6 ms·

The Bourne shell lets you set variables in if expressions

67 points by justaj 5y ago

1337shadow 5y ago
Like many other languages such as PHP, Python, JS, Go ...
- wrnr 5y ago
  There is legit use of this in the wild, my favourite example is from prosemirror: https://github.com/ProseMirror/prosemirror-example-setup/blob/master/src/keymap.js#L67 https://github.com/ProseMirror/prosemirror-example-setup/blo...
rwmj 5y ago
As a regular shell user I find this quite surprising. The article omits the important point that the shell variable persists after the conditional expression: $ foo= ; if foo=bar; then echo $foo ; fi ; echo $foo bar bar However this work as expected. foo is only set as an environment variable for the duration of the command (true) in the condition. $ foo= ; if foo=bar true; then echo $foo ; fi ; echo $foo (prints two blank lines) (My examples above use bash rather than the plain Bourne shell). Now I'm wondering if the shell special-cases this. Edit: No it doesn't. It just executes the command as normal using execute_command in the same context as the "if" statement, so any assignments take effect in the script, not in any sub-context. https://git.savannah.gnu.org/cgit/bash.git/tree/execute_cmd.c#n3686 https://git.savannah.gnu.org/cgit/bash.git/tree/execute_cmd....
twooster 5y ago
You should almost always be running Bash in `-e` (exit-on-error) mode. This necessitates precisely the construct mentioned in this article. For example: set -e var="$( false )" if [ $? -eq 0 ] ; then echo Ok: "$var" else echo Not ok: $? fi If you run this program, neither "Ok" or "Not ok" will be echoed, because the program will exit with an error on the `var=` line. (Not to mention the $? on the not-ok line won't work because it will be the exit code of the `[` test command in the conditional, not the exit code of the captured subshell command). Instead the following will work: set -e if var="$( false )" ; then echo Ok: "$var" else echo Not ok: $? fi Note that this will _not_ work: if ! var="$( false )"; then echo Not ok: $? fi Your output will be "Not ok: 0". This is because negation impacts the exit code of the previous command.
- Dr_Emann 5y ago
  For simple cases, I'll do that, but for longer commands, I've taken to doing rc=0 long command || rc = $? if (( rc == 0 ))...
- matvore 5y ago
  `set -e` has a couple of surprising corner cases and in the details is pretty hard to understand. The documentation in `man bash` for the `set -e` flag is 28 lines in my terminal, and the other flags are 2 or 3 lines. One such corner case is in pipelines. Only the last command in a pipeline can cause the script to terminate. Another corner case is `foo && bar`, often used as an abbreviated `if`, will not exit when `foo` fails. It is not a significant task to just add `|| exit $?` after any command whose failure should cause an abort.
  - mgerdts 5y ago
    My scripts tend to start with set -euo pipefail This addresses the pipeline case you mention and also notices use of unitialized variables.
    - shawnz 5y ago
      Unfortunately Ubuntu's new bash replacement "dash" doesn't support "-o pipefail" and will error if it's present
      - throw0101a 5y ago
        dash is not a bash "replacement". dash is an implementation of the Bourne/POSIX shell. bash, when called as sh, also implements the Bourne shell as well (but badly, because it leaks bash-isms), but when bash is called as bash it is a super-set of Bourne. * https://en.wikipedia.org/wiki/Almquist_shell#dash https://en.wikipedia.org/wiki/Almquist_shell#dash * https://en.wikipedia.org/wiki/Bourne_shell https://en.wikipedia.org/wiki/Bourne_shell * https://en.wikipedia.org/wiki/Comparison_of_command_shells https://en.wikipedia.org/wiki/Comparison_of_command_shells If you want to use super-set functionality adjust your shebang accordingly.
erdos4d 5y ago
Bash rarely impresses me but this is a feature I love about it. I write Python normally, I really wish it had this.
- frumiousirc 5y ago
  https://www.python.org/dev/peps/pep-0572/ https://www.python.org/dev/peps/pep-0572/
  - nijave 5y ago
    tldr; `:=` assignment does this in Python 3.8+ while data := something_that_may_be_false(): # do something with data
- matvore 5y ago
  This idea doesn't apply so cleanly in Python. The meaning of this: if x=$(foo); then ... fi is that the stdout of running `foo` replaces the contents of $x and the `if` evaluates the exit code of `foo`. Python doesn't see stdout as some kind of first-class language construct. I also don't think this could ever work on Windows if you expect `foo` to be a simple function that has access to the interpreter state.
posix_me_less 5y ago
You don't need quotes in res="$(... whatever ...)" This is sufficient: res=$(... whatever ...)
- adolph 5y ago
  It depends on what is being returned and how the result is used. In my inexpert experience cat $(find -name <some file with spaces in the name>) will fail.
  - jolmg 5y ago
    They mean specifically on assignment. Though, when in doubt, quoting never hurts.
  - nijave 5y ago
    It's specifically an edge case with variable assignment. If it were export val="$( ... )" you'd need the quotes again. You don't need quotes in plain variable assignments and afaik it goes back to how space splitting works in Bash (whether or not you're in a "splitting context" or whatever the docs call it). There's a semi-related set of rules for when * gets expanded and when it's passed through to the underlying command as a literal '*' character
    - jolmg 5y ago
      > If it were `export val="$( ... )"` you'd need the quotes again. No. That's also recognized as an assignment by bash[1]: > PARAMETERS... A variable may be assigned to by a statement of the form `name=[value]`... Word splitting is not performed... Assignment statements may also appear as arguments to the alias, declare, typeset, export, readonly, and local builtin commands (declaration commands). [1] https://www.gnu.org/software/bash/manual/bash.html#Shell-Parameters https://www.gnu.org/software/bash/manual/bash.html#Shell-Par...
ipnon 5y ago
I use this to great affect in Elixir to name magic numbers as one off arguments. Process.sleep(_minute = 1000 * 60)

