Difference between revisions of "Bash notes"

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<pre>
 
<pre>
 
while sleep 1; do lsof|grep -i Maildir; done
 
while sleep 1; do lsof|grep -i Maildir; done
 +
</pre>
 +
 +
== check if running as root ==
 +
 +
<pre>
 +
if [ `id -u` = "0" ]; then
 +
    echo "You are root."
 +
    exit 0
 +
fi
 
</pre>
 
</pre>

Revision as of 03:57, 22 October 2008


Turn off bash history for a session

set +o history

Rename a group of files by extension

For example, rename all images from foo.jpg to foo_2.jpg.

This is somewhat more clear:

for filename in *.jpg ; do mv $filename `basename $filename .jpg`_2.jpg; done

This is more "correct" and doesn't require `basename`:

for filename in *.jpg ; do mv $filename ${filename%.jpg}_2.jpg; done

Variable Expansion and Substitution

Bash can do some freaky things with variables. It can do lots of other substitutions. See "Parameter Expansion" in the Bash man page.

  • ${foo#pattern} - deletes the shortest possible match from the left
  • ${foo##pattern} - deletes the longest possible match from the left
  • ${foo%pattern} - deletes the shortest possible match from the right
  • ${foo%%pattern} - deletes the longest possible match from the right
  • ${foo=text} - If $foo exists and is not null then return $foo. If $foo doesn't exist then create it and set value to text.

Statements

Loop on filenames in a directory

for foo in *; do {
  echo ${foo}
}; done

Loop on lines in a file

for foo in $(cat data_file.txt); do {
  echo ${foo}
}; done

while loop

This is kind of like `watch`:

while sleep 1; do lsof|grep -i Maildir; done

check if running as root

if [ `id -u` = "0" ]; then
    echo "You are root."
    exit 0
fi