From: (Joe Halpin)
Subject: FAQ - Answers to Frequently Asked Questions
Summary: This posting answers questions which come up with some
         frequency on It should be read by
         anyone with a question about shell programming before
         posting questions to the newsgroup.

Archive-name: unix-faq/shell/sh
Posting-Frequency: monthly
Version: $Id: cus-faq.html,v 1.10 2005/05/22 12:29:50 jhalpin Exp jhalpin $
Maintainer: Joe Halpin 

This FAQ list contains the answers to some Frequently Asked Questions
often seen in It spells "unix" in lower case letters
to avoid arguments about whether or not Linux, FreeBSD, etc are
unix. That's not the point of this FAQ, and I'm ignoring the issue.

This document as a whole is Copyright (c) 2003 Joe Halpin. It may be
copied freely. Exceptions are noted in individual answers.

Suggestions, complaints, et al, should be sent to the maintainer at or posted to

There are two levels of questions about shells.

One is the use of the shell itself as an interface to the operating
system. For example, "how do I run a program in the background, and go
on with other things?". Or "how do I setup environmental variables
when I log in?". 

The other level is how to write shell scripts. This often involves
having the shell execute unix utilities to perform part of the work
the shell script needs to accomplish, and requires knowledge of these
utilities, which isn't nominally in the scope of shell
programming. However, unless the question involves something other
than standard unix utilities, it should be included in this FAQ.

Standard unix utilities are defined by either POSIX or the Single Unix
Specification. These are now joined and are normally abbreviated as
"POSIX/SUS". This specification can be found at

The man pages found on that web page define standard behavior for any
given utility (or the shell itself). However, you should also check
the man page on your system for any utility or shell you need to
use. There isn't always a perfect correspondence between the standard
and a particular implementation (in fact, I'm not sure there's any
case in which they perfectly correspond).

There is also an Austin Group FAQ, which describes the standardization
effort in more detail ls

Other good web sites that provide information about shells and shell
programming (including OS utilities) include:

This FAQ is available at

The predictable legal stuff

The answers given in this FAQ list are provided with the best
intentions, but they may not be accurate for any particular
shell/os. They may be completely wrong for any shell/os. If you don't
test the answers, that's a bug in your procedures.

There are no guarantees for the answers or recommendations given in
this document. In fact, I don't even claim to have tested any or all
of them myself. Many of the answers here have been contributed by one
or more regular participants in the newsgroup, who I believe to be
competent (certainly more competent than I am), but THERE ARE NO

Did I really need to make that all uppercase? Hopefully not, but there
are a lot of lawyers around with too much time on their hands, so I
want to make it clear that THERE ARE NO GUARANTEES
about the accuracy of answers in this FAQ list. This is, hopefully, an
aid to people trying to learn shell programming, but it is NOT a
supported product. You have to figure out for yourself whether or not
the answers here work for what you're trying to do.

Under no circumstances will the maintainer of this FAQ list, or any
contributors to it, be held liable for any mistakes in this
document. If the answers work for you, well and good. If not, please
tell me and I'll modify them appropriately so that this will be more

If you don't agree to that, don't read any farther than this. Reading
beyond this point indicates your agreement.

If you do test the answers and find a problem, please send email to
the maintainer (see above), so it can be corrected, or (preferably)
post a question to the newsgroup so it can be discussed and corrected
if there's a problem. 

A number of people have contributed to this FAQ, knowingly or
unknowingly. Some of the answers were taken from previous postings in
the group, and other people contributed questions and answers
directly to the maintainer, which you are welcome to do as well.

Among the contributors is Heiner Steven, who also provided the
momentum to get this FAQ list started. He maintains a web site about
shell programming that has a lot of good stuff in it.




