[Scons-users] SCons misses a dependency which results in incorrect output.

Bill Deegan bill at baddogconsulting.com
Wed Jul 16 15:45:07 EDT 2025 

Arguably your command.sh is incorrectly written.

 It copies the file and then checks it's contents and exits with error
status if there improper.

So a quick fix would be to check the input before copying the file.

>From the URL you posted in other email here's the command.sh
#!/bin/bash
# Usage: ./command input > output

# Copy source to destination
cp "$1" "$2"

# If the input file stats with 'bad' inject an error AFTER creating the
output file.

first_line=$(head -n 1 "$1")

if [[ "$first_line" == bad* ]]; then
  exit 1
fi

exit 0

Change it to this:
#!/bin/bash
# Usage: ./command input > output

# If the input file stats with 'bad' inject an error AFTER creating the
output file.

first_line=$(head -n 1 "$1")

if [[ "$first_line" == bad* ]]; then
  exit 1
fi

# Copy source to destination
cp "$1" "$2"

exit 0

Results:

% ./run.sh

----- Iteration 1: file1 = 'good' -----

scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
scons: building `file2' because it doesn't exist
./command.sh file1 file2
+-file2
  +-file1
scons: done building targets.

File1
MD5 (file1) = d7f986677d9f563bd1794b09d82206a3
     1 good

File2
MD5 (file2) = d7f986677d9f563bd1794b09d82206a3
     1 good

DBlite:
=== .:
file1: d7f986677d9f563bd1794b09d82206a3 1752694898 5
file2: d7f986677d9f563bd1794b09d82206a3 1752694898 5
        file1: d7f986677d9f563bd1794b09d82206a3 1752694898 5
        2dbc2dce125a753309a27b7d5157aaaa [./command.sh $SOURCE $TARGET]

----- Iteration 2: file1 = 'bad' -----

scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
scons: rebuilding `file2' because `file1' changed
./command.sh file1 file2
scons: *** [file2] Error 1
+-file2
  +-file1
scons: building terminated because of errors.

File1
MD5 (file1) = df207dc9143c6fabf60b69b9c3035103
     1 bad

File2
md5: file2: No such file or directory
cat: file2: No such file or directory

DBlite:
=== .:
file1: df207dc9143c6fabf60b69b9c3035103 1752694898 4
file2: d7f986677d9f563bd1794b09d82206a3 1752694898 5
        file1: d7f986677d9f563bd1794b09d82206a3 1752694898 5
        2dbc2dce125a753309a27b7d5157aaaa [./command.sh $SOURCE $TARGET]

----- Iteration 3: file1 = 'good' -----

scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
scons: building `file2' because it doesn't exist
./command.sh file1 file2
+-file2
  +-file1
scons: done building targets.

File1
MD5 (file1) = d7f986677d9f563bd1794b09d82206a3
     1 good

File2
MD5 (file2) = d7f986677d9f563bd1794b09d82206a3
     1 good

DBlite:
=== .:
file1: d7f986677d9f563bd1794b09d82206a3 1752694899 5
file2: d7f986677d9f563bd1794b09d82206a3 1752694899 5
        file1: d7f986677d9f563bd1794b09d82206a3 1752694899 5
        2dbc2dce125a753309a27b7d5157aaaa [./command.sh $SOURCE $TARGET]

There's not presently logic in SCons to delete target files if the
associated action yields an error.

I think what you want is equivalent to makes .DELETE_ON_FAILURE, there's
actually a SO question on this:
https://stackoverflow.com/questions/29546276/scons-delete-target-on-failure-of-any-action

Please go ahead and file an enhancement request to add equivalent
to DELETE_ON_FAILURE, please include your reproducer scripts.

-Bill

On Wed, Jul 16, 2025 at 10:44 AM Tal Dayan <tal at zapta.com> wrote:

