Changing Directories

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Always check the error status of chdir, to avoid running commands in the wrong directory. Alternatively, use fully qualified paths to obviate the need for chdir(2).

#!/bin/sh
cd $nosuchdir || exit 1
rsync elsewhere:/foo .

Without || exit 1 to abort the script should cd fail, the subsequent rsync command could move files to the wrong location or fill up the wrong partition.

• Redirection.

Redirection can take place (almost) anywhere, not just at the end.

$ echo a b >c
$ echo >c a b
$ >c echo a b

Placing the filename at the beginning allows easier editing of the search term at the end of the command.

$ </var/log/messages grep foo
$ </var/log/messages grep bar
$ </var/log/messages grep user1

• Pipe to command arguments.

I use xargs(1) frequently to convert output from something (file, or another program) to arguments to another command. For instance, to commit only modified files in a cvs sandbox where there may be conflicted, new, or other troublesome files mixed in, use the following.

$ cvs up | perl -ne 'print if s/M //' | xargs cvs ci

Depending on the editor, one can use concept above to open certain files for editing, for example, files in a cvs sandbox that have conflicts.

$ cvs up | perl -ne 'print if s/C //' | xargs vi
ex/vi: Vi's standard input and output must be a terminal
 
$ cvs up | perl -ne 'print if s/C //' | xargs emacs
emacs: standard input is not a tty
 
$ cvs up | perl -ne 'print if s/C //' | xargs bbedit

• Stubborn Files

Dealing with files that have odd characters in their names can often be a chore on Unix, as one cannot type in the names in question. One could use a graphical file manager tool, but I find those cumbersome, ill suited to dealing with large numbers of files, and usually not installed on server systems.

To simply delete the bad filenames, there are a few options.

• Qualify the path.

Files that being with a hyphen (such as a file -rf) will trigger option processing, as the shell is very stupid. These can be avoided by either disabling option processing, or prefixing a directory name to the file path.

$ ls
-rf
$ rm -rf
$ ls
-rf
$ rm *
$ ls
-rf
$ rm -- -rf
$ ls
$ touch ./-rf
$ ls
-rf
$ rm ./-rf
$ ls
$

The -- argument only works on systems whose getopt(3) library supports the syntax. On other systems, or for portability, the qualified path option must be used.

• Find inode and delete by that.

Each file on a Unix filesystem has a inode number associated with it; knowing the inode number of the bad file allows us to search for and delete it.

$ ls -i *
615383 foo
$ find . -inum 615383 -exec rm {} \;