0a. Glossary
    POSIX/SUS ("the standard")
    race condition

0b. Notes about using echo
1.  How can I send e-mails with attached files?
2.  How can I generate random numbers in shell scripts?
3.  How can I automatically transfer files using FTP with error checking?
4.  How can I remove whitespace characters within file names?
5.  How can I automate a telnet session?
6.  How do I do date arithmetic?
7.  Why did someone tell me to RTFM?
8.  How do I create a lock file?
9. How can I convert DOS text files to unix, and vice versa?
10. How can a shell prompt be set up to change the title of xterm?
11. How do I get the exit code of cmd1 in cmd1|cmd2
12. Why do I get " not found"
13. Why doesn't echo do what I want?
14. How do I loop through files with spaces in their name?
15. how do I change my login shell?
16. When should I use a shell instead of perl/python/ruby/tcl...
17. Why shouldn't I use csh?
18. How do I reverse a file?
19. How do I remove last n lines?
20. how do I get file size, or file modification time?
21. How do I get a process id given a process name? Or, how do I find out if a process is still running, given a process ID?
22. How do I get a script to update my current environment?
23. how do I rename *.foo to *.bar?
24. How do I use shell variables in awk scripts
25. How do I input the user with a timeout?
26. How do I get one character input from the user?
27. why isn't my .profile read?
28. why do I get "[5" not found in "[$1 -eq 2]"?
29. How do I exactly display the content of $var (with a \n appended).
30. How do I split a pathname into the directory and file?
31. How do I make an alias take an argument?
32. How do I deal with a file whose name begins with a weird character
33. Why do I lose the value of global variables that are set in a loop
34. How do I batch an FTP download/upload?
Appendix A: Some example scripts
Appendix B: References. These
correspond with numbers in square brackets (e.g. [1]) which may appear
in the text. 




   Some contributors may copyright their submissions and license them
   differently than this document.

   [1] Chris F.A. Johnson. Examples marked with COPYING[1] were
       contributed by Chris F.A. Johnson. He has copyrighted these
       examples, and licensed them under the GNU General Public
       License (GPL). Copying them directly into another script will
       cause that script to also come under the GPL. For details see



      Google is one of the search engines on the Internet. It took
      over dejanews some years ago, and now is the standard reference
      when directing someone to a past thread one some topic. This is
      a very good place to start when researching a question about
      shell programming (and just about anything else).

   POSIX/SUS ("the standard")

      POSIX (Portable Operating System Interface) and SUS (Single Unix
      Specification) have been joined into one standard. This is what
      people usually mean when they refer to "the standard" in
      discussions about unix. When people in this group refer to the
      POSIX shell, they are talking about the shell prescribed by this
      specification. You can find this standard at 

      This is short for "Useless use of cat". It's used to point out
      that some example script has used cat when it could have used
      redirection instead. It's more efficient to redirect input than
      it is to spawn a process to run cat. For example

        $ cat file | tr -d 'xyz'

      runs two processes, one for cat and one for tr. This is less
      efficient than

        $ tr -d 'xyz' < file

      In general, "cat file | somecommand" can be more efficiently
      replaced by "somecommand < file"

      or (especially for multi-file input)

        $ somecommand file [file ...]

      but check the man page for "somecommand" to find out if it will
      accept this syntax.

      For more details about this, as well as other things like it, see


      This refers to a file which starts with '.' (a dot). These files
      are not shown in directory listings without the -a (or -A in
      newer versions of ls - check the man page on your system) option
      to ls. Often they are configuration files, subdirectories used
      by applications to store configuration files, NFS swap files, et


      The word "portable" means different things to different people,
      in different situations, which is to say, there isn't one
      definition of "portable".

      At one extreme, a portable script is one which will work under
      any shell, on any operating system. At this end of the spectrum,
      there is no such thing as a portable shell script (some
      operating systems don't even have shells). If we confine the
      operating system to unix (which would make sense since this is, the only truly portable scripts are those
      which make no use of built-in shell facilities or syntax, but
      which only call external utilities. For example

        echo Hello World

      would probably qualify. However, that doesn't do anyone much

      Given that there are probably few (if any) scripts which have to
      meet such a standard, a more frequent use of the word "portable"
      indicates the degree to which a script will run under different
      shells and/or different operating environments.

      For example, if you're writing an installation script for an
      application, and the platforms on which that application runs
      are defined, then the problem is pretty well bounded. The choice
      of shell is one which is available on all required platforms,
      and the syntax to be used is the smallest subset of all the
      variants of that shell on the target platforms.

