4.4.8 Shell special characters (metacharacters)
Normally, these characters have special meaning to the shell:
\ ' " ' < > | ; <Space> <Tab> <Newline> ( ) [ ] ? # $ ^ & * =
Here is the meaning of some of them:
\ ' " and ' are used for quoting and were described before.
< and > are used for input/output redirection and were described before.
| pipes the output of the command to the left of the pipe symbol "|" to the input of the command on the right of the pipe symbol.
; separates multiple commands written on a single line.
<Space> and <Tab> separate the command words.
<Newline> completes a command or set of commands.
( ) enclose command(s) to be launched in a separate shell (subshell). E.g. ( dir ).
{ } enclose a group of commands to be launched by the current shell. E.g. { dir }. It needs the spaces.
& causes the preceding command to execute in the background (i.e., asynchronously, as its own separate process) so that the next command does not wait for its completion.
* when a filename is expected, it matches any filename except those starting with a dot (or any part of a filename, except the initial dot).
? when a filename is expected, it matches any single character.
[ ] when a filename is expected, it maches any single character enclosed inside the pair of [ ].
&& is an "AND" connecting two commands. command1 && command2 will execute command2 only if command1 exits with the exit status 0 (no error). For example: cat file1 && cat file2 will display file2 only if displaying file1 succeeded.
|| is an "OR" connecting two commands. command1 || command2 will execute command2 only if command1 exits with the exit status of non-zero (with an error). For example: cat file1 || cat file2 will display file2 only if displaying file1 didn't succeed.
= assigns a value to a variable. Example. The command:
me=blahblah
assigns the value "blahblah" to the variable called "me". I can print the name of the variable using:
echo $me
$ preceeds the name of a variable to be expanded. The variables are either assigned using "=" or are one of the pre-defined variables (which cannot be assigned to):
$0 name of the shell or the shell script being executed.
$# number of the positional parameters to the command
$1 the value of the first positional parameter passed to the command. $2 is the second positional parameter passed to the command. etc. up to $9.
$* expands to all positional parameters passed to the command
$@ expands to all positional parameters passed to the command, but individually quoted when "$@" is used.
See man bash if you really need more.
* License
