Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
# range action
# 1 s/.* //p
# The line number is in the range, so it executes the action:
# tries to substitute the longest string ending with a space in the line
# ("0000000 ") with nothing (//), and if it succeeds, prints the result
# ("p" is a flag to the "s" command here, this is different from the "p" command).
# sed is now ready to continue reading its input. (Note that before
# continuing, if −n option had not been passed, sed would have printed
# the line once again).
# Now, sed reads the remainder of the characters, and finds the end of the file.
# It is now ready to process its 2nd line (which is also numbered '$' as
# it's the last one).
# It sees it is not matched by any , so its job is done.
# In few word this sed commmand means:
# "On the first line only, remove any character up to the right−most space,
# then print it."
# A better way to do this would have been:
# sed −e 's/.* //;q'
# Here, two s (could have been written
# sed −e 's/.* //' −e q):
# range action
# nothing (matches line) s/.* //
# nothing (matches line) q (quit)
# Here, sed only reads its first line of input.
# It performs both actions, and prints the line (substituted) before quitting
# (because of the "q" action) since the "−n" option is not passed.
# =================================================================== #
# A simpler altenative to the above 1−line script would be:
# head −c4 /dev/urandom| od −An −tu4
exit 0
tail
See also Example 12−30.
lists the end of a file to stdout (the default is 10 lines). Commonly used to keep track of changes to
a system logfile, using the −f option, which outputs lines appended to the file.
Example 12−11. Using tail to monitor the system log
#!/bin/bash
filename=sys.log
cat /dev/null > $filename; echo "Creating / cleaning out file."
# Creates file if it does not already exist,
#+ and truncates it to zero length if it does.
# : > filename and > filename also work.
tail /var/log/messages > $filename
Chapter 12. External Filters, Programs and Commands 165