      The degree to which your shell script needs to be portable has
      to be determined by you, or the requirements you've been given
      for the script.

   race condition

      This is a situation in which two entities (processes, threads,
      etc) are trying to access a shared resource, or perform the same
      action, and the result depends on the order of execution of the
      two entities.


      This is the first line of a shell script, which indicates to the
      operating system which interpreter (shell) it should invoke to
      interpret the script. It has the form

      #!/path/to/shell [ argument ]

      where /path/to/shell might be /bin/sh, /usr/local/bin/bash, etc.

      This line is only interpreted by the operating system. That is,
      if a shell script ( is executable and run from the
      command line by typing its name and giving the script as an
      argument, as in

      $ sh

      then sh interprets For sh, the shebang line is simply a
      comment, and is ignored.


   A number of shells are discussed in this group, including

   These (and more) are names of shells which are referenced in the
   group. A comparison of some of these is available at

   However, it does not make specific the differences between
   ksh88, ksh93 and pdksh, which are not entirely compatible.


   A. top posting
   Q. What's the most irritating way to respond on usenet?

   Please see the following:

0b. Notes about using echo

   This isn't really a FAQ, but discussions about using echo come up
   often enough that it seems reasonable to have something about it in
   the FAQ list.

   The echo command is not consistent in the handling of its arguments
   from implementation to implementation. Sometimes a string with
   backslash quoted characters will be interpreted in one way, and
   sometimes another. 

   Also, if the string being echoed wasn't built into the script
   itself, then it could have shell metacharacters in it, which could
   confuse things. In cases where external input is used to build a
   string to be echoed the string typically should be quoted.

   For example

   s="a string with\na newline and\ta tab"

   Following are some results with various shells:

   $ echo "$s"
   a string with\na newline and\ta tab
   $ echo -e "$s"
   a string with
   a newline and        a tab

   $ echo "$s"
   a string with
   a newline and        a tab

   $ echo -e "$s"
   $ echo "$s"
   a string with
   a newline and        a tab

   $ echo "$s"
   a string with
   a newline and        a tab

   $ echo -e "$s"
   -e a string with
   a newline and        a tab

   $ echo "$s"
   a string with\na newline and\ta tab

   $ echo -e "$s"
   -e a string with\na newline and\ta tab

   Note that ksh93 makes the handling of arguments system dependent
   when they contain '\', and/or the first argument begins with '-'.

   POSIX does not allow the -e option. It also makes the result of
   using -n or any string with '\' in it implementation-defined.
   However, on XSI-conforming systems, it disallows options, and
   defines the use of backslash-quoted characters.

   In general, the behavior of echo is system and/or shell dependent
   if its arguments contain a backslash, or its first argument is -n
   or -e.

   The biggest problem with echo is when using it to output strings
   that the script got externally (e.g. user input, or reading from a
   file). These strings may have '\' characters in them for
   example. In this case, results may not be what you expect.

   print is available in some shells, although printf(1) is perhaps
   more portable. Additionally, a here document will give predictable
   results in that it will not expand escape sequences.

   cat <<EOF


   a string with\na newline and\ta tab

   So consider not using echo unless you are sure what will happen,
   given the shell you're using.   


1. How can I send e-mails with attached files?

   a. Use uuencode
      This is the simplest way to do this. For example

      $ uuencode surfing.jpeg surfing.jpeg | mail someone@some.where

      To send regular text as well

      $ (cat mailtext; uuencode surfing.jpeg surfing.jpeg) |
        mail someone@some.where

   b. Use MIME

      $ metasend -b -t someone@some.where -s "Hear our son!" \
        -m audio/basic -f

      These examples are adapted from which
      goes into much more detail about this.

   c. Use pine (with a patch) or mutt



2. How can I generate random numbers in shell scripts?

   This depends on the shell, and the facilities available from the

   a. Some shells have a variable called RANDOM, which evaluates to a
      different value every time you dereference it. If your shell has
      this variable,

        $ number=$RANDOM will produce a random number. 

   b. Some systems have a /dev/urandom device, which generates a
      stream of bits. This can be accessed using the dd(1) utility. An
      example of this (from a more extensive discussion of different
      techniques at

        n=`dd if=/dev/urandom bs=1 count=4 2>/dev/null | od -t u4 | \
        awk 'NR==1 {print $2}'` 


        od -vAn -N4 -tu4 < /dev/urandom

   c. Use a utility such as awk(1), which has random number generation
      included. This approach is the most portable between shells and
      operating systems.

        awk 'BEGIN {srand();print rand()}'

      Note that this doesn't work with older versions of awk. This
      requires a version supporting the POSIX spec for srand(). For
      example, on Solaris this will not work with /usr/bin/awk, but
      will with nawk or /usr/xpg4/bin/awk.

