5 ms·

> Ughh. Let's improve! I really don't understand what is the problem with that simple if statement to receive this kind of reaction. :shrug:

waynesonfire 6y ago
I agree. Also, not sure what's being improved. I also would have expected to run into a reference of double and single brackets used in if statements, e.g. [[ ]] and [ ] -- which I always seem to find confusing.
- prussian 6y ago
  [[ ]] is effectively an entirely different parser than the surrounding shell, similar to (( )). so you can do things like [[ 1 < 2 ]] && echo true # where < is now not redirect, like to (( 1 < 2 )) [[ "x" == 'y' || x == x ]] # note how || is not behaving the same as [ '' || 'y' ] `man 1 test` explains the more conventional `[ ]` where the bash man page would better explain the [[ case.
- stonesweep 6y ago
  The [ character is actually the program called "test"[1], so you can think of it as just running grep or sed or any other external binary/program, whereas the [[ replacement is shell-specific builtin; this blog is not using "bash-ism" but instead the external test operator from `coreutils` which generally would work from sh, zsh and whatever but on zsh, it doesn't work the same (kinda, it's more that the comparison EQUALS operator doesn't work well with [ but it's fine with [[). Here it is in pdksh: # [ "Z" == "z" ] && echo Bob # [ "Z" == "Z" ] && echo Bob Bob I chose pdksh on purpose - because _it_ also supports [[ (builtin like bash)[2], so while [[ is a "bash-ism" it's actually present in some/many other shells as well. However, the operational aspect is not identical between [ and [[, both in the bash and pdksh implementations and let's add zsh to show the same line of shell break: # [[ "Z" == "Z" && "A" == "A" ]] && echo Bob Bob # [ "Z" == "Z" && "A" == "A" ] && echo Bob pdksh: [: missing ] # [[ "Z" == "Z" && "A" == "A" ]] && echo Bob Bob # [ "Z" == "Z" && "A" == "A" ] && echo Bob -bash: [: missing `]' # [[ "Z" == "Z" && "A" == "A" ]] && echo Bob Bob # [ "Z" == "Z" && "A" == "A" ] && echo Bob zsh: = not found The [ operator requires that you use the bash/pdksh method to combine them with && "outside the brackets" in a more POSIX like use, like so: # if ([ "Z" == "Z" ] && [ "A" == "A" ]); then echo Bob; fi; Bob ...except on zsh, because (surprise!) zsh has decided to internalize the [ command rather than use the external one (which works OK); but it also has a problem with the "=" sign being used like this[3] with the test operator. zsh requires we then handle the == sign by quoting it (or unsetting a value internally): # if [ "Z" == "Z" ] && [ "A" == "A" ]; then echo Bob; fi; zsh: = not found # if [ "Z" '==' "Z" ] && [ "A" '==' "A" ]; then echo Bob; fi; Bob This last example is a subtle point - the use of [[ is actually _more_ compatible between bash/pdksh/zsh than the use of [ due to the way zsh handles the input which is different than bash/pdksh and even old school POSIX Bourne shell (/bin/sh). This got kinda long sorry, hope it helps. [1] there's actually a binary "[" and a binary "test" in the `coreutils` package on most systems, however I'm not sure why they have different binary file sizes to be honest [2] https://linux.die.net/man/1/pdksh https://linux.die.net/man/1/pdksh search "[[" [3] https://www.zsh.org/mla/users/2011/msg00161.html https://www.zsh.org/mla/users/2011/msg00161.html
pizza234 6y ago
The traditional format of Bash's if/then has `if` and `then` on separate lines: if command_or_condition then branch fi I suppose that to many, this looks wasteful and consequently ugly, so one looks for alternatives. I always inline the `then`, which requires a semicolon: if command_or_condition; then branch fi If I was forced to use the traditional format, I'd be definitely annoyed :)
- a1369209993 6y ago
  I use: if command_or_condition then branch more_branch fi Given the relationship between ; and newline, and the positions of required ;s in if/then/fi, I assumed that was intended.
- joana035 6y ago
  How swapping a '\n' char for a ';' is less "wasteful" or "ugly"?

