Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
sleep 3 h
# Pauses 3 hours!
usleep
The watch command may be a better choice than sleep for running commands at
timed intervals.
Microsleep (the "u" may be read as the Greek "mu", or micro− prefix). This is the same as sleep,
above, but "sleeps" in microsecond intervals. It can be used for fine−grain timing, or for polling an
ongoing process at very frequent intervals.
usleep 30
# Pauses 30 microseconds.
This command is part of the Red Hat initscripts / rc−scripts package.
The usleep command does not provide particularly accurate timing, and is therefore
unsuitable for critical timing loops.
hwclock, clock
The hwclock command accesses or adjusts the machine's hardware clock. Some options require root
privileges. The /etc/rc.d/rc.sysinit startup file uses hwclock to set the system time from
the hardware clock at bootup.
The clock command is a synonym for hwclock.
12.4. Text Processing Commands
Commands affecting text and text files
sort
tsort
uniq
File sorter, often used as a filter in a pipe. This command sorts a text stream or file forwards or
backwards, or according to various keys or character positions. Using the −m option, it merges
presorted input files. The info page lists its many capabilities and options. See Example 10−9,
Example 10−10, and Example A−9.
Topological sort, reading in pairs of whitespace−separated strings and sorting according to input
patterns.
This filter removes duplicate lines from a sorted file. It is often seen in a pipe coupled with sort.
cat list−1 list−2 list−3 | sort | uniq > final.list
# Concatenates the list files,
# sorts them,
# removes duplicate lines,
# and finally writes the result to an output file.
The useful −c option prefixes each line of the input file with its number of occurrences.
bash$ cat testfile
This line occurs only once.
This line occurs twice.
This line occurs twice.
Chapter 12. External Filters, Programs and Commands 160