      Also, if you call this line more than once within the same
      second, you'll get the same number you did the previous time.


3. How can I automatically transfer files using FTP with error

    First, there are tools to do that: curl, wget, lftp, ncftp. But,
    they are generally not part of the base system (you need to
    install them).
    zsh (version 4 and above) provides a FTP facility, see "info -f
    zsh -n 'zsh/zftp Module'"
    #! /usr/bin/zsh
    zftp open host user passwd || exit
    zftp get /remote/file > /local/file; r=$?
    zftp close && exit r
    With your system "ftp" command, two ways:
    1- using "ftp -n". Without the -n option, ftp expects user
    interaction to enter the password, so you'd need to use
    "expect". With "-n", you provide the user and password as any
    other FTP command.
    #! /bin/sh
    ftp -n << EOF
    user anonymous ${LOGNAME:-`who am i`}@
    get /remote/file /local/file
    The error checking can't be made correctly (if "open" fails, the
    "user" command will be still sent even if it shouldn't).
    2- using ~/.netrc
    If you put:
    login mylogin
    password mypasswd
    macdef init
      get /remote/file /local/file
    (with the trailing empty line) in your ~/.netrc (ensure it's not
    world readable) and then run "ftp", ftp will find
    the matching "machine" entry in your ~/.netrc and use the
    parameters provided there to make the ftp transaction.
    Those work at least on Linux, FreeBSD, Solaris, HPUX


4. How can I remove whitespace characters within file names?

   File names in unix can contain all kinds of whitespace characters,
   not just spaces. The following examples only work with spaces,
   adjust accordingly.

   a. Use the substitution capabilities of awk, sed, et al.

        f=`printf '%s\n' "$filename" | sed 's/ /_/g'`

        f=`printf '%s\n' "$filename" | awk '{gsub(" ","_");print $0}'`

        f=`printf '%s\n' "$filename" | tr ' ' _`

      Add characters to the tr command line as needed (see the man
      page for tr to find out the available escape sequences).

      Additionally (although not exactly a one-liner)

        f=`tr ' ' _ <<EOF

      See section 0a "Notes about using echo" for why echo is not used

   b. Use the substitution capabilities of the shell if it has
      them. Check the man page for your shell (probably under a
      section named something like "Parameter expansion") to see. For

      f=${filename// /_}

      With zsh:

      autoload -U zmv
      zmv '* *' '$f:gs/ /_/'

      It should be noted that the zmv solution renames the files (call
      mv internally and adress several problems that may arise) while
      the other solutions only update a variable (and then, renaming
      the files may involve a quite complicated script to do it


5. How can I automate a telnet session?

   This is outside the realm of shell programing, per se. You need
   a more special purpose scripting language such as expect. See 

   Perl scripts can also do this with the Telnet module from CPAN.


6. How do I do date arithmetic?

   This depends on exactly what you have in mind.

   a. Finding yesterday's date

      The GNU version of date has some nice features in this
      respect. For example

        To find yesterday's date

          $ date --date yesterday

        To find tomorrow's date

          $ date --date tomorrow

         See the man page for GNU date for other options. It can also
         provide dates more than one day in the past/future.

       The FreeBSD version of date also provides extensions that can
       do things like this.

