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# Bash

### Factoid:
The extremely serious ShellShock bug was present in Bash since 1989, and remained undiscovered for a quarter of a century. Much of the joy of writing Bash is coming to grips with its many idiosyncracies and inconsistencies.

### Length 1 snippet:

[
An alias for the ```test``` builtin, allowing code of the format ```if [ a == b ]; then``` - in reality ```[``` is a standalone command, not a syntactical element, and ```]``` is purely decorative (although required by [, its requirement is arbitrary and you can do away with it by using ```alias [=test```).

### Length 2 snippet:
 ||
Like logical ```or``` in most languages, but for processes. Will execute the command after the ```||``` only if the command before it returns non-zero.

### Length 3 snippet:
 x=y
Assignment. Nice and predictable... but unlike most other languages, extra spaces aren't allowed. Which is kind of funny because you can stick extra spaces pretty much everywhere else between things in bash, just not around the ```=```.

### Length 4 snippet:
 $IFS
**Internal Field Separator** - this variable affects how Bash splits data for many built-in actions, such as iterating in for loops and populating arrays from strings. Used correctly it can be very powerful; but more often it's the cause of subtle and unpredictable bugs.

### Length 5 snippet:
 ${x^}
Substitute the string in x, but with the first character capitalised! Such a frequently used feature that it has its own dedicated piece of language syntax.

### Length 6 snippet:
 x=($y)
Fill an array, x, from a string or list of elements y, splitting on whatever the IFS is currently set to - by default, whitespace. A very useful feature for advanced bash programming.

### Length 7 snippet:
 ${x[y]}
Arrays! But wait, what's that... y is a string, not a numerical index? Yes indeed, Bash supports associative arrays! Many people don't know this. You just need to ```declare -A x``` first.

### Length 8 snippet:
 ${x##*,}
Substitute x, everything up until the last ```,``` character (or whatever you specify). Useful to get the last field of a csv - this is something you can't so easily do with ```cut```, which only counts fields from the left. % and %% allows the same to cut from the right; % and # were chosen for their location on the US keyboard so it would be clear which means left and which means right, but that doesn't hold much value for everyone not using a US keyboard :)

### Length 9 snippet:
 [ a = b ]
In most other languages, a single equals in a comparison operation would produce unintended behaviour in the form of an assignment. Not in Bash, though. Just don't omit any of the spaces, whatever you do!

### Length 10 snippet:
 if [ a=b ]
This is what happens if you forget about the mandatory spaces. Will not throw an error. **Will always return true** - even if ```a``` and ```b``` are variables that are unset, or whatever they're set to, doesn't matter - it'll always return true. Think about code like ```if [ "$password"="$correctpass" ]``` to see the fun potential of this "feature".

### Length 11 snippet:
 x=${y//a/b}
Regex-style substring replacement! Set x to the value of y but with every instance of a replaced with b.

### Length 12 snippet:
 [[:upper:]]*
Pattern matching - you aren't limited to just using the * wildcard in the shell, you can use any POSIX standard match such as ```alnum```, ```alpha```, ```digit``` etc.

### Length 13 snippet:
 function x(){
A bit of C syntax has mysteriously crept in! One of many completely different uses for curly braces, and another example of optional decorative elements to make Bash look more like other languages - either ```()``` or ```function``` can be omitted here (but not both). Also more fun with inconsistent spaces - a space after the ```{``` is mandatory, but **not** before the closing ```}```, as in ```function x { y;}```

### Length 14 snippet:
 echo {Z..A..3}
Yet another totally unrelated use of curly braces. Expands a range with a specified step. In this case, will produce every 3rd letter from Z to A. Useful for generating sequences without using seq, but cannot be used with variables, so has limited functionality.

### Length 15 snippet:
 echo {a,b,c,d}x
Another similar but not identical use for curly braces in sequence generation; prints ```ax bx cx dx```, and is useful for generating a list of strings from a sequence or list in a single command. Again however, limited in usefulness as it can't be used with variables inside the braces.

If this is an answer to a challenge…

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\$\begingroup\$ Actually, ] is not purely decorative. [ will refuse to function if its last argument isn't ]. \$\endgroup\$

FUZxxl
– FUZxxl

2015年01月26日 21:36:41 +00:00
Commented Jan 26, 2015 at 21:36
\$\begingroup\$ Yes, but it serves no purpose other than cosmetics; and if you substitute [ with its other form test, then the ] can be omitted without changing anything else in the call - I'm simply making the point that it's not actual bash syntax, just visual sugar. \$\endgroup\$

Riot
– Riot

2015年01月26日 21:39:39 +00:00
Commented Jan 26, 2015 at 21:39
\$\begingroup\$ You're right in that it's not bash syntax, but the trailing ] is [ syntax and you have to provide it just like how you have to terminate a statement in C with a semicolon. \$\endgroup\$

FUZxxl
– FUZxxl

2015年01月26日 21:40:30 +00:00
Commented Jan 26, 2015 at 21:40
2

\$\begingroup\$ Regarding the function x(){ syntax: You can drop the parens, as you say, but you can also just drop the function part. In fact, that's how POSIX shell defines functions, so it's more portable that way. You could define a full function in 13 characters. For example: x(){ startx;} \$\endgroup\$

kojiro
– kojiro

2015年01月29日 02:02:54 +00:00
Commented Jan 29, 2015 at 2:02
1

\$\begingroup\$ If you run alias [=test then while [ 1 = 1 works, [ 1 = 1 ] will complain bash: test: too many arguments. I don't suggest breaking things that might load later, which means you'd want something like test2(){ local x=;for i in "$@";do test -z "$x"&&x=1&&set --;test "$i" != \]&&set -- "$@" "$i";done;test "$@";};alias [=test2 — though this seems pedantic and against the spirit of the language. I do not recommend it. \$\endgroup\$

Adam Katz
– Adam Katz

2021年10月06日 17:50:46 +00:00
Commented Oct 6, 2021 at 17:50

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