Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
TESTCHARS=2
SHABANG='#!'
# Test first 2 characters.
# Scripts begin with a "sha−bang."
for file in * # Traverse all the files in current directory.
do
if [[ `head −c$TESTCHARS "$file"` = "$SHABANG" ]]
# head −c2 #!
# The '−c' option to "head" outputs a specified
#+ number of characters, rather than lines (the default).
then
echo "File \"$file\" is a script."
else
echo "File \"$file\" is *not* a script."
fi
done
exit 0
Example 12−10. Generating 10−digit random numbers
#!/bin/bash
# rnd.sh: Outputs a 10−digit random number
# Script by Stephane Chazelas.
head −c4 /dev/urandom | od −N4 −tu4 | sed −ne '1s/.* //p'
# =================================================================== #
# Analysis
# −−−−−−−−
# head:
# −c4 option takes first 4 bytes.
# od:
# −N4 option limits output to 4 bytes.
# −tu4 option selects unsigned decimal format for output.
# sed:
# −n option, in combination with "p" flag to the "s" command,
# outputs only matched lines.
# The author of this script explains the action of 'sed', as follows.
# head −c4 /dev/urandom | od −N4 −tu4 | sed −ne '1s/.* //p'
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−> |
# Assume output up to "sed" −−−−−−−−> |
# is 0000000 1198195154\n
# sed begins reading characters: 0000000 1198195154\n.
# Here it finds a newline character,
# so it is ready to process the first line (0000000 1198195154).
# It looks at its s. The first and only one is
Chapter 12. External Filters, Programs and Commands 164