Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
echo "Part 2 of script."
echo $a # Value of $a stays at 1.
[6] This allows some cute tricks.
#!/bin/rm
# Self−deleting script.
# Nothing much seems to happen when you run this... except that the file disappears.
WHATEVER=65
echo "This line will never print (betcha!)."
exit $WHATEVER # Doesn't matter. The script will not exit here.
Also, try starting a README file with a #!/bin/more, and making it executable. The result is a
self−listing documentation file.
[7] Portable Operating System Interface, an attempt to standardize UNIX−like OSes.
[8] Caution: invoking a Bash script by sh scriptname turns off Bash−specific extensions, and the
script may therefore fail to execute.
[9] A script needs read, as well as execute permission for it to run, since the shell needs to be able to read
it.
[10] Why not simply invoke the script with scriptname? If the directory you are in ($PWD) is where
scriptname is located, why doesn't this work? This fails because, for security reasons, the current
directory, "." is not included in a user's $PATH. It is therefore necessary to explicitly invoke the script
in the current directory with a ./scriptname.
[11] The shell does the brace expansion. The command itself acts upon the result of the expansion.
[12] Exception: a code block in braces as part of a pipe may be run as a subshell.
ls | { read firstline; read secondline; }
# Error. The code block in braces runs as a subshell,
# so the output of "ls" cannot be passed to variables within the block.
echo "First line is $firstline; second line is $secondline" # Will not work.
# Thanks, S.C.
[13] The process calling the script sets the $0 parameter. By convention, this parameter is the name of the
script. See the manpage for execv.
[14] Encapsulating "!" within double quotes gives an error when used from the command line. Apparently
this is interpreted as a history command. Within a script, though, this problem does not occur.
Of more concern is the inconsistent behavior of "\" within double quotes.
bash$ echo hello\!
hello!
bash$ echo "hello\!"
hello\!
Appendix J. Copyright 441