67 points by justaj 5y ago

1337shadow 5y ago

Like many other languages such as PHP, Python, JS, Go ...

wrnr 5y ago
There is legit use of this in the wild, my favourite example is from prosemirror: https://github.com/ProseMirror/prosemirror-example-setup/blob/master/src/keymap.js#L67 https://github.com/ProseMirror/prosemirror-example-setup/blo...

rwmj 5y ago

As a regular shell user I find this quite surprising. The article omits the important point that the shell variable persists after the conditional expression: $ foo= ; if foo=bar; then echo $foo ; fi ; echo $foo bar bar However this work as expected. foo is only set as an environment variable for the duration of the command (true) in the condition. $ foo= ; if foo=bar true; then echo $foo ; fi ; echo $foo (prints two blank lines) (My examples above use bash rather than the plain Bourne shell). Now I'm wondering if the shell special-cases this. Edit: No it doesn't. It just executes the command as normal using execute_command in the same context as the "if" statement, so any assignments take effect in the script, not in any sub-context. https://git.savannah.gnu.org/cgit/bash.git/tree/execute_cmd.c#n3686 https://git.savannah.gnu.org/cgit/bash.git/tree/execute_cmd....

twooster 5y ago

You should almost always be running Bash in `-e` (exit-on-error) mode. This necessitates precisely the construct mentioned in this article. For example: set -e var="$( false )" if [ $? -eq 0 ] ; then echo Ok: "$var" else echo Not ok: $? fi If you run this program, neither "Ok" or "Not ok" will be echoed, because the program will exit with an error on the `var=` line. (Not to mention the $? on the not-ok line won't work because it will be the exit code of the `[` test command in the conditional, not the exit code of the captured subshell command). Instead the following will work: set -e if var="$( false )" ; then echo Ok: "$var" else echo Not ok: $? fi Note that this will _not_ work: if ! var="$( false )"; then echo Not ok: $? fi Your output will be "Not ok: 0". This is because negation impacts the exit code of the previous command.

