Bash Basics

From #LinuxWiki

Contents 1 Mathematics 2 Strings 2.1 Search/Replace 2.1.1 Single Occurrance 2.1.2 Multiple Occurances 2.2 Trimming 2.2.1 Right Trim 2.2.2 Left Trim 2.2.3 Useful Examples

[edit] Mathematics

Bash includes full mathematical functionality (theres no need to use expr at all). Just be sure to include all your math work inside double brackets.

N=0 ; TOTAL=0
while ((N<100))
do
 ((TOTAL=TOTAL+N))
 ((N=N+1))
done
echo "Total is $TOTAL"
echo "Average is $((TOTAL/(N-1)))"

Total is 4950
Average is 50

[edit] Strings

[edit] Search/Replace

[edit] Single Occurrance

A="Windows Rules"
echo ${A/Windows/Linux}

Linux Rules

[edit] Multiple Occurances

A="The cob sob on the mob"
echo ${A//ob/at}

The cat sob on the mob

[edit] Trimming

Bash allows you to remove from the left and right of strings using the ${#} and ${%} functions.

[edit] Right Trim

# Rename all files ending in .doc to end in .odf
for FN in *.doc
do
 mv $FN ${FN%.doc}.odf
done

${%} matches against the shortest match when a wildcard is specified - for example -

A="Get Rid of the Xs XXXXXXXXXXXXXXXX"
echo ${A%X*}

Get Rid of the Xs XXXXXXXXXXXXXXX

Whereas ${%%} removes the largest match -

A="Get Rid of the Xs XXXXXXXXXXXXXXXX"
echo ${A%%X*}

Get Rid of the 

[edit] Left Trim

${#} works similarly to ${%} only working on the left side of a string.

MOBILE="+447123456789"
NEWMOBILE="0${MOBILE#+44}"
echo "$MOBILE is now $NEWMOBILE"

+447123456789 is now 07123456789

[edit] Useful Examples

RUNME=/usr/local/bin/runme
PATH=${RUNME%/*}
BASENAME=${RUNME##*/}
echo "PATH is $PATH, Basename is $BASENAME"

PATH is /usr/local/bin, Basename is runme