         $ date
         Wed Oct 22 13:48:29 CDT 2003
         $ date -v-1d
         Tue Oct 21 13:45:16 CDT 2003

       Playing with the TZ variable isn't a reliable method. If you
       need to do something like this, but don't have GNU or FreeBSD
       date available, see section g. "Arbitrary date arithmetic".

   b. Determining relative ages of files

      If you want to determine whether or not one file is older than
      another, you can (with bash, pdksh, ksh93) do

        $ [[ file1 -ot file2 ]] && echo file1 is older

      or you can use find to search a directory tree for files that
      are newer/older than some file:

        $ find . -name '*.c' -newer test.c

   c. Finding elapsed time

      If you want to find elapsed time, perhaps because you want to
      know when some operation has timed out, some shells (bash, ksh
      zsh [,??])  have a SECONDS variable which tell how many seconds
      have elapsed since the invocation of the shell, or since the
      last time it was set.

      ksh93 has a floating point SECONDS which is locale dependent.

      In zsh 4.1 and above one can be made floating point with: float

      zsh 4.1 and above also has $EPOCHSECONDS for seconds since
      1970-1-1 0:0:0 UTC (see zsh/datetime module).

   d. Determining leap year

      A leap year in the Gregorian calendar is defined as a year which
      is evenly divisible by 4, however, if it's also evenly divisible
      by 100 then it's not a leap year unless it's also evenly
      divisible by 400.  It gets worse than that, actually, but this
      is as far as I go :-).

      In the Julain calendar which was used before in Europe, only the
      years divisible by 4 where leap years.

      The standard "cal" utility performed the switch between Julian
      and Gregorian calendar in september 1752 (see cal 9 1752) which
      corresponds to the date used in England. The Gregorian calendar
      (created by Pope Gregory III) was first used in 1582 in many
      other countries.

      One possibility for a ksh function to do this (after 1600 AD/CE)

          four=$(( $y % 4 ))
          hundred=$(( $y % 100 ))
          fourhundred=$(( $y % 400 ))
          if [ $four -eq 0 ];then
            if [ $hundred -eq 0 ];then
              if [ $fourhundred -eq 0 ];then
                echo leap year
                echo not a leap year
              echo leap year
           echo not a leap year 

      Or, valid with any date with the same calendar switch day as
      POSIX cal's (POSIX syntax):

        is_leap_year() # args: year
        # NB: year before year 1 is year -1, not 0.
          [ "$1" -lt 0 ] && set -- "$(($1 + 1))"
            [ "$(($1 % 4))" -eq 0 ] && {
                [ "$(($1 % 100))" -ne 0 ] || [ "$(($1 % 400))" -eq 0 ] \
                      || [ "$1" -le 1752 ]

      Or in any Bourne shell (see COPYING[1]):

        is_leap_year() { ## USAGE: is_leap_year [year]
          isl_year=${1:-`date +%Y`}
          case $isl_year in
            *0[48] |\
            *[2468][048] |\
            *[13579][26] |\
            *[2468][048]00 |\
            *[13579][26]00 ) _IS_LEAP_YEAR=1
               return 0 ;;
            *) _IS_LEAP_YEAR=0
               return 1 ;;

      On FreeBSD, use the -f option to date(1) to pass in the
      (supposed) February 29 in the current year and then print it the
      day of the month again to see if there really is such a date
      (note that you need -j as well as -f, otherwise date(1) thinks
      you want to set the clock):

      if [ $(date -jf%Y%m%d $(date +%Y0229) +%d) = 29 ]; then 
        echo Leap year!

   e. Determining the last day of a month. 

      There are a number of possibilities for doing this which have
      been mentioned in the group. The following is a sampling:

      In any Bourne-type shell (in conjunction with is_leap_year() as
      given above, when month is February) (see COPYING[1]):

        days_in_month() { ## USAGE: days_in_month [month [year]]
          if [ -n "$1" ] 
            eval `date "+dim_m=%m dim_y=%Y"`
          case $dim_m in
            9|09|4|04|6|06|11) _DAYS_IN_MONTH=30 ;;
            1|01|3|03|5|05|7|07|8|08|10|12) _DAYS_IN_MONTH=31 ;;
            2|02) is_leap_year ${dim_y:-`date +%Y`} &&
              _DAYS_IN_MONTH=29 || _DAYS_IN_MONTH=28 ;;
          [ ${SILENT_FUNCS:-0} -eq 1 ] || echo $_DAYS_IN_MONTH

