list (§2.9.4.1), which returns the exit status of the underlying compound-list (see above).
A conditional construct (case, §2.9.4.3 or if, §2.9.4.4) or a loop (for, §2.9.4.2; while, §2.9.4.5; until, §2.9.4.6). If the body of this construct is not executed, it returns an exit status of zero.
A function definition (§2.9.5), which returns an exit status of zero if the definition is accepted, and a non-zero status otherwise.
And finally, §2.9.4.4 ‘The if Conditional Construct’ defines the condition and the body of the if construct to be a compound-list, described by §2.9.3 mentioned above. This means that the body of an if construct will always contain at least one command that will clobber $?. The specification does not even leave any wiggle room for ‘implementation-defined behaviour’.
All of the above means that the only way to write a no-op command that preserves $? is by reading out the value in $? and returning it back. There are three constructs in POSIX shell that are capable of this, two of which are already mentioned in the question body: an exit from a subshell or a function invocation.
The third is .: the shell sourcing statement mentioned in #Jens’ answer. By sourcing a script containing just a return "$?" command, the value of $? can be preserved. However, this requires you to arrange that a suitable script is found at some known location, which I assume to be just as inconvenient as ensuring that a no-op function has been defined early enough in the file (if not in fact more).
If you bend the strict POSIX requirement a little, even this can be overcome, though:
. /dev/stdin <<EOF
return "$?"
EOF
/dev/stdin is not a POSIX feature, but is widely available; it is explicitly listed in IEEE Std 1003.1-2017 vol. 1 §2.1.1 as an extension. The above snippet has been tested to work in bash 5.0.8, dash 0.5.11 and busybox sh 1.30.1 on Linux (with appropriately set-up /dev and so on).
Ultimate Goal
Essentially I have a while loop going through each command-line argument passed to the script, but want to set default behaviour if no arguments are passed.
Current solution idea
My plan for this is to test if there are any arguments prior to this while loop and if there aren't any, simply add the default arguments (flags in this case) to the argument array and let the script execute from there as normal.
Problem I'm having
Where I'm tripping up a little here is figuring out the syntax to add these default arguments to the arguments array itself (I can get values from it no problem).
This is what I have so far:
if test $# -eq 0
then
# ADD --default TO ARGUMENTS ARRAY HERE
fi
while test $# -gt 0
do
case "$1" in
--opt1) DO STUFF
;;
--opt2) DO MORE / OTHER STUFF
;;
--default) DO DEFAULT STUFF
;;
esac
shift
done
Following and altering examples using user-defined arrays I have tried:
if test $# -eq 0
then
$+=('--default')
fi
But just get the syntax error
syntax error near unexpected token `'--default''
sh does not have arrays other than the built-in command-line argument list, and thus no += operator for arrays.
You also seem to be missing a character; what you wrote evaluates to assigning (default) to the value of the variable $+ but this variable is not valid (though because there are no arrays, the parentheses cause an error already).
The shell allows you to use set to manipulate the built-in argument list.
if test $# -eq 0
then
set -- --default
fi
The condition requires the array to be empty, so we simply populate it with the single element you provided; in the general case, you could add stuff to the end with
set -- "$#" --default
or correspondingly to the beginning by putting something before "$#" (which of course contains the previous value of the argument list).
-- Edit : Resolved. See answer.
Background:
I'm writing a shell that will perform some extra actions required on our system when someone resizes a database.
The shell is written in ksh (requirement), the OS is Solaris 5.10 .
The problem is with one of the checks, which verifies there's enough free space on the underlying OS.
Problem:
The check reads the df -k line for root, which is what I check in this step, and prints it to a file. I then "read" the contents into variables which I use in calculations.
Unfortunately, when I try to run an arithmetic operation on one of the variables, I get an error indicating it is null. And a debug output line I've placed after that line verifies that it is null... It lost it's value...
I've tried every method of doing this I could find online, they work when I run it manually, but not inside the shell file.
(* The file does have #!/usr/bin/ksh)
Code:
df -k | grep "rpool/ROOT" > dftest.out
RPOOL_NAME=""; declare -i TOTAL_SIZE=0; USED_SPACE=0; AVAILABLE_SPACE=0; AVAILABLE_PERCENT=0; RSIGN=""
read RPOOL_NAME TOTAL_SIZE USED_SPACE AVAILABLE_SPACE AVAILABLE_PERCENT RSIGN < dftest.out
\rm dftest.out
echo $RPOOL_NAME $TOTAL_SIZE $USED_SPACE $AVAILABLE_SPACE $AVAILABLE_PERCENT $RSIGN
((TOTAL_SIZE=$TOTAL_SIZE/1024))
This is the result:
DBResize.sh[11]: TOTAL_SIZE=/1024: syntax error
I'm pulling hairs at this point, any help would be appreciated.
The code you posted cannot produce the output you posted. Most obviously, the error is signalled at line 11 but you posted fewer than 11 lines of code. The previous lines may matter. Always post complete code when you ask for help.
More concretely, the declare command doesn't exist in ksh, it's a bash thing. You can achieve the same result with typeset (declare is a bash equivalent to typeset, but not all options are the same). Either you're executing this script with bash, or there's another error message about declare, or you've defined some additional commands including declare which may change the behavior of this code.
