Linux Commands Examples

A great documentation place for Linux commands


The Perl 5 language interpreter



-sTtuUWX ]

-hv ] [ -V[:configvar] ]

-cw ] [ -d[t][:debugger] ] [ -D[number/list] ]

-pna ] [ -Fpattern ] [ -l[octal] ] [ -0[octal/hexadecimal] ]

-Idir ] [ -m[-]module ] [ -M[-]’module...’ ] [ -f ]

-C [number/list]

-S ]

-x[dir] ]

-i[extension] ]

[ [-e|-E’command’ ] [ -- ] [ programfile ] [ argument ]...

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perl -Isrc/perl/lib "$@"

How do I remove a file in Linux whose name looks like it's ONLY a hyphen, as in "-"

Should be able to do rm -f '-' worked for me in cygwin

For that matter, rm - also seems to work on red hat.


Remove duplicates in each line of a file

Since ruby comes with any Linux distribution I know of:

ruby -e 'STDIN.readlines.each { |l| l.split(" ").uniq.each { |e| print "#{e} " }; print "\n" }' < test

Here, test is the file that contains the elements.

To explain what this command does—although Ruby can almost be read from left to right:

  • Read the input (which comes from < test through your shell)
  • Go through each line of the input
  • Split the line based on one space separating the items, into an array (split(" "))
  • Get the unique elements from this array (in-order)
  • For each unique element, print it, including a space (print "#{e} ")
  • Print a newline once we're done with the unique elements

Shrinking file size under Linux

You may want to use the truncate command:

truncate --size=1G test.txt

SIZE can be specified as bytes, KB, K, MB, M, etc. I assume you can calculate the desired size by hand; if not, you could probably use the stat command to get information about the file's current size.


unix - split a huge .gz file by line

I'd consider using split.

split a file into pieces


When to use Bash and when to use Perl/Python/Ruby?

Bash is a Unix shell it includes a scripting language. It is rather command processor. you control the way how you run commands, you actually run them.

Perl/Ruby/Python are general purpose languages.

When you want a shell script, you use Bash

If you want more complex task or not related to shell. Use Python etc.

I would never compare these languages actually. Python etc. are portable. You can run them anywhere. Bash is for Unix only.

Python etc. have tons of reusable libraries solving millions of tasks.

It's almost the same if you ask. "When to use Paint and when to use Photoshop"

For processing emails I would use Ruby, again, because it has a lot of reusable libraries.

But the best way would be combine bash and ruby. That would be right. Like you create a email lrocessor in ruby and bash script would invoke that ruby script and run other commans ds.

So whenever you need command processor you use bash. You run unix commands and control them.


Output numbers from a file in a different order

Try doing this in

 perl -lane '$c=0; for (@F){ print "$F[$c]\t$F[$c+=1]" if $F[$c+1]}' file.txt

Or decomposed :

perl -lane '
    for (@F) {
        print "$F[$c]\t$F[$c+=1]"
            if $F[$c+1];
' file.txt


  • lane switchs means : l=newlines ; a=autosplit in @F array ; n=like while (<>) magic diamond operator ; e=basic switch to run a command
  • $c=0 assign 0 to a counter
  • for (@F) { for each element of the current line
  • print "$F[$c]\t$F[$c+=1]" : print array element with indice $c + tab + $c+1
  • if $F[$c+1]; : apply last line only if $F[$c+1] is not null

Or using (same algorithm), maybe more human readable for beginners :

while read a; do
    arr=( $a )
    for ((i=0; i< ${#arr[@]}; i++)); do
        [[ ${arr[i+1]} ]] && echo "${arr[i]} ${arr[i+1]}"
done < file.txt

How do you manage perl modules on a Debian system?

Depending on the modules, I'm fairly sure you can use the apt-get command to install some of the more common modules.

apt-get install perl5-crypt (maybe its p5-crypt - its been a while).

It depends a lot on what modules are required though, many of them are not in the apt packages.


convert .dbx (outlook express) mail files to a mbox format (for thunderbird) w/o Windows?

Since no one posted a solution that was implemented nor throughly documented, I will post my solution.

