Bash: Difference between revisions

== For Loops ==

The bash one-liner for doing a for loop looks something like this:

for FILE in $(ls); do [COMMAND]; done

Here is a real example that will lowercase the names of all files in the current directory:

for FILE in $(ls); do mv $FILE $(echo $FILE | tr [A-Z] [a-z]); done

To do simple range looping, use the '''seq''' command:

for i in `seq 1 10`; do echo "$i, "; done

== Arrays ==

Simple arrays  work like the following:

#!/bin/bash

echo "we have ${#vegetables[@]} vegetables available"

printf "%s\n" "${vegetables[@]}"

== If construct ==

The <code>then</code> can go on the same line as the <code>if</code> as long as you use a semi-colon to terminate the if clause.  Alternately, you can put the <code>then</code> on it's own line

if EXPR; then

   # do stuff

fi

is equivalent to

if EXPR

then

   # do stuff

fi

Adding an <code>else</code> clause

if EXPR; then

   # do stuff

   # do other stuff

fi

Adding multiple <code>else</code> clauses with <code>elif; then</code>

if EXPR; then

   # do stuff

   # final else

fi

Note: sometimes you want to comment out a section of an if/else block, or maybe it does nothing at all.  In this case, you'll get an error.  To avoid the error, you can use the [http://www.gnu.org/software/bash/manual/html_node/Bourne-Shell-Builtins.html bash built-in] <code>:</code> (colon command)

Do nothing beyond expanding arguments and performing redirections. The return status is zero.

if [ -f "/tmp/Non-existing-file.txt" ] ; then

   echo "I found the non-existing file"

   : # the colon command prevents an error if there are no other statements in this block

fi

== Using Find ==

=== Mime report ===

This example finds and counts files by their extension.  A "poor-man's mime-report"

for x in \

   $(find . -maxdepth 1 -type d | \

   uniq -c ; \

done

Breakdown:

<pre>

But it looks more impressive as a one-liner:

=== Prune ===

In case you are trying to figure out the prune option so that you can efficiently scan a directory for something while also ignoring .svn metadata, here is an example:

find ./ -name .svn -prune -o -name "*html*"

Or, a more complex example: 'wcgrep', from the contrib section of the svn repo:

#!/bin/bash

-type f -print0 | xargs -r0 ${WCGREP_GREP:-grep} ${WCGREP_GREPARGS:--HnI} \

$grepargs "$pattern"

== Examples ==

This script sets the svn:executable property on a number of files

#!/bin/bash

echo "Finished fixing websites"

echo

Sometimes when using find, you end up with "Permission denied" errors that add noise to your output.  There are a couple solutions for this.  Use the '''prune''' option to skip entire trees that you should avoid (e.g. /proc).  Use shell redirection to ignore remaining error messages (e.g. 2>/dev/null).

The following example searches all of my hard drive starting at / but skips over the backups directory I have in my external disk drive and also skips over the "process" directory.  Any errors like files in /var that I do not have permission to see are discarded by redirecting STDERR to the bitbucket.

   find / -path /media/disk/backups -prune -o -path /proc -prune -o -type d -name soffice.cfg 2>/dev/null

/home/greg/.openoffice.org2/user/config/soffice.cfg

/proc

/media/disk/backups

=== Move a directory up one level ===   

Sometimes you can end up with a directory which is nested inside it's intended destination.  For example, <code>drush archive-restore (arr)</code> can leave you with <tt>/var/www/drush/drush</tt> and you want the contents of the sub-directory to be at the location of it's parent.  Using the BASH shell options for glob control, you can set dotglob and later unset it to be able to move * up.  This worked for me on one host, and didn't work on another.  For the one that didn't work, mv kept complaining that destination directories were not empty.  I don't care if the destination directories exist.... that's the whole point.  Uggh!

cd /var/www/drupal/drupal/

shopt -s dotglob

mv -- * ..

shopt -u dotglob

==Resources==

* [http://penguinpetes.com/b2evo/index.php?title=how_the_one_liner_for_loop_in_bash_goes Penguin Pete]

* [[wp:Bash|Wikipedia page]]

[[Category:System Administration]]