[SOLVED] Bash `if` statement versus conditional execution logic

Issue

This works, as it should

$ bash -cx 'echo $BASH_VERSION; set -euo pipefail; if true; then (echo good reaction, bad result >&2; exit 1); else echo bad reaction; fi'
+ echo '5.2.21(2)-release'
5.2.21(2)-release
+ set -euo pipefail
+ true
+ echo good reaction, bad result
good reaction, bad result
+ exit 1

But a similar snippet using conditionals instead doesn’t. I understand why it doesn’t (although I’m not sure why the errfail setting doesn’t stop execution), but would like to know if there’s a way to make conditional execution work like an if statement

$ bash -cx 'echo $BASH_VERSION; set -euo pipefail; true && ( (echo good reaction, bad result >&2; exit 1); ) || { echo bad reaction; }'
+ echo '5.2.21(2)-release'
5.2.21(2)-release
+ set -euo pipefail
+ true
+ echo good reaction, bad result
good reaction, bad result
+ exit 1
+ echo bad reaction
bad reaction

Solution

Assuming you're going to take the advice to not use set -euo pipefail then I think the question is really just how to use conditionals to mimic:

if command1; then command2; else command3; fi

so that'd be the following:

command1 && { command2; es="$?"; } || { command3; es="$?"; }; (exit "$es")

That works because even if command2 fails, the subsequent es="$?" resets the exit status to success so that command3 doesn't get executed and then the (exit "$es") at the end spawns a subshell just to get an exit status that's the contents of es but, as others have mentioned, it's not worth the effort.

The above assumes you can guarantee that assigning to es can't fail (e.g. as it could if es was readonly)

---

Here's a test script to show the equivalence of the above structure to the if...then...else code, including verifying that for each line:

1. The same commands execute.
2. Variables being set at each step are retained in the current shell once the line ends.
3. We have the correct exit status in the current shell at the end of the line.
4. The current shell continues to execute after the line ends.

$ cat tst.sh
#!/usr/bin/env bash

# run each line in a subshell just so variables x, y, and z, all start as null for testing

( x=T; if true;  then y=T; true;  else z=T; true;  fi; echo "$x?$y:$z -> $?" )
( x=T; if true;  then y=T; true;  else z=F; false; fi; echo "$x?$y:$z -> $?" )
( x=T; if true;  then y=F; false; else z=T; true;  fi; echo "$x?$y:$z -> $?" )
( x=T; if true;  then y=F; false; else z=F; false; fi; echo "$x?$y:$z -> $?" )
( x=F; if false; then y=T; true;  else z=T; true;  fi; echo "$x?$y:$z -> $?" )
( x=F; if false; then y=T; true;  else z=F; false; fi; echo "$x?$y:$z -> $?" )
( x=F; if false; then y=F; false; else z=T; true;  fi; echo "$x?$y:$z -> $?" )
( x=F; if false; then y=F; false; else z=F; false; fi; echo "$x?$y:$z -> $?" )

printf -- '--------\n'

( { x=T; true;  } && { y=T; true;  es=$?; } || { z=T; true;  es=$?; }; (exit "$es"); echo "$x?$y:$z -> $?" )
( { x=T; true;  } && { y=T; true;  es=$?; } || { z=F; false; es=$?; }; (exit "$es"); echo "$x?$y:$z -> $?" )
( { x=T; true;  } && { y=F; false; es=$?; } || { z=T; true;  es=$?; }; (exit "$es"); echo "$x?$y:$z -> $?" )
( { x=T; true;  } && { y=F; false; es=$?; } || { z=F; false; es=$?; }; (exit "$es"); echo "$x?$y:$z -> $?" )
( { x=F; false; } && { y=T; true;  es=$?; } || { z=T; true;  es=$?; }; (exit "$es"); echo "$x?$y:$z -> $?" )
( { x=F; false; } && { y=T; true;  es=$?; } || { z=F; false; es=$?; }; (exit "$es"); echo "$x?$y:$z -> $?" )
( { x=F; false; } && { y=F; false; es=$?; } || { z=T; true;  es=$?; }; (exit "$es"); echo "$x?$y:$z -> $?" )
( { x=F; false; } && { y=F; false; es=$?; } || { z=F; false; es=$?; }; (exit "$es"); echo "$x?$y:$z -> $?" )

$ ./tst.sh
T?T: -> 0
T?T: -> 0
T?F: -> 1
T?F: -> 1
F?:T -> 0
F?:F -> 1
F?:T -> 0
F?:F -> 1
--------
T?T: -> 0
T?T: -> 0
T?F: -> 1
T?F: -> 1
F?:T -> 0
F?:F -> 1
F?:T -> 0
F?:F -> 1

Answered By - Ed Morton

Answer Checked By - Katrina (WPSolving Volunteer)