As above, I simply went to a friends house who has windows installed and did the conversion the way that everyone else does it... on windows...

Another way to do this would be to run a virtual machine to do what you need but I did not have my discs as this wasn't a solution for me.


Shell script to find all types of files in a directory with their count

find . -type f -name '*.*' -exec sh -c 'echo ${0##*.}' {} \; | sort | uniq -c | sort -nr

The echo ${0##*.} gives you the extension of the file. We pipe the output to sort and then count the unique lines with uniq.

Some additions:

  • Since the output of uniq is not sorted according to the number of occurrences, you'll have to pipe again into a numeric sort (-nr) if you want it sorted.
  • If you want to search your current directory only, add -maxdepth 1 to your find command.
  • Pipe into awk '{print $2, $1}' to show the count after the extensions.

Renaming files in batch with Unix RENAME

rename doesn't do sed-style substitutions. This very short Perl script will let you do regmv *.sam s/indel/snp/:

#!/usr/bin/perl -w
# regmv - Rename files using a regular expression
# This program renames files by using a regular expression to
# determine their new name.
# Usage: regmv file [file ...] regexp
# $Id: regmv,v 1.1 1998/10/14 17:07:55 blrfl Exp $

sub at_clean()
    my $message = $@;
    $message =~ s|\s+at /.*$||s;
    return $message;

(@ARGV > 1)
    || die "Usage: regmv [options] file [file ...] regexp\n";

my $expr = pop @ARGV;

$_ = 'SomeValue';
eval "\$_ =~ $expr";
die "Yuck: " . at_clean() if $@;

foreach (@ARGV)
    my $old = $_;

    eval "\$_ =~ $expr";

    next if $_ eq $old;

    (rename $old, $_)
        || die "Rename failed: $!\n";

Linux / Perl - What happens when a process is forked?

From Advanced Programming in the UNIX Environment by W. Richard Stevens (pg. 188):

8.3 fork function

The only way a new process is created by the Unix kernel is when an existing process calls the fork function. (This doesn't apply to the special processes that we mentioned in the previous section—the swapper, init, and pagedaemon. These processes are created specially by the kernel as part of bootstrapping.)

#include <sys/types.h>
#include <unistd.h>

pid_t fork(void);
/* Returns: 0 in child, process ID of child in parent, -1 on error */

The new process created by fork is called the child process. The function is called once but returns twice. The only difference in the returns is that the return value in the child is 0 while the return value in the parent is the process ID of the new child. The reason the child's process ID is returned to the parent is because a process can have more than one child, so there is no function that allows a process to obtain the process IDs of its children. The reason fork returns 0 to the child is because a process can have only a single parent, so the child can always call getppid to obtain the process ID of its parent. (Process ID 0 is always in use by the swapper, so it's not possible for 0 to be the process ID of a child.)

Both the child and parent continue executing with the instruction that follows the call to fork. The child is a copy of the parent. For example, the child gets a copy of the parent's data space, heap, and stack. Note that this is a copy for the child—the parent and child do not share these portions of memory. Often the parent and child share the text segment (Section 7.6), if it is read-only.

On Linux, Perl's fork operator calls the system's fork and returns undef on failure rather than -1.

Stevens gives lists (pg. 192) of inherited properties and differences between parents processes and their forked children:

Besides open files, there are numerous other properties of the parent that are inherited by the child:

  • real user ID, real group ID, effective user ID, effective group ID
  • supplementary group IDs
  • process group ID
  • session ID
  • controlling terminal
  • set-user-ID flag and set-group-ID flag
  • current working directory
  • root directory
  • file mode creation mask
  • signal mask and dispositions
  • the close-on-exec flag for any open file descriptors
  • environment
  • attached shared memory segments
  • resource limits

The differences between the parent and child are

  • the return value from fork
  • the process IDs are different
  • the two processes have different parent process IDs—the parent process ID of the child is the parent; the parent process ID of the parent doesn't change
  • the child's values for tms_utime, tms_stime, tms_cutime, and tms_ustime are set to 0
  • file locks set by the parent are not inherited by the child
  • pending alarms are cleared for the child
  • the set of pending signals for the child is set to the empty set

How can I identify non-ASCII characters from the shell?

