Control characters are codes or numbers in a character set that does not represent a written symbol. They are building blocks of Bash scripts which are like metacharacters having a special meaning. Below are the common separators and Control characters used in Bash scripts.
| Control Characters | Description |
|---|---|
| | pipe will take the first commands stdout as the second commands stdin. |
|| | OR if first command is false, it will take the second. |
|= | OR IS (mostly used in if statements) |
&& | AND if first command is true, it will execute the second one. |
! | NOT (mostly used in if and test statements), but as a shell-command it opens a shell to run the command (ex. ! echo foo) |
!= | NOT IS (mostly used in if statements) |
!$ | last commands last argument |
!! | repeat last command |
= | IS (mostly used in if statements) |
; | will separate 2 commands as if they were written on separate command lines |
;; | end of a case function in a case statement. |
$ | prefix to a variable like “$customVariable” |
$! | PID of the last child process. |
$$ | PID of the current process (PID is Process ID) |
$0 | Shows program that owns the current process. |
$1 | First argument supplied after the program/function on execution. |
$2 | Second argument supplied after the program/function on execution. ($3 etc.) |
$# | Shows the number of arguments. |
$? | Any argument (good to use in if statements) |
$- | current option flags |
$_ | Last argument/Command |
$* | All arguments |
$@ | All arguments |
# | commented line, anything on a line after “#” will be processed by the script |
{ | start braces (starts a function) |
} | end braces (ends a function) |
[ | start bracket (multiple-argument specifiers) |
] | end bracket (multiple-argument specifiers) |
@ | $@ is equivalent to “$1” “$2” etc. (all arguments) |
* | wild card (* can substitute any number of characters) |
? | wild card (? can substitute any single character) |
' | precise quote. (Will even include “‘s in the quote) |
. | dot will read and execute commands from a file, ( . .bashrc ) |
& | and. as suffix to executed file makes it go to the background(./program &) |
1> | stdout stream director (standard output) |
2> | stderr stream director (standard error output) |
% | job character, %1 = fg job 1, %2 = fg job 2, etc. |
>> | stream director append to a file |
<< | stdin stream director. (cat > file << EOF ; anything ; EOF) |
> | stream director that will start at the top of the file (in if statements < and > may be used as greater-then and lesser-then, as: if [ “$1” >= “2” ]) |
\ | back-slash, takes away any special meaning with a character |
>& | stream director to stream director, ie. echo “a” 1>/dev/null 2>&1 this directs 2> to the same place as 1> |