waynesonfire 6y ago

I agree. Also, not sure what's being improved. I also would have expected to run into a reference of double and single brackets used in if statements, e.g. [[ ]] and [ ] -- which I always seem to find confusing.

prussian 6y ago
[[ ]] is effectively an entirely different parser than the surrounding shell, similar to (( )). so you can do things like [[ 1 < 2 ]] && echo true # where < is now not redirect, like to (( 1 < 2 )) [[ "x" == 'y' || x == x ]] # note how || is not behaving the same as [ '' || 'y' ] `man 1 test` explains the more conventional `[ ]` where the bash man page would better explain the [[ case.
stonesweep 6y ago
The [ character is actually the program called "test"[1], so you can think of it as just running grep or sed or any other external binary/program, whereas the [[ replacement is shell-specific builtin; this blog is not using "bash-ism" but instead the external test operator from `coreutils` which generally would work from sh, zsh and whatever but on zsh, it doesn't work the same (kinda, it's more that the comparison EQUALS operator doesn't work well with [ but it's fine with [[). Here it is in pdksh: # [ "Z" == "z" ] && echo Bob # [ "Z" == "Z" ] && echo Bob Bob I chose pdksh on purpose - because _it_ also supports [[ (builtin like bash)[2], so while [[ is a "bash-ism" it's actually present in some/many other shells as well. However, the operational aspect is not identical between [ and [[, both in the bash and pdksh implementations and let's add zsh to show the same line of shell break: # [[ "Z" == "Z" && "A" == "A" ]] && echo Bob Bob # [ "Z" == "Z" && "A" == "A" ] && echo Bob pdksh: [: missing ] # [[ "Z" == "Z" && "A" == "A" ]] && echo Bob Bob # [ "Z" == "Z" && "A" == "A" ] && echo Bob -bash: [: missing `]' # [[ "Z" == "Z" && "A" == "A" ]] && echo Bob Bob # [ "Z" == "Z" && "A" == "A" ] && echo Bob zsh: = not found The [ operator requires that you use the bash/pdksh method to combine them with && "outside the brackets" in a more POSIX like use, like so: # if ([ "Z" == "Z" ] && [ "A" == "A" ]); then echo Bob; fi; Bob ...except on zsh, because (surprise!) zsh has decided to internalize the [ command rather than use the external one (which works OK); but it also has a problem with the "=" sign being used like this[3] with the test operator. zsh requires we then handle the == sign by quoting it (or unsetting a value internally): # if [ "Z" == "Z" ] && [ "A" == "A" ]; then echo Bob; fi; zsh: = not found # if [ "Z" '==' "Z" ] && [ "A" '==' "A" ]; then echo Bob; fi; Bob This last example is a subtle point - the use of [[ is actually _more_ compatible between bash/pdksh/zsh than the use of [ due to the way zsh handles the input which is different than bash/pdksh and even old school POSIX Bourne shell (/bin/sh). This got kinda long sorry, hope it helps. [1] there's actually a binary "[" and a binary "test" in the `coreutils` package on most systems, however I'm not sure why they have different binary file sizes to be honest [2] https://linux.die.net/man/1/pdksh https://linux.die.net/man/1/pdksh search "[[" [3] https://www.zsh.org/mla/users/2011/msg00161.html https://www.zsh.org/mla/users/2011/msg00161.html

pizza234 6y ago

The traditional format of Bash's if/then has `if` and `then` on separate lines: if command_or_condition then branch fi I suppose that to many, this looks wasteful and consequently ugly, so one looks for alternatives. I always inline the `then`, which requires a semicolon: if command_or_condition; then branch fi If I was forced to use the traditional format, I'd be definitely annoyed :)

a1369209993 6y ago
I use: if command_or_condition then branch more_branch fi Given the relationship between ; and newline, and the positions of required ;s in if/then/fi, I assumed that was intended.
joana035 6y ago
How swapping a '\n' char for a ';' is less "wasteful" or "ugly"?