$ perl -ne 'print "$. $_" if m/[\x80-\xFF]/'  utf8.txt
2 Pour être ou ne pas être
4 By? ?i neby?
5 ???


$ grep -n -P '[\x80-\xFF]' utf8.txt
2:Pour être ou ne pas être
4:By? ?i neby?

where utf8.txt is

$ cat utf8.txt
To be or not to be.
Pour être ou ne pas être
Om of niet zijn
By? ?i neby?

How can uptime be used to trigger an action?

You can easily determine the uptime in days with awk:

# Print days of uptime, or zero if less than 1 day.
uptime | awk '/days?/ {print $3; next}; {print 0}'

You can use this with command substitution to perform any action you like based on the results. For example:

days () { uptime | awk '/days?/ {print $3; next}; {print 0}'; }
if [ $(days) -ge $UPTIME_THRESHOLD ]; then
    : # Take some action.

Obviously, the action you take is up to you. You can mail yourself messages, schedule a reboot with the at command, or anything else that you feel necessary to do.

You may also wish to set this script up as a daily cron job, so that it will trigger your defined action when the uptime threshold has been exceeded. If you have root access, you could simply drop the script into /etc/cron.daily/, or you might set up your personal crontab to call this script once a day.


Force wget proxy to non environment proxy

You can set the http_proxy environment variable just for a specific invocation of wget:

http_proxy=http://specific-squid-proxy-host:3129/ wget http://server/page


Perl officially stands for Practical Extraction and Report Language, except when it doesn’t.

Perl was originally a language optimized for scanning arbitrary text files, extracting information from those text files, and printing reports based on that information. It quickly became a good language for many system management tasks. Over the years, Perl has grown into a general-purpose programming language. It’s widely used for everything from quick "one-liners" to full-scale application development.

The language is intended to be practical (easy to use, efficient, complete) rather than beautiful (tiny, elegant, minimal).

Perl combines (in the author’s opinion, anyway) some of the best features of C, sed, awk, and sh, so people familiar with those languages should have little difficulty with it. (Language historians will also note some vestiges of csh, Pascal, and even BASIC-PLUS.) Expression syntax corresponds closely to C expression syntax. Unlike most Unix utilities, Perl does not arbitrarily limit the size of your data--if you’ve got the memory, Perl can slurp in your whole file as a single string. Recursion is of unlimited depth. And the tables used by hashes (sometimes called "associative arrays") grow as necessary to prevent degraded performance. Perl can use sophisticated pattern matching techniques to scan large amounts of data quickly. Although optimized for scanning text, Perl also has many excellent tools for slicing and dicing binary data.

But wait, there’s more...

Begun in 1993 (see perlhist), Perl version 5 is nearly a complete rewrite that provides the following additional benefits:

modularity and reusability using innumerable modules

Described in perlmod, perlmodlib, and perlmodinstall.

embeddable and extensible

Described in perlembed, perlxstut, perlxs, perlcall, perlguts, and xsubpp.

roll-your-own magic variables (including multiple simultaneous DBM implementations)

Described in perltie and AnyDBM_File.

subroutines can now be overridden, autoloaded, and prototyped

Described in perlsub.

arbitrarily nested data structures and anonymous functions

Described in perlreftut, perlref, perldsc, and perllol.

object-oriented programming

Described in perlobj, perlboot, perltoot, perltooc, and perlbot.

support for light-weight processes (threads)

Described in perlthrtut and threads.

support for Unicode, internationalization, and localization

Described in perluniintro, perllocale and Locale::Maketext.

lexical scoping

Described in perlsub.

regular expression enhancements

Described in perlre, with additional examples in perlop.

enhanced debugger and interactive Perl environment, with integrated editor support

Described in perldebtut, perldebug and perldebguts.

POSIX 1003.1 compliant library

Described in POSIX .

Okay, that’s definitely enough hype.


Perl is available for most operating systems, including virtually all Unix-like platforms. See "Supported Platforms" in perlport for a listing.


