Thursday, June 27, 2013

Unix: Choosing the easy path

Wise men say that you should never choose the easy path but, instead, live life fully. But when it comes to moving around the Unix file system, easy is good. And bash's builtin shopt command can make maneuvering even the most complicated file system paths easier.

Some of us have spent decades moving around our Unix file systems with the cd command and maybe even using file name completion so that we don't have to type very letter in every directory name. Even so, there may be some bash options that you might not be aware of and the shopt command has some options that might surprise you.
The shopt built-in provides numerous options for changing optional shell behavior. To view these options, just type shopt on the command line. Oh, and don't mentally parse that as "shop t", but "sh opt" as in "shell options". That should make it easier to remember.
Depending on the version of bash that you are using, you will see a variable list of options that you can set and unset with shopt -s (set) and shopt -u (unset) commands. Some of these can be used to change bash's behavior when you issue cd commands.
$ bash -version
GNU bash, version 3.2.25(1)-release (i386-redhat-linux-gnu)
Copyright (C) 2005 Free Software Foundation, Inc.
Typing shopt on a line by itself will list the current settings.
$ shopt
cdable_vars     off
cdspell         off
checkhash       off
checkwinsize    on
cmdhist         on
dotglob         off
execfail        off
expand_aliases  on
extdebug        off
extglob         off
extquote        on
failglob        off
force_fignore   on
gnu_errfmt      off
histappend      off
histreedit      off
histverify      off
hostcomplete    on
huponexit       off
interactive_comments    on
lithist         off
login_shell     on
mailwarn        off
no_empty_cmd_completion off
nocaseglob      off
nocasematch     off
nullglob        off
progcomp        on
promptvars      on
restricted_shell        off
shift_verbose   off
sourcepath      on
xpg_echo        off
Notice that this list also tells you if the option in question is enabled (on) or disabled (off).
If autocd is in your list (bash 4.1 and newer), you can use this option to make it possible to cd to a directory without typing the cd command. Instead, you just type a directory name. The shell then sees that the "command" you entered is not a command, but the name of a directory and moves you there.
$ Lab12
$ pwd
A similar option, available in older (pre 4.1) versions of bash is cdable_vars. This is easier to parse than "shopt". Think of it as "cd-able variables". In other words, if a variable contains the name of a directory, you can move into the directory by using the variable with the cd command.
$ shopt -s cdable_vars
$ lab=Lab12
$ cd $lab
$ pwd
By the way, this doesn't work if your variable is set to something like ~/Lab12 or ~jdoe/Lab12. The tilde isn't interpreted as referring to your home directory.

  But $HOME/Lab12 works as expected.
The cdable_vars option essentially provides a way for you to set up shortcuts to directories without having to create symbolic links.
Another useful option that helps with directory changes is cdspell. This one allows the shell to compensate for minor typos in the paths you type. Meant to go to /tmp, but you typed cd /tpm, no problem!
$ shopt -s cdspell
$ cd /tpm
The force_fignore, set by default, uses whatever value you have assigned to $FIGNORE to prevent path completion using that name. If, for example, you have two directories -- ignore.yes and -- and you set FIGNORE to "yes", you will be able to use path completion for, but the shell will act as if ignore.yes doesn't exist -- for path completion anyway. You have to type each letter to cd into it.
$ FIGNORE="yes"
$ mkdir ignore.yes
$ mkdir
$ cd ignor[TAB]
$ pwd
The force_fignore variable can be used when there are particular directories that you don't want your uses to accidentally enter when using path name completion. The FIGNORE variable can be set to a complete directory name or, as in the example, to a file extension or right substring of the file name.
Choose your own path, but let shopt make it easier.

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