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3

I am writing an automated homework grader in bash. The grader compiles a program and runs it. If the program fails to compile or fails to run (e.g. due to a segmentation fault) the grade should be a fixed small number, e.g. 5. Otherwise, the grade is the last line output by the program.

To get the last line, I can do:

grade=$( ./a.out | tail -1 )

But this always gives an exit code of 0, even if a.out fails to run (e.g. not found), so I cannot tell whether the program failed to exist.

Another option is to use a temporary file:

./a.out > temp
if [ $? -ne 0 ]
then
 grade=0
else
 grade=$( tail -1 temp )
fi

However, this might be problematic if there are many different processes doing the same simultaneously. Even with one process, it is wasteful to keep all output in a file (the output might be large) when I only need the last line.

Is there a solution that does not use a temporary file?

bash scripting

share|improve this question

edited yesterday

Rich

369212

asked 2 days ago

Erel Segal-HaleviErel Segal-Halevi

3201210

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Any objections to running it twice? Once to ensure a zero exit code, subsequently for the output?
  
  – Jeff Schaller♦
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @JeffSchaller This is also an option, but it is quite inefficient, especially when there are many simultanous submissions. Consider 100 students submitting simultaneously. There is a noticeable difference between 100 runs and 200 runs.
  
  – Erel Segal-Halevi
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  In bash, you can use PIPESTATUS instead of $?: exit 13 | tail -n1; echo $PIPESTATUS[0]
  
  – mosvy
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @mosvy strangely, this does not work when I capture the output into a variable: *** $ grade=$( ./a.out | tail -1 ) *** bash: ./a.out: No such file or directory *** $ echo $PIPESTATUS[0] *** 0
  
  – Erel Segal-Halevi
  2 days ago
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  command substitutions are run in a subshell; you should check the PIPESTATUS inside that subshell: grade=$(./a.out | tail -1; test $PIPESTATUS[0] = 0 || echo 5)
  
  – mosvy
  2 days ago
  
  

  
  
  
  

 | 
show 3 more comments

3

I am writing an automated homework grader in bash. The grader compiles a program and runs it. If the program fails to compile or fails to run (e.g. due to a segmentation fault) the grade should be a fixed small number, e.g. 5. Otherwise, the grade is the last line output by the program.

To get the last line, I can do:

grade=$( ./a.out | tail -1 )

But this always gives an exit code of 0, even if a.out fails to run (e.g. not found), so I cannot tell whether the program failed to exist.

Another option is to use a temporary file:

./a.out > temp
if [ $? -ne 0 ]
then
 grade=0
else
 grade=$( tail -1 temp )
fi

However, this might be problematic if there are many different processes doing the same simultaneously. Even with one process, it is wasteful to keep all output in a file (the output might be large) when I only need the last line.

Is there a solution that does not use a temporary file?

bash scripting

share|improve this question

edited yesterday

Rich

369212

asked 2 days ago

Erel Segal-HaleviErel Segal-Halevi

3201210

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Any objections to running it twice? Once to ensure a zero exit code, subsequently for the output?
  
  – Jeff Schaller♦
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @JeffSchaller This is also an option, but it is quite inefficient, especially when there are many simultanous submissions. Consider 100 students submitting simultaneously. There is a noticeable difference between 100 runs and 200 runs.
  
  – Erel Segal-Halevi
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  In bash, you can use PIPESTATUS instead of $?: exit 13 | tail -n1; echo $PIPESTATUS[0]
  
  – mosvy
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @mosvy strangely, this does not work when I capture the output into a variable: *** $ grade=$( ./a.out | tail -1 ) *** bash: ./a.out: No such file or directory *** $ echo $PIPESTATUS[0] *** 0
  
  – Erel Segal-Halevi
  2 days ago
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  command substitutions are run in a subshell; you should check the PIPESTATUS inside that subshell: grade=$(./a.out | tail -1; test $PIPESTATUS[0] = 0 || echo 5)
  
  – mosvy
  2 days ago
  
  

  
  
  
  

 | 
show 3 more comments

I am writing an automated homework grader in bash. The grader compiles a program and runs it. If the program fails to compile or fails to run (e.g. due to a segmentation fault) the grade should be a fixed small number, e.g. 5. Otherwise, the grade is the last line output by the program.