The "use warnings" pragma (and the -w switch) produces some lovely diagnostics.

See perldiag for explanations of all Perl’s diagnostics. The "use diagnostics" pragma automatically turns Perl’s normally terse warnings and errors into these longer forms.

Compilation errors will tell you the line number of the error, with an indication of the next token or token type that was to be examined. (In a script passed to Perl via -e switches, each -e is counted as one line.)

Setuid scripts have additional constraints that can produce error messages such as "Insecure dependency". See perlsec.

Did we mention that you should definitely consider using the -w switch?


See perlrun.


 "@INC"                 locations of perl libraries

getting help

The perldoc program gives you access to all the documentation that comes with Perl. You can get more documentation, tutorials and community support online at <>.

If you’re new to Perl, you should start by running "perldoc perlintro", which is a general intro for beginners and provides some background to help you navigate the rest of Perl’s extensive documentation. Run "perldoc perldoc" to learn more things you can do with perldoc.

For ease of access, the Perl manual has been split up into several sections.


    perl                Perl overview (this section)
    perlintro           Perl introduction for beginners
    perltoc             Perl documentation table of contents


    perlreftut          Perl references short introduction
    perldsc             Perl data structures intro
    perllol             Perl data structures: arrays of arrays
    perlrequick         Perl regular expressions quick start
    perlretut           Perl regular expressions tutorial
    perlboot            Perl OO tutorial for beginners
    perltoot            Perl OO tutorial, part 1
    perltooc            Perl OO tutorial, part 2
    perlbot             Perl OO tricks and examples
    perlperf            Perl Performance and Optimization Techniques
    perlstyle           Perl style guide
    perlcheat           Perl cheat sheet
    perltrap            Perl traps for the unwary
    perldebtut          Perl debugging tutorial
    perlfaq             Perl frequently asked questions
      perlfaq1          General Questions About Perl
      perlfaq2          Obtaining and Learning about Perl
      perlfaq3          Programming Tools
      perlfaq4          Data Manipulation
      perlfaq5          Files and Formats
      perlfaq6          Regexes
      perlfaq7          Perl Language Issues
      perlfaq8          System Interaction
      perlfaq9          Networking

Reference Manual

    perlsyn             Perl syntax
    perldata            Perl data structures
    perlop              Perl operators and precedence
    perlsub             Perl subroutines
    perlfunc            Perl built-in functions
      perlopentut       Perl open() tutorial
      perlpacktut       Perl pack() and unpack() tutorial
    perlpod             Perl plain old documentation
    perlpodspec         Perl plain old documentation format specification
    perlpodstyle        Perl POD style guide
    perlrun             Perl execution and options
    perldiag            Perl diagnostic messages
    perllexwarn         Perl warnings and their control
    perldebug           Perl debugging
    perlvar             Perl predefined variables
    perlre              Perl regular expressions, the rest of the story
    perlrebackslash     Perl regular expression backslash sequences
    perlrecharclass     Perl regular expression character classes
    perlreref           Perl regular expressions quick reference
    perlref             Perl references, the rest of the story
    perlform            Perl formats
    perlobj             Perl objects
    perltie             Perl objects hidden behind simple variables
      perldbmfilter     Perl DBM filters
    perlipc             Perl interprocess communication
    perlfork            Perl fork() information
    perlnumber          Perl number semantics
    perlthrtut          Perl threads tutorial
    perlport            Perl portability guide
    perllocale          Perl locale support
    perluniintro        Perl Unicode introduction
    perlunicode         Perl Unicode support
    perlunifaq          Perl Unicode FAQ
    perluniprops        Index of Unicode Version 6.0.0 properties in Perl
    perlunitut          Perl Unicode tutorial
    perlebcdic          Considerations for running Perl on EBCDIC platforms
    perlsec             Perl security
    perlmod             Perl modules: how they work
    perlmodlib          Perl modules: how to write and use
    perlmodstyle        Perl modules: how to write modules with style
    perlmodinstall      Perl modules: how to install from CPAN
    perlnewmod          Perl modules: preparing a new module for distribution
    perlpragma          Perl modules: writing a user pragma
    perlutil            utilities packaged with the Perl distribution
    perlcompile         Perl compiler suite intro
    perlfilter          Perl source filters
    perlglossary        Perl Glossary

