Shell Tips

booleans:

DOTOUCH=true
$DOTOUCH >> touch $SIGFIL

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find extension of filename:

$filename="test.file.jpg"
...
EXT=${filename#*.}	# gives ".jpg"
BASE=${filename%.*}	# gives "test.file"

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String replacement (once and multiple):

$startstring="This is some nice text."
NEW="${startstring/nice/different}	# gives: This is some different text.
NEW="${startstring//i/a}"		# gives: Thas is some nace text.

Special handy: remove superfluous spaces:

x="This   is a spacious     string"
y=${x/ /}
echo $y
This is a spacious string

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command substitution:

FOUND=$(grep searchpattern $(find . -name filename) 2> /dev/null)

This supports nesting (backticks do not)

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parameter default:

PARAMETER=${uservalue:-defaultvalue)

If uservalue is empty OR not defined, results in defaultvalue

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compare file-times:

FILE1=...
FILE2=...
FILETIME1=$(stat -c %Y "$FILE1")
FILETIME2=$(stat -c %Y "$FILE2")
if [ $FILETIME2 -lt $FILETIME1 ]
then
	...

stat option %Y gives modification time in seconds since epoch

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delete unnecessary files:

find . -name Thumbs.db -print0 | xargs --null rm -f

-print0 makes 'find' and 'xargs' work with null-terminated strings, so accept filenames with spaces and <CR>-characters.

find large files (>1GB):
or:

find . -mtime +31 -name Thumbs.db -exec rm -f {} \;

find / -size +1000000k -xdev | xargs ls -lh

or:

find / -size +1000000k -xdev | xargs -i ls -lh {}

(-i: means inline replacement / {} will be replaced )

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find problems with E-mail:

egrep '(reject|warning|error|fatal|panic):' /var/log/mail.log

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Check in a logfile what happend yesterday:

fgrep -e "`(export TZ=FJH+24; date "+%a %b %e")`" some-logfile

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Print the number of bytes used in the current directory:

ls -l | awk 'BEGIN { c=0 }{ c+=$5 } END { print c }'

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Make your script behave nicely:

renice +19 -p $$

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Append stderr to stdout:
Append stderr to stdout to file:

2>&1
bad_command >>filename 2>&1