Shell Special Characters

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 > are used for input/output redirection.

• | 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.