Internals and C Language Interface

    perlembed           Perl ways to embed perl in your C or C++ application
    perldebguts         Perl debugging guts and tips
    perlxstut           Perl XS tutorial
    perlxs              Perl XS application programming interface
    perlclib            Internal replacements for standard C library functions
    perlguts            Perl internal functions for those doing extensions
    perlcall            Perl calling conventions from C
    perlmroapi          Perl method resolution plugin interface
    perlreapi           Perl regular expression plugin interface
    perlreguts          Perl regular expression engine internals
    perlapi             Perl API listing (autogenerated)
    perlintern          Perl internal functions (autogenerated)
    perliol             C API for Perl's implementation of IO in Layers
    perlapio            Perl internal IO abstraction interface
    perlhack            Perl hackers guide
    perlsource          Guide to the Perl source tree
    perlinterp          Overview of the Perl intepreter source and how it works
    perlhacktut         Walk through the creation of a simple C code patch
    perlhacktips        Tips for Perl core C code hacking
    perlpolicy          Perl development policies
    perlgit             Using git with the Perl repository


    perlbook            Perl book information
    perlcommunity       Perl community information
    perltodo            Perl things to do
    perldoc             Look up Perl documentation in Pod format
    perlhist            Perl history records
    perldelta           Perl changes since previous version
    perl5141delta       Perl changes in version 5.14.1
    perl5140delta       Perl changes in version 5.14.0
    perl51311delta      Perl changes in version 5.13.11
    perl51310delta      Perl changes in version 5.13.10
    perl5139delta       Perl changes in version 5.13.9
    perl5138delta       Perl changes in version 5.13.8
    perl5137delta       Perl changes in version 5.13.7
    perl5136delta       Perl changes in version 5.13.6
    perl5135delta       Perl changes in version 5.13.5
    perl5134delta       Perl changes in version 5.13.4
    perl5133delta       Perl changes in version 5.13.3
    perl5132delta       Perl changes in version 5.13.2
    perl5131delta       Perl changes in version 5.13.1
    perl5130delta       Perl changes in version 5.13.0
    perl5123delta       Perl changes in version 5.12.3
    perl5122delta       Perl changes in version 5.12.2
    perl5121delta       Perl changes in version 5.12.1
    perl5120delta       Perl changes in version 5.12.0
    perl5115delta       Perl changes in version 5.11.5
    perl5114delta       Perl changes in version 5.11.4
    perl5113delta       Perl changes in version 5.11.3
    perl5112delta       Perl changes in version 5.11.2
    perl5111delta       Perl changes in version 5.11.1
    perl5110delta       Perl changes in version 5.11.0
    perl5101delta       Perl changes in version 5.10.1
    perl5100delta       Perl changes in version 5.10.0
    perl595delta        Perl changes in version 5.9.5
    perl594delta        Perl changes in version 5.9.4
    perl593delta        Perl changes in version 5.9.3
    perl592delta        Perl changes in version 5.9.2
    perl591delta        Perl changes in version 5.9.1
    perl590delta        Perl changes in version 5.9.0
    perl589delta        Perl changes in version 5.8.9
    perl588delta        Perl changes in version 5.8.8
    perl587delta        Perl changes in version 5.8.7
    perl586delta        Perl changes in version 5.8.6
    perl585delta        Perl changes in version 5.8.5
    perl584delta        Perl changes in version 5.8.4
    perl583delta        Perl changes in version 5.8.3
    perl582delta        Perl changes in version 5.8.2
    perl581delta        Perl changes in version 5.8.1
    perl58delta         Perl changes in version 5.8.0
    perl573delta        Perl changes in version 5.7.3
    perl572delta        Perl changes in version 5.7.2
    perl571delta        Perl changes in version 5.7.1
    perl570delta        Perl changes in version 5.7.0
    perl561delta        Perl changes in version 5.6.1
    perl56delta         Perl changes in version 5.6
    perl5005delta       Perl changes in version 5.005
    perl5004delta       Perl changes in version 5.004
    perlartistic        Perl Artistic License
    perlgpl             GNU General Public License


