Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
watch
bash$ ldd /bin/ls
libc.so.6 => /lib/libc.so.6 (0x4000c000)
/lib/ld−linux.so.2 => /lib/ld−linux.so.2 (0x80000000)
Run a command repeatedly, at specified time intervals.
The default is two−second intervals, but this may be changed with the −n option.
strip
nm
rdist
watch −n 5 tail /var/log/messages
# Shows tail end of system log, /var/log/messages, every five seconds.
Remove the debugging symbolic references from an executable binary. This decreases its size, but
makes debugging it impossible.
This command often occurs in a Makefile, but rarely in a shell script.
List symbols in an unstripped compiled binary.
Remote distribution client: synchronizes, clones, or backs up a file system on a remote server.
Using our knowledge of administrative commands, let us examine a system script. One of the shortest and
simplest to understand scripts is killall, used to suspend running processes at system shutdown.
Example 13−8. killall, from /etc/rc.d/init.d
#!/bin/sh
# −−> Comments added by the author of this document marked by "# −−>".
# −−> This is part of the 'rc' script package
# −−> by Miquel van Smoorenburg,
# −−> This particular script seems to be Red Hat specific
# −−> (may not be present in other distributions).
# Bring down all unneeded services that are still running (there shouldn't
# be any, so this is just a sanity check)
for i in /var/lock/subsys/*; do
# −−> Standard for/in loop, but since "do" is on same line,
# −−> it is necessary to add ";".
# Check if the script is there.
[ ! −f $i ] && continue
# −−> This is a clever use of an "and list", equivalent to:
# −−> if [ ! −f "$i" ]; then continue
# Get the subsystem name.
subsys=${i#/var/lock/subsys/}
# −−> Match variable name, which, in this case, is the file name.
# −−> This is the exact equivalent of subsys=`basename $i`.
# −−> It gets it from the lock file name (if there is a lock file,
# −−>+ that's proof the process has been running).
# −−> See the "lockfile" entry, above.
Chapter 13. System and Administrative Commands 234