In Bash, test
and [
are shell builtins.
The double bracket, which is a shell keyword, enables additional functionality. For example, you can use &&
and ||
instead of -a
and -o
and there's a regular expression matching operator =~
.
Also, in a simple test, double square brackets seem to evaluate quite a lot quicker than single ones.
$ time for ((i=0; i<10000000; i++)); do [[ "$i" = 1000 ]]; done
real 0m24.548s
user 0m24.337s
sys 0m0.036s
$ time for ((i=0; i<10000000; i++)); do [ "$i" = 1000 ]; done
real 0m33.478s
user 0m33.478s
sys 0m0.000s
The braces, in addition to delimiting a variable name are used for parameter expansion so you can do things like:
Truncate the contents of a variable
$ var="abcde"; echo ${var%d*}
abc
Make substitutions similar to sed
$ var="abcde"; echo ${var/de/12}
abc12
Use a default value
$ default="hello"; unset var; echo ${var:-$default}
hello
and several more
Also, brace expansions create lists of strings which are typically iterated over in loops:
$ echo f{oo,ee,a}d
food feed fad
$ mv error.log{,.OLD}
(error.log is renamed to error.log.OLD because the brace expression
expands to "mv error.log error.log.OLD")
$ for num in {000..2}; do echo "$num"; done
000
001
002
$ echo {00..8..2}
00 02 04 06 08
$ echo {D..T..4}
D H L P T
Note that the leading zero and increment features weren't available before Bash 4.
Thanks to gboffi for reminding me about brace expansions.
Double parentheses are used for arithmetic operations:
((a++))
((meaning = 42))
for ((i=0; i<10; i++))
echo $((a + b + (14 * c)))
and they enable you to omit the dollar signs on integer and array variables and include spaces around operators for readability.
Single brackets are also used for array indices:
array[4]="hello"
element=${array[index]}
Curly brace are required for (most/all?) array references on the right hand side.
ephemient's comment reminded me that parentheses are also used for subshells. And that they are used to create arrays.
array=(1 2 3)
echo ${array[1]}
2
You may have a bad connection in your xib.
I've had this error many times. While TechZen's answer is absolutely right in this case, another common cause is when you change the name of a IBOutlet property in your .h/.m which you've already connected up to File's Owner in the nib.
From your nib:
- Select the object in IB and go to the 'Connections Inspector'.
Under 'Referencing Outlets' make sure that your object isn't still connected to the old property name... if it is, click the small 'x' to delete the reference and build again.
Another common cause if you are using Storyboard, your UIButton might have more then one assignings (Solution is almost the same as for nib):
- Open your storyboard and right click the UIButton
You will see that there is more than one assign/ref to this button.
Remove one of the "Main..." greyed windows with the small "x":
Best Answer
By default,
*.sh
files are opened in a text editor (Xcode or TextEdit). To create a shell script that will execute in Terminal when you open it, name it with the “command” extension, e.g.,file.command
. By default, these are sent to Terminal, which will execute the file as a shell script.You will also need to ensure the file is executable, e.g.:
Without this, Terminal will refuse to execute it.
Note that the script does not have to begin with a
#!
prefix in this specific scenario, because Terminal specifically arranges to execute it with your default shell. (Of course, you can add a#!
line if you want to customize which shell is used or if you want to ensure that you can execute it from the command line while using a different shell.)Also note that Terminal executes the shell script without changing the working directory. You’ll need to begin your script with a
cd
command if you actually need it to run with a particular working directory.