return value from eval
Solution 1
When you do the following -
`nonsense`
echo $?
You basically are asking "Tell me the exit status when I try to get the output of the command nonsense" the answer to that is "command not found" or 127
But when you do the following
eval `nonsense`
echo $?
You are asking "tell me the exit status of eval when I evaluate an empty string" (the output of command nonsense) which is equal to running eval
without arguments.
eval
has no problems in running without arguments and its exit status becomes 0
Solution 2
Actually, it's more the:
$ `nonsense`
bash: nonsense: command not found
$ echo "$?"
127
That is surprising here.
We're asking bash
to run the command that results of the split+glob operator on the stdout of nonsense
. Since nonsense
produces no output, it runs no command, so you may think the exit status should be 0.
But actually, when a simple command line has no argument, only assignment or redirection, the exit status is that of the last command substitution in assignment and normal words (not in redirection targets) that was run (though failure in redirections will also affect the exit status).
That's specially useful with assignments.
In:
output=$(grep pattern file)
status=$?
You can get both the output and exit status of grep
, which you couldn't do if $?
was otherwise the exit status of that non-command.
In:
output=$(cmd1) cmd2
That is where there are both assignment words and argument words, the exit status of cmd1
is ignored. $?
will contain the exit status of cmd2
.
And, also $output
will only be set for cmd2
only. Exception to that is when cmd2
is a special builtin.
eval
is such a special builtin.
$ a=0; a=1 eval; echo "$a"
1
In bash
and most modern POSIX shells.
a=`exit 5` eval; echo "$?"
or
eval `exit 5`; echo "$?"
Would output 0, as it's the result of running eval
with no argument. But that was not the case in the Bourne shell or ksh88, where for special builtins you'd get the exit status of exit 5
there.
In those shells, you'll also find:
$ a=`exit 3` set x; echo "$?"
3
As set
is another special builtin.
.
is another special builtin. In the Bourne shell and ksh88:
$ . /some/file `exit 4`; echo "$?"
4
(as long as /some/file
doesn't run any command)
Viesturs
Updated on September 18, 2022Comments
-
Viesturs almost 2 years
The bash manual states:
eval [arg ...] The args are read and concatenated together into a single com- mand. This command is then read and executed by the shell, and its exit status is returned as the value of eval. If there are no args, or only null arguments, eval returns 0.
I try
eval `nonsense` echo $?
The result is
0
.Whereas when I execute the back-quoted command separately:
`nonsense` echo $?
The result is
127
.From what is written in the bash manual I would expect
eval
to return127
when taking the back-quotednonsense
as argument.How to obtain the exit status of the argument of
eval
?-
Ignacio Vazquez-Abrams over 6 years
`nonsense`
doesn't return anything ifnonsense
doesn't exist, so there are no arguments toeval
. -
Weijun Zhou over 6 yearsYou should run
eval nonsense
, then you will get 127. -
Viesturs over 6 years@WeijunZhou, for the specific case at hand
nonsense
comes in back-quotes, -
Weijun Zhou over 6 yearsOK, I upvoted it because it has provided some insight to the problem.
-
-
Charles Duffy over 6 yearsCode formatting should not be used for English prose.
-
amisax over 6 years@CharlesDuffy .. thanks for the edit tip, made the changes accordingly
-
codenoob over 4 yearsI'm confused. The OP provided context, and that is what is being responded to, but where is the answer to the actual question posited which was: "How to obtain the exit status of the argument of eval?" I guess I need to read more carefully?
-
codenoob over 4 yearsI'm confused. The OP provided context, and that is what is being responded to, but where is the answer to the actual question posited which was: "How to obtain the exit status of the argument of eval?" I guess I need to read more carefully?