None of this should have an impact on the particular problem that you're posting about, however. The variables created by read remain assigned until the end of the subshell, i.e. until the code hits a ), the end of a pipe (left-hand side of the pipe only in ksh), etc.
About the use of declare or typeset, note that you're only declaring TOTAL_SIZE as an integer. For the other variables, you're just assigning a value which happens to consist exclusively of digits. It doesn't matter for the code you posted, but it's probably not what you meant.
One thing that may be happening is that grep matches nothing, and therefore read reads an empty line. You should check for errors. Use set -e in scripts to exit at the first error. (There are cases where set -e doesn't catch errors, but it's a good start.)
Another thing that may be happening is that df is splitting its output onto multiple lines because the first column containing the filesystem name is too large. To prevent this splitting, pass the option -P.
Using a temporary file is fragile: the code may be executed in a read-only directory, another process may want to access the same file at the same time... Here a temporary file is useless. Just pipe directly into read. In ksh (unlike most other sh variants including bash), the right-hand side of a pipe runs in the main shell, so assignments to variables in the right-hand side of a pipe remain available in the following commands.
It doesn't matter in this particular script, but you can use a variable without $ in an arithmetic expression. Using $ substitutes a string which can have confusing results, e.g. a='1+2'; $((a*3)) expands to 7. Not using $ uses the numerical value (in ksh, a='1+2'; $((a*3)) expands to 9; in some sh implementations you get an error because a's value is not numeric).
#!/usr/bin/ksh
set -e
typeset -i TOTAL_SIZE=0 USED_SPACE=0 AVAILABLE_SPACE=0 AVAILABLE_PERCENT=0
df -Pk | grep "rpool/ROOT" | read RPOOL_NAME TOTAL_SIZE USED_SPACE AVAILABLE_SPACE AVAILABLE_PERCENT RSIGN
echo $RPOOL_NAME $TOTAL_SIZE $USED_SPACE $AVAILABLE_SPACE $AVAILABLE_PERCENT $RSIGN
((TOTAL_SIZE=TOTAL_SIZE/1024))
Strange...when I get rid of your "declare" line, your original code seems to work perfectly well (at least with ksh on Linux)
The code :
#!/bin/ksh
df -k | grep "/home" > dftest.out
read RPOOL_NAME TOTAL_SIZE USED_SPACE AVAILABLE_SPACE AVAILABLE_PERCENT RSIGN < dftest.out
\rm dftest.out
echo $RPOOL_NAME $TOTAL_SIZE $USED_SPACE $AVAILABLE_SPACE $AVAILABLE_PERCENT $RSIGN
((TOTAL_SIZE=$TOTAL_SIZE/1024))
print $TOTAL_SIZE
The result :
32962416 5732492 25552588 19% /home
5598
Which are the value a simple df -k is returning. The variables seem to last.
For those interested, I have figured out that it is not possible to use "read" the way I was using it.
The variable values assigned by "read" simply "do not last".
To remedy this, I have applied the less than ideal solution of using the standard "while read" format, and inside the loop, echo selected variables into a variable file.
Once said file was created, I just "loaded" it.
(pseudo code:)
LOOP START
echo "VAR_A="$VAR_A"; VAR_B="$VAR_B";" > somefile.out
LOOP END
. somefile.out
I wrote configuration of fish shell like this:
# One or more argument(s) will be given
function run
set -l src $argv[1]
set -l var
switch "$src"
case *
set var "$src"
end
echo $var
end
I expected the first argument is printed in any case if one or more argument is given. However, $var becomes $argv[1] if file with the same name as $argv[1] exists, otherwise it becomes empty string.
Could someone tell me why?
The * in case * is interpreted as a glob. Quote it if you do not want that.
Faho already answered your question but I wanted to point out that your approach is more complicated than necessary. If you just want to print the first argument if one or more were provided do this:
set -q argv[1]
and echo $argv[1]
The first statement checks if argv has at least one value. The second echoes it if the previous statement returned success (i.e., $status set to zero).
Using fish shell, I'm writing very simple script that checks the command execution
#!/usr/bin/fish
command
if $status
echo "Oops error"
else
echo "Worked OK"
#...
end
And get the error message:
fish: Variables may not be used as commands. Instead, define a function like “function status; 0 $argv; end”. See the help section for the function command by typing “help function”.
The message looks pretty straight forward but no "defining function like..." nor "help function" helps solving the problem.
There is also a 'test' command, that sounds promising. But docs say it is to be used to check files...
How this simple thing should be done with fish shell?
Heh... And why all documentation is SO misleading?..
P.S. Please, don't write about 'and' command.
Fish's test command currently works exactly like POSIX test (i.e. the one you'll find in bash or similar shells). It has a couple of operations, including "-gt", "-eq", "-lt" to check if a number is bigger, equal or less than another number, respectively.
So if you want to use test, you'll do if test $status -eq 0 (a 0 traditionally denotes success). Otherwise, you can check the return value of a command by putting it in the if clause directly like if command (which will be true if the command returns 0) - that's what fish is trying to do here, which is why it complains about a variable being used in place of a command.