To get the last line, I can do:

grade=$( ./a.out | tail -1 )

But this always gives an exit code of 0, even if a.out fails to run (e.g. not found), so I cannot tell whether the program failed to exist.

Another option is to use a temporary file:

./a.out > temp
if [ $? -ne 0 ]
then
 grade=0
else
 grade=$( tail -1 temp )
fi

However, this might be problematic if there are many different processes doing the same simultaneously. Even with one process, it is wasteful to keep all output in a file (the output might be large) when I only need the last line.

Is there a solution that does not use a temporary file?

bash scripting

share|improve this question

edited yesterday

Rich

369212

asked 2 days ago

Erel Segal-HaleviErel Segal-Halevi

3201210

grade=$( ./a.out | tail -1 )

./a.out > temp
if [ $? -ne 0 ]
then
 grade=0
else
 grade=$( tail -1 temp )
fi

share|improve this question

edited yesterday

Rich

369212

asked 2 days ago

Erel Segal-HaleviErel Segal-Halevi

3201210

share|improve this question

edited yesterday

Rich

369212

asked 2 days ago

Erel Segal-HaleviErel Segal-Halevi

3201210

edited yesterday

Rich

369212

edited yesterday

Rich

369212

asked 2 days ago

Erel Segal-HaleviErel Segal-Halevi

3201210

asked 2 days ago

Erel Segal-HaleviErel Segal-Halevi

3201210

2 Answers
2

active

oldest

votes

2

Your problem boils down to: "I want to get the last line of output AND detect abnormal exit without using temporary files". In that case, set -o pipefail is your friend.

Here's a simple script that executes its arguments and records the last line of the output on normal exit:

#!/bin/bash

[[ -x $1 ]] || 
 echo >&2 "Usage $0##*/ <PROGRAM> [ARG1] ..."
 exit 1


set -o pipefail
program=$1
shift;
x=$($program "$@" | tail -1)

if [[ $? -eq 0 ]]; then
 echo "Pass: $x"
else
 echo "Fail: $?"
fi

I'll leave the problem of assigning a grade to you.

share|improve this answer

answered yesterday

Chris WilliamsChris Williams

411

New contributor

Chris Williams is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

add a comment | 

2

grade=$( echo 0; | tail -n 1 )

This would try to execute ./a.out and then add a line with a single 0 to its output if that program exited with a non-zero exit status or failed to execute at all. The 0 would be caught by tail -n 1 and placed in $grade.

If ./a.out executed correctly and terminated with a zero exit status, the echo would not be triggered.

Remove the redirection of standard error to /dev/null if you are interested in seeing diagnostic messages related to running ./a.out.

Change the 0 to "$?" to get the exit code instead. To be able to differentiate a number from an error, you may want to use NaN instead, or some error string.

share|improve this answer

edited yesterday

answered yesterday

Kusalananda♦Kusalananda

139k17259430

add a comment | 

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2

Your problem boils down to: "I want to get the last line of output AND detect abnormal exit without using temporary files". In that case, set -o pipefail is your friend.

Here's a simple script that executes its arguments and records the last line of the output on normal exit:

#!/bin/bash

[[ -x $1 ]] || 
 echo >&2 "Usage $0##*/ <PROGRAM> [ARG1] ..."
 exit 1


set -o pipefail
program=$1
shift;
x=$($program "$@" | tail -1)

if [[ $? -eq 0 ]]; then
 echo "Pass: $x"
else
 echo "Fail: $?"
fi

I'll leave the problem of assigning a grade to you.

share|improve this answer

answered yesterday

Chris WilliamsChris Williams

411

New contributor

Chris Williams is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

add a comment | 

2

Your problem boils down to: "I want to get the last line of output AND detect abnormal exit without using temporary files". In that case, set -o pipefail is your friend.

