: ${HOSTNAME?} ${USER?} ${MAIL?}
#Prints error message if one or more of essential environmental variables not set.
Variable expansion / substring replacement.
In combination with the > redirection operator, truncates a file to zero length, without changing its
permissions. If the file did not previously exist, creates it.
: > data.xxx # File "data.xxx" now empty.
# Same effect as cat /dev/null >data.xxx
# However, this does not fork a new process, since ":" is a builtin.
See also Example 12−14.
In combination with the >> redirection operator, has no effect on a pre−existing target file (: >>
target_file). If the file did not previously exist, creates it.
This applies to regular files, not pipes, symlinks, and certain special files.
May be used to begin a comment line, although this is not recommended. Using # for a comment
turns off error checking for the remainder of that line, so almost anything may be appear in a
comment. However, this is not the case with :.
: This is a comment that generates an error, ( if [ $x −eq 3] ).
The ":" also serves as a field separator, in /etc/passwd, and in the $PATH variable.
bash$ echo $PATH
/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/sbin:/usr/sbin:/usr/games
!
reverse (or negate) the sense of a test or exit status. The ! operator inverts the exit status of the
command to which it is applied (see Example 6−2). It also inverts the meaning of a test operator. This
can, for example, change the sense of "equal" ( = ) to "not−equal" ( != ). The ! operator is a Bash
keyword.
In a different context, the ! also appears in indirect variable references.
In yet another context, from the command line, the ! invokes the Bash history mechanism (see
Appendix I). Note that within a script, the history mechanism is disabled.
*
wild card [asterisk]. The * character serves as a "wild card" for filename expansion in globbing. By
itself, it matches every filename in a given directory.
bash$ echo *
abs−book.sgml add−drive.sh agram.sh alias.sh
The * also represents any number (or zero) characters in a regular expression.
*
arithmetic operator. In the context of arithmetic operations, the * denotes multiplication.
A double asterisk, **, is the exponentiation operator.
?
test operator. Within certain expressions, the ? indicates a test for a condition.
Advanced Bash−Scripting Guide
Chapter 3. Special Characters 11
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