Dr_Emann 5y ago
For simple cases, I'll do that, but for longer commands, I've taken to doing rc=0 long command || rc = $? if (( rc == 0 ))...
matvore 5y ago
`set -e` has a couple of surprising corner cases and in the details is pretty hard to understand. The documentation in `man bash` for the `set -e` flag is 28 lines in my terminal, and the other flags are 2 or 3 lines. One such corner case is in pipelines. Only the last command in a pipeline can cause the script to terminate. Another corner case is `foo && bar`, often used as an abbreviated `if`, will not exit when `foo` fails. It is not a significant task to just add `|| exit $?` after any command whose failure should cause an abort.
- mgerdts 5y ago
  My scripts tend to start with set -euo pipefail This addresses the pipeline case you mention and also notices use of unitialized variables.
  - shawnz 5y ago
    Unfortunately Ubuntu's new bash replacement "dash" doesn't support "-o pipefail" and will error if it's present
    - throw0101a 5y ago
      dash is not a bash "replacement". dash is an implementation of the Bourne/POSIX shell. bash, when called as sh, also implements the Bourne shell as well (but badly, because it leaks bash-isms), but when bash is called as bash it is a super-set of Bourne. * https://en.wikipedia.org/wiki/Almquist_shell#dash https://en.wikipedia.org/wiki/Almquist_shell#dash * https://en.wikipedia.org/wiki/Bourne_shell https://en.wikipedia.org/wiki/Bourne_shell * https://en.wikipedia.org/wiki/Comparison_of_command_shells https://en.wikipedia.org/wiki/Comparison_of_command_shells If you want to use super-set functionality adjust your shebang accordingly.

erdos4d 5y ago

Bash rarely impresses me but this is a feature I love about it. I write Python normally, I really wish it had this.

frumiousirc 5y ago
https://www.python.org/dev/peps/pep-0572/ https://www.python.org/dev/peps/pep-0572/
- nijave 5y ago
  tldr; `:=` assignment does this in Python 3.8+ while data := something_that_may_be_false(): # do something with data
matvore 5y ago
This idea doesn't apply so cleanly in Python. The meaning of this: if x=$(foo); then ... fi is that the stdout of running `foo` replaces the contents of $x and the `if` evaluates the exit code of `foo`. Python doesn't see stdout as some kind of first-class language construct. I also don't think this could ever work on Windows if you expect `foo` to be a simple function that has access to the interpreter state.

posix_me_less 5y ago

You don't need quotes in res="$(... whatever ...)" This is sufficient: res=$(... whatever ...)

adolph 5y ago
It depends on what is being returned and how the result is used. In my inexpert experience cat $(find -name <some file with spaces in the name>) will fail.
- jolmg 5y ago
  They mean specifically on assignment. Though, when in doubt, quoting never hurts.
- nijave 5y ago
  It's specifically an edge case with variable assignment. If it were export val="$( ... )" you'd need the quotes again. You don't need quotes in plain variable assignments and afaik it goes back to how space splitting works in Bash (whether or not you're in a "splitting context" or whatever the docs call it). There's a semi-related set of rules for when * gets expanded and when it's passed through to the underlying command as a literal '*' character
  - jolmg 5y ago
    > If it were `export val="$( ... )"` you'd need the quotes again. No. That's also recognized as an assignment by bash[1]: > PARAMETERS... A variable may be assigned to by a statement of the form `name=[value]`... Word splitting is not performed... Assignment statements may also appear as arguments to the alias, declare, typeset, export, readonly, and local builtin commands (declaration commands). [1] https://www.gnu.org/software/bash/manual/bash.html#Shell-Parameters https://www.gnu.org/software/bash/manual/bash.html#Shell-Par...

ipnon 5y ago

I use this to great affect in Elixir to name magic numbers as one off arguments. Process.sleep(_minute = 1000 * 60)