    perlcn              Perl for Simplified Chinese (in EUC-CN)
    perljp              Perl for Japanese (in EUC-JP)
    perlko              Perl for Korean (in EUC-KR)
    perltw              Perl for Traditional Chinese (in Big5)


    perlaix             Perl notes for AIX
    perlamiga           Perl notes for AmigaOS
    perlbeos            Perl notes for BeOS
    perlbs2000          Perl notes for POSIX-BC BS2000
    perlce              Perl notes for WinCE
    perlcygwin          Perl notes for Cygwin
    perldgux            Perl notes for DG/UX
    perldos             Perl notes for DOS
    perlepoc            Perl notes for EPOC
    perlfreebsd         Perl notes for FreeBSD
    perlhaiku           Perl notes for Haiku
    perlhpux            Perl notes for HP-UX
    perlhurd            Perl notes for Hurd
    perlirix            Perl notes for Irix
    perllinux           Perl notes for Linux
    perlmacos           Perl notes for Mac OS (Classic)
    perlmacosx          Perl notes for Mac OS X
    perlmpeix           Perl notes for MPE/iX
    perlnetware         Perl notes for NetWare
    perlopenbsd         Perl notes for OpenBSD
    perlos2             Perl notes for OS/2
    perlos390           Perl notes for OS/390
    perlos400           Perl notes for OS/400
    perlplan9           Perl notes for Plan 9
    perlqnx             Perl notes for QNX
    perlriscos          Perl notes for RISC OS
    perlsolaris         Perl notes for Solaris
    perlsymbian         Perl notes for Symbian
    perltru64           Perl notes for Tru64
    perluts             Perl notes for UTS
    perlvmesa           Perl notes for VM/ESA
    perlvms             Perl notes for VMS
    perlvos             Perl notes for Stratus VOS
    perlwin32           Perl notes for Windows

On Debian systems, you need to install the perl-doc package which contains the majority of the standard Perl documentation and the perldoc program.

Extensive additional documentation for Perl modules is available, both those distributed with Perl and third-party modules which are packaged or locally installed.

You should be able to view Perl’s documentation with your man(1) program or perldoc(1).

In general, if something strange has gone wrong with your program and you’re not sure where you should look for help, try the -w switch first. It will often point out exactly where the trouble is.


The Perl motto is "There’s more than one way to do it." Divining how many more is left as an exercise to the reader.

The three principal virtues of a programmer are Laziness, Impatience, and Hubris. See the Camel Book for why.


The -w switch is not mandatory.

Perl is at the mercy of your machine’s definitions of various operations such as type casting, atof(), and floating-point output with sprintf().

If your stdio requires a seek or eof between reads and writes on a particular stream, so does Perl. (This doesn’t apply to sysread() and syswrite().)

While none of the built-in data types have any arbitrary size limits (apart from memory size), there are still a few arbitrary limits: a given variable name may not be longer than 251 characters. Line numbers displayed by diagnostics are internally stored as short integers, so they are limited to a maximum of 65535 (higher numbers usually being affected by wraparound).

You may mail your bug reports (be sure to include full configuration information as output by the myconfig program in the perl source tree, or by "perl -V") to perlbug[:at:]perl[:dot:]org . If you’ve succeeded in compiling perl, the perlbug script in the utils/ subdirectory can be used to help mail in a bug report.

Perl actually stands for Pathologically Eclectic Rubbish Lister, but don’t tell anyone I said that.

see also       the Perl homepage       Perl articles (O'Reilly)       the Comprehensive Perl Archive         the Perl Mongers


Larry Wall <larry[:at:]wall[:dot:]org>, with the help of oodles of other folks.

If your Perl success stories and testimonials may be of help to others who wish to advocate the use of Perl in their applications, or if you wish to simply express your gratitude to Larry and the Perl developers, please write to perl-thanks[:at:]perl[:dot:]org .

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