Here's a simple script that executes its arguments and records the last line of the output on normal exit:

#!/bin/bash

[[ -x $1 ]] || 
 echo >&2 "Usage $0##*/ <PROGRAM> [ARG1] ..."
 exit 1


set -o pipefail
program=$1
shift;
x=$($program "$@" | tail -1)

if [[ $? -eq 0 ]]; then
 echo "Pass: $x"
else
 echo "Fail: $?"
fi

I'll leave the problem of assigning a grade to you.

share|improve this answer

answered yesterday

Chris WilliamsChris Williams

411

New contributor

Chris Williams is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

add a comment | 

Your problem boils down to: "I want to get the last line of output AND detect abnormal exit without using temporary files". In that case, set -o pipefail is your friend.

Here's a simple script that executes its arguments and records the last line of the output on normal exit:

#!/bin/bash

[[ -x $1 ]] || 
 echo >&2 "Usage $0##*/ <PROGRAM> [ARG1] ..."
 exit 1


set -o pipefail
program=$1
shift;
x=$($program "$@" | tail -1)

if [[ $? -eq 0 ]]; then
 echo "Pass: $x"
else
 echo "Fail: $?"
fi

I'll leave the problem of assigning a grade to you.

share|improve this answer

answered yesterday

Chris WilliamsChris Williams

411

New contributor

Chris Williams is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

#!/bin/bash

[[ -x $1 ]] || 
 echo >&2 "Usage $0##*/ <PROGRAM> [ARG1] ..."
 exit 1


set -o pipefail
program=$1
shift;
x=$($program "$@" | tail -1)

if [[ $? -eq 0 ]]; then
 echo "Pass: $x"
else
 echo "Fail: $?"
fi

share|improve this answer

answered yesterday

Chris WilliamsChris Williams

411

New contributor

Chris Williams is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

answered yesterday

Chris WilliamsChris Williams

411

New contributor

Chris Williams is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

answered yesterday

Chris WilliamsChris Williams

411

2

grade=$( echo 0; | tail -n 1 )

This would try to execute ./a.out and then add a line with a single 0 to its output if that program exited with a non-zero exit status or failed to execute at all. The 0 would be caught by tail -n 1 and placed in $grade.

If ./a.out executed correctly and terminated with a zero exit status, the echo would not be triggered.

Remove the redirection of standard error to /dev/null if you are interested in seeing diagnostic messages related to running ./a.out.

Change the 0 to "$?" to get the exit code instead. To be able to differentiate a number from an error, you may want to use NaN instead, or some error string.

share|improve this answer

edited yesterday

answered yesterday

Kusalananda♦Kusalananda

139k17259430

add a comment | 

2

grade=$( echo 0; | tail -n 1 )

This would try to execute ./a.out and then add a line with a single 0 to its output if that program exited with a non-zero exit status or failed to execute at all. The 0 would be caught by tail -n 1 and placed in $grade.

If ./a.out executed correctly and terminated with a zero exit status, the echo would not be triggered.

Remove the redirection of standard error to /dev/null if you are interested in seeing diagnostic messages related to running ./a.out.

Change the 0 to "$?" to get the exit code instead. To be able to differentiate a number from an error, you may want to use NaN instead, or some error string.

share|improve this answer

edited yesterday

answered yesterday

Kusalananda♦Kusalananda

139k17259430

add a comment | 

grade=$( echo 0; | tail -n 1 )

This would try to execute ./a.out and then add a line with a single 0 to its output if that program exited with a non-zero exit status or failed to execute at all. The 0 would be caught by tail -n 1 and placed in $grade.

If ./a.out executed correctly and terminated with a zero exit status, the echo would not be triggered.

Remove the redirection of standard error to /dev/null if you are interested in seeing diagnostic messages related to running ./a.out.

Change the 0 to "$?" to get the exit code instead. To be able to differentiate a number from an error, you may want to use NaN instead, or some error string.

share|improve this answer

edited yesterday

answered yesterday

Kusalananda♦Kusalananda

139k17259430

grade=$( echo 0; | tail -n 1 )

share|improve this answer

edited yesterday

answered yesterday

Kusalananda♦Kusalananda

139k17259430

answered yesterday

Kusalananda♦Kusalananda

139k17259430

answered yesterday

Kusalananda♦Kusalananda

139k17259430