      With GNU date:

        date -d "$year/$month/1 +1 month -1 day" +%d

      With FreeBSD date use the -v-1d option to date(1) to get the day
      before the first day of the next month:

        $ MONTH=12
        $ date -v-1d -jf%Y-%m-%d $(date +%Y-$(((MONTH+1)%12))-01) +%d
      In the shell using cal (But beware of implementations of cal
      which print more than one month at a time):

        month=9 ; year=2003   # adjust
        for lday in `cal $month $year` ; do : ; done
        echo $lday

        ## or
        set -- `cal $month $year` ; eval lday=\${$#}
        echo $lday
      In ksh, bash and zsh:

        : $(cal)

      In zsh:


   f. Determining the day of the week for a given date.

      This algorithm is known as Zeller's congruence. An explanation
      of it is available from the Dictionary of Algorithms and Data
      Structures web page at NIST:

      Also, a fuller explanation is available at

      An example in C, with a short explanation, is given at

      A shell (ksh93) implementation of a homework assignment (given
      for illustration only - don't turn this in as yours - you might
      be sorry if it's wrong :-) 

          # Implementation of a homework assignment given at
          # call with day:   1 - 31
          #           month: March = 1, Jan & Feb are months 11 and
          #                  12 of the previous year.
          #           year:  The year of the century
          #           c:     The previous century
          # For example, for July 4, 1989, 
          #   m = 5,  d = 4,  y = 89, and c = 19,
          # while for January 25, 1989,
          #   m = 11, d = 25, y = 88, and c = 19.
          # The output is the day of the week with Sunday = 0, 
          # Monday = 1, etc.


          A=$(( ($m * 13 - 1) / 5 ))
          B=$(( $y / 4 ))
          C=$(( $c / 4 ))
          D=$(( $A + $B + $C + $d + $y - ($c * 2) ))
          echo $(( $D % 7 ))

      On FreeBSD, use the -f option to date(1) to pass in the date of
      interest and +%A to print the day of the week:

        $ date -jf%Y-%m-%d 2000-01-01 +%A

      (Use +%u or +%w if you want the weekday as a number.  See the
      strftime(3) manpage for details.)

   g. Arbitrary date arithmetic

      To do arbitrary date calculations is more complicated. One
      possibility is to call an external utility, or a program in
      another scripting language, which has this built in. For
      example, perl has wrappers for the unix time functions built in,
      so it can provide some relief in this regard. C programs can
      also be easily written to do date arithmetic (see the examples
      section). One thing to keep in mind, however, is that unix time
      functions are, strictly speaking, limited to the range of time
      between January 1 1970 at midnight, and January 19, 2038 at
      3:14:07. C/Perl programs which calculate dates outside this
      range might work, or they might not, that would depend on the

      To do arbitrary date arithmetic in the shell itself is also
      possible. An article provided on the web by SysAdmin magazine
      describes one way to do this.

      Another possibility is given in the examples section, from

      See also

      On FreeBSD, the -f and -v options to date(1) cover most things
      you might want to do, with the caveat that only dates within the
      range mentioned above are defined. Dates outside that range are
      not guaranteed to work.

      Also, zsh 4.1 and above has the zsh/datetime module that
      provides the $EPOCHSECONDS and the strftime function.

   h. Getting the number of seconds since the epoch

    - GNU date has the %s format option which returns the epoch

    - More portably, use awk.

        awk 'BEGIN {srand(); printf("%d\n", srand())}'

      This works because srand() sets its seed value with the
      current epoch time if not given an argument. It also returns
      the previous seed value, so the second call gives the epoch

      Note that this doesn't work with older versions of awk. This
      requires a version supporting the POSIX spec for srand(). For
      example, on Solaris this will not work with /usr/bin/awk, but
      will with nawk or /usr/xpg4/bin/awk.

      Depending on scheduling, when the call is actually executed,
      etc, this might be off by a second.

    - Another way is to use perl if you have it.

        perl -le 'print time'

    - Also, zsh 4.1 and above has the zsh/datetime module that
      provides the $EPOCHSECONDS and the strftime function.