> Looking at the end state of scons after invocation #3, the actual md5 of
> file2 doesn't match its md5 in the dblite.
>
> https://i.imgur.com/NGco3yQ.png
>
> On Wed, Jul 16, 2025 at 10:33 AM Tal Dayan <tal at zapta.com> wrote:
>
>> Hi Keith, I updated the example files here
>> https://github.com/FPGAwars/apio/issues/676
>>
>> They now include the md5 of the files and a dump of .sconsign.dblite
>>
>> On Wed, Jul 16, 2025 at 10:08 AM Keith Prussing <kprussing74 at gmail.com>
>> wrote:
>>
>>> I suspect it's because `file1` has the same hash in the
>>> .sconsign.dblite as the last "good" build (i.e. the first one). Thus
>>> you get the line "scons: `file2' is up to date." on the third run.
>>> However, I am not an expert in the specifics of SCons' hashing
>>> methods.
>>>
>>> On Wed, Jul 16, 2025 at 12:38 PM Tal Dayan <tal at zapta.com> wrote:
>>> >
>>> > Hi all,
>>> >
>>> > We encountered this problem with the nextpnr tool and created here a
>>> small and independent example that demonstrates it.
>>> >
>>> > In the example below, a shell script 'command.sh' reads the source
>>> file 'file1' and writes it to the target file 'file2'. However, if the
>>> > source file starts with 'bad' it exits with an error code, *after*
>>> creating the target file.
>>> >
>>> > The script `run.sh', runs scons three times with these values of the
>>> source file file1 'good', 'bad', and 'good'.  The expectation is that after
>>> the third scons run, file2 should contain the value 'good' but it contains
>>> the value 'bad'.
>>> >
>>> > Do we miss anything or is it simply a bug?
>>> >
>>> > SConstruct:
>>> >
>>> > ----------------------------------------------
>>> >
>>> > # SCons environment
>>> >
>>> > env = Environment()
>>> >
>>> >
>>> > # Copy file1 → file2 using command.sh
>>> >
>>> > # Inject an error if file1 starts with 'bad"
>>> >
>>> > file2 = env.Command(
>>> >
>>> >     target='file2',
>>> >
>>> >     source='file1',
>>> >
>>> >     action='./command.sh $SOURCE > $TARGET'
>>> >
>>> > )
>>> >
>>> >
>>> > # Make 'file2' the default target
>>> >
>>> > Default(file2)
>>> >
>>> > ----------------------------------------------
>>> >
>>> >
>>> > command.sh:
>>> >
>>> > ----------------------------------------------
>>> >
>>> > #!/bin/bash
>>> >
>>> > # Usage: ./command input > output
>>> >
>>> >
>>> > # Read from the first argument and copy to stdout
>>> >
>>> > cat "$1"
>>> >
>>> >
>>> > # If the input file starts with 'bad', inject an error AFTER creating
>>> the output file.
>>> >
>>> >
>>> > first_line=$(head -n 1 "$1")
>>> >
>>> >
>>> > if [[ "$first_line" == bad* ]]; then
>>> >
>>> >   exit 1
>>> >
>>> > fi
>>> >
>>> >
>>> > exit 0
>>> >
>>> > ----------------------------------------------
>>> >
>>> >
>>> >
>>> > run.sh
>>> >
>>> > ----------------------------------------------
>>> >
>>> > #!/bin/bash
>>> >
>>> >
>>> > # Clean up.
>>> >
>>> > rm -f .sconsign.dblite
>>> >
>>> > rm -f file[12]
>>> >
>>> >
>>> > echo
>>> >
>>> > echo "---- Iteration 1: file1 = 'good'"
>>> >
>>> > echo "good" > file1
>>> >
>>> > scons
>>> >
>>> > echo
>>> >
>>> >
>>> > echo "File1"
>>> >
>>> > cat -n file1
>>> >
>>> >
>>> > echo "File2"
>>> >
>>> > cat -n file2
>>> >
>>> >
>>> >
>>> > echo
>>> >
>>> > echo "---- Iteration 2: file1 = 'bad'"
>>> >
>>> > echo "bad" > file1
>>> >
>>> > cat -n file1
>>> >
>>> > scons
>>> >
>>> > echo
>>> >
>>> >
>>> > echo "File1"
>>> >
>>> > cat -n file1
>>> >
>>> >
>>> > echo "File2"
>>> >
>>> > cat -n file2
>>> >
>>> >
>>> > echo
>>> >
>>> > echo "---- Iteration 3: file1 = 'good'"
>>> >
>>> > echo "good" > file1
>>> >
>>> > cat -n file1
>>> >
>>> > scons
>>> >
>>> > echo
>>> >
>>> >
>>> > echo "File1"
>>> >
>>> > cat -n file1
>>> >
>>> >
>>> > echo "File2"
>>> >
>>> > cat -n file2
>>> >
>>> > ----------------------------------------------
>>> >
>>> >
>>> >
>>> > Run log:
>>> >
>>> > ----------------------------------------------
>>> >
>>> > $ ./run.sh
>>> >
>>> >
>>> > ---- Iteration 1: file1 = 'good'
>>> >
>>> > scons: Reading SConscript files ...
>>> >
>>> > scons: done reading SConscript files.
>>> >
>>> > scons: Building targets ...
>>> >
>>> > ./command.sh file1 > file2
>>> >
>>> > scons: done building targets.
>>> >
>>> >
>>> > File1
>>> >
>>> >      1 good
>>> >
>>> > File2
>>> >
>>> >      1 good
>>> >
>>> >
>>> > ---- Iteration 2: file1 = 'bad'
>>> >
>>> >      1 bad
>>> >
>>> > scons: Reading SConscript files ...
>>> >
>>> > scons: done reading SConscript files.
>>> >
>>> > scons: Building targets ...
>>> >
>>> > ./command.sh file1 > file2
>>> >
>>> > scons: *** [file2] Error 1
>>> >
>>> > scons: building terminated because of errors.
>>> >
>>> >
>>> > File1
>>> >
>>> >      1 bad
>>> >
>>> > File2
>>> >
>>> >      1 bad
>>> >
>>> >
>>> > ---- Iteration 1: file1 = 'good'
>>> >
>>> >      1 good
>>> >
>>> > scons: Reading SConscript files ...
>>> >
>>> > scons: done reading SConscript files.
>>> >
>>> > scons: Building targets ...
>>> >
>>> > scons: `file2' is up to date.
>>> >
>>> > scons: done building targets.
>>> >
>>> >
>>> > File1
>>> >
>>> >      1 good
>>> >
>>> > File2
>>> >
>>> >      1 bad
>>> >
>>> > ----------------------------------------------
>>> >
>>> > _______________________________________________
>>> > Scons-users mailing list
>>> > Scons-users at scons.org
>>> > https://pairlist4.pair.net/mailman/listinfo/scons-users
>>>
>>>
>>>
>>> --
>>> Keith Prussing
>>> _______________________________________________
>>> Scons-users mailing list
>>> Scons-users at scons.org
>>> https://pairlist4.pair.net/mailman/listinfo/scons-users
>>>
>> _______________________________________________
> Scons-users mailing list
> Scons-users at scons.org
> https://pairlist4.pair.net/mailman/listinfo/scons-users
>
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