Bash snippets

I guess I will continue my current trend of having a single post be a clearing house for tips relating to a specific language.

 Math

$ x=0; let x+=3; let x-=1; echo $x
2

Generate range of numbers
$ for i in {1..5}; do echo -n $i; done
12345

Alternatively
$ for i in $(seq 1 5); do echo $i; done
12345

Iteration, etc...

Change password expiration data

$ chage -d now user

Printf

$ printf  "%02d %02d\n" 5 6
05 06

Echo list of folder names

$ echo */

home/ Desktop/ ...

Iterating through a file line by line

IN_FILE=$1
OUT_FILE=$2
<!-- Add a generated password to end of line -->
while read s; do
    echo "$s, `apg -n 1`" >> $OUT_FILE
done < $IN_FILE

Or as a one liner (even works if lines have spaces)

$ while read s; do echo "$s"; done < broke.txt

Dereference variables

$ var1='some text'
$ var2='var1'
$ echo ${!var2}
some text

String and array manipulation

From tldp.org we have two articles covering similar things named Issue18 and string-manipulation:

Creating an array and indexing into arrays:

$ arr=(hi there dude)
$ echo "Middle value: ${arr[1]}"

Middle value: there
$ echo ${arr[*]}
hi there dude

Indexing into strings: ${variable_name:start_position:length}

$ var="Hello" && echo ${var:1:3}
ell

Separate a path and its base filename.

$ FILE="/some/file/with/path.txt"
$ basename $FILE
path.txt
$ dirname $FILE

/some/file/with

Another way to do this (the first doesn't always seem to work)

$ echo "${FILE##*/}"
path.txt

 I use this in scripts within a usage() function to print the name of the script

$ usage() { echo "Usage: ${0##*/} <options>" ... }