7. Why did someone tell me to RTFM?

   Because you didn't :-)

   RTFM is part of Usenet lingo, and means "Read The F-ing Manual".
   Generally people say this when someone asks a question that is
   asked so often, and is answered plainly in some relevant man page,
   that they're tired of seeing it asked.

   So RTFM, and the FAQs first before asking. Also, if you're new to
   the group, search Google Groups 

   before asking questions. And please don't post your homework
   questions to the group unless you've tried to figure them out, and
   have some specific questions. People will generally be happy to
   help you with your homework if you post what you've got and ask
   specific questions.


8. How do I create a lock file?

   Very carefully :-)

   The scheduler can stop one process in the middle of a non-atomic
   operation, and run another one, which wants to perform the same
   operation. The second one, having a full timeslice, might finish
   the operation. When control returns to the first process, confusion
   will reign.

   The trick is to do something atomic, so that this won't
   happen. There are a couple ways to do this. One is to create a
   directory instead of a file, the other is to create a symbolic
   link. Both operations are defined to be atomic by POSIX/SUS, by
   virtue of the fact that they both require invocation of the
   corresponding system calls, which are atomic.

   Beware of trying to create ANY kind of lock file on an NFS
   partition. NFS pretty much eliminates anything like atomicity.  If
   you're going to create a lock file, make sure you're doing it on a
   local partition, such as /tmp.

   Netscape/Mozilla uses the symbolic link method for its lockfile (in
   spite of the fact that it creates it in the user's home directory,
   which may be NFS mounted). When it starts up it creates a file
   named for the IP address of the machine it's running on, and the
   pid of the creating process. Then it tries to create a symbolic
   link named "lock", which points to that file. If this symlink
   already exists, link(2) will return an error. In a script this
   would work something like

   touch /tmp/xxx
   ln -s /tmp/xxx /tmp/lockfile 2>/dev/null
   rm /tmp/xxx
   if [ $ret -ne 0 ];then
     echo lockfile already exists
     exit 1
     echo success

   If you have procmail installed, another possibility is the
   lockfile(1) command that comes with it.


9. How can I convert DOS text files to unix, and vice versa?

    Unix text files consist of lines delimited by an LF ("line-feed")
    character (ASCII 10). DOS uses the two characters CR LF ("carriage
    return", "line feed"; ASCII 13, 10) for the same purpose.

    To convert a DOS text into unix text format, the CR characters
    (control-M) at the end of a line have to be removed. To create a
    DOS text file, the CR character should be added.

    A couple ways to remove CR characters:

      sed 's/^M$//' dos.txt > unix.txt

      tr -d '\r' < dosfile > unixfile

    To add them:    

      sed 's/$/^M/' unix.txt > dos.txt

    Note that "^M" in this case is an embedded control character, (CR,
    ASCII 13). Many shells allow embedding control characters by
    entering ^V first (control-V), resulting in the sequence


    for entering "^M".

    However, zsh, bash or ksh93 allow for:

      sed $'s/$/\r/'

    There is one special case to be considered: DOS text files
    sometimes contain an explicit end-of-file character ^Z (ASCII 26,
    or octal 32), which has no correspondent character for unix text
    files, where the end-of-file condition is determined
    implicitly. To remove that as well as the CR characters:
      tr -d '\r\032' < dosfile > unixfile

    Note that sed does not understand that notation, but awk does, and
    one simple way to do the opposite conversion is

      $ awk '{printf "%s\r\n" $0}END{printf "%c", 26}' unixfile > dosfile

    This assume a not-quite-ancient awk, in practice anything
    but Solaris /bin/awk (use nawk or /usr/xpg4/bin/awk in Solaris).

    Finally, your system may come with utilities named something like
    dos2unix and unix2dos, or d2u dos2unix fromdos non-standard
    utilities, and GNU recode: recode /CRLF


10. How can a shell prompt be set up to change the title of xterm?

    Gives escape sequences for xterm. For example, to change the name
    of the current window to "XXX" (in bash), do

      $ echo -en "\033]2;XXX\007"

      or, more portably:

      $ printf '%b' '\e]2;XXX\a'

    See also "Why doesn't echo do what I want?"