Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
Example A−15. password: Generating random 8−character passwords
#!/bin/bash
# May need to be invoked with #!/bin/bash2 on older machines.
#
# Random password generator for bash 2.x by Antek Sawicki ,
# who generously gave permission to the document author to use it here.
#
# ==> Comments added by document author ==>
MATRIX="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
LENGTH="8"
# ==> May change 'LENGTH' for longer password, of course.
while [ "${n:=1}" −le "$LENGTH" ]
# ==> Recall that := is "default substitution" operator.
# ==> So, if 'n' has not been initialized, set it to 1.
do
PASS="$PASS${MATRIX:$(($RANDOM%${#MATRIX})):1}"
# ==> Very clever, but tricky.
# ==> Starting from the innermost nesting...
# ==> ${#MATRIX} returns length of array MATRIX.
# ==> $RANDOM%${#MATRIX} returns random number between 1
# ==> and length of MATRIX − 1.
# ==> ${MATRIX:$(($RANDOM%${#MATRIX})):1}
# ==> returns expansion of MATRIX at random position, by length 1.
# ==> See {var:pos:len} parameter substitution in Section 3.3.1
# ==> and following examples.
# ==> PASS=... simply pastes this result onto previous PASS (concatenation).
# ==> To visualize this more clearly, uncomment the following line
# ==> echo "$PASS"
# ==> to see PASS being built up,
# ==> one character at a time, each iteration of the loop.
done
let n+=1
# ==> Increment 'n' for next pass.
echo "$PASS"
# ==> Or, redirect to file, as desired.
exit 0
+
James R. Van Zandt contributed this script, which uses named pipes and, in his words, "really exercises
quoting and escaping".
Example A−16. fifo: Making daily backups, using named pipes
#!/bin/bash
# ==> Script by James R. Van Zandt, and used here with his permission.
Appendix A. Contributed Scripts 388