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Old 10-25-2008, 09:48 PM   #1
bartonski
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Learning how to use Bash Productively


I've been using linux in one job or another for about 10 years. I used to be a C programmer, and I'm decent with perl and passable with ruby. These days, I do a lot of text data analysis. I tend to work on fairly short term projects, generally too short and too limited to justify writing a perl script, but I do need to work fast. I work with lots of different kinds of text data: fixed width files, CSV files, delimited files, xml files...

I'm always looking for slick little time savers.

chunks of my day are spent writing something like this at the command line:

Code:
for i in $(blah)
do
   foo $i | grep bar | sort | uniq -c | grep -v "        1\>"
done
My latest find is process substitution in bash.

here's what the man pages have to say about process substitution:

Code:
   Process Substitution
       Process substitution is supported on systems that support  named  pipes
       (FIFOs)  or the /dev/fd method of naming open files.  It takes the form
       of <(list) or >(list).  The process list is run with its input or  out-
       put connected to a FIFO or some file in /dev/fd.  The name of this file
       is passed as an argument to the current command as the  result  of  the
       expansion.   If the >(list) form is used, writing to the file will pro-
       vide input for list.  If the <(list) form is used, the file  passed  as
       an argument should be read to obtain the output of list.

       When  available,  process substitution is performed simultaneously with
       parameter and variable expansion, command substitution, and  arithmetic
       expansion.
Here's what this means in real life:

I have two text files which are sorted differently, let's call them 'a' and 'b'. I've done a 'wc -l' on the files, and I know that file b is 5 lines longer than a. In the past, I've always had to sort a and b into a couple of temp files, say a.tmp and b.tmp, then diff a.tmp and b.tmp. Using process substitution, I can eliminate the need for the temp files:

Code:
diff <(sort a) <(sort b)
so... I'm fishing around for other people's little time savers...
 
Old 10-26-2008, 03:05 PM   #2
H_TeXMeX_H
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Thanks, I didn't know about this, I'll have to look into it.
 
Old 10-26-2008, 03:32 PM   #3
i92guboj
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I spend quite a lot of time on the shell, and I absolutely love how the linux shells work. Now, I wonder how did I survive my DOS years with such a limited functionality. But well, to the topic.

These are some of the little things I use on my bashrc.

Code:
# Function to add a path
# Usage: add_to_path <path>
add_path () {
  if [ "$PATH" == "${PATH/$1/}" ]
  then
    PATH="$1:$PATH"
    export PATH
  fi
}

# Function to search on the bash history
function recal () {
  if [ -z "$1" ]; then
    echo "Uso: recal <cadena>, para buscar <cadena> en el historial de bash."
  else
    history | grep "$1" | grep -v recal
  fi
}

# Function to rename files to lower case
function lowcase () {
  NEW_NAME="$(echo "$1" | tr '[A-Z]' '[a-z]')"
  if [ ! "$1" == "$NEW_NAME" ]
  then
    mv "$1" "$NEW_NAME"
  fi
}

# Function to join two parts of a movie. Both must be encoded using the same codec,
# otherwise you are going to have problems to play it in anything that's not mplayer.
# This just copies the streams and recalculates the indexes, no re-encoding is done,
# and hence this is quite fast.
function avijoin () {
  local part1="$1"
  local part2="$2"
  local out_f="$3"
  mencoder -oac copy -ovc copy -idx "$1" "$2" -o "$3"
}
That, and a bunch of aliases which saves me some typing.

I also use a lot the AND and OR operators for many purposes, including when I don't want to bother an if...else...fi block for something simple.

Code:
$ [ -h /bin/sh ] && echo "/bin/sh is a symlink" || echo "/bin/sh is not a symlink"
/bin/sh is a symlink
Nice and neat. It saves some typing as well.

Another small trick I use quite a lot is "exec bash", when I want to reload .bashrc or something like that. It will close the actual shell and spawn a new one. That way you don't need to close and login again, or close urxvt and launch it again.
 
Old 10-26-2008, 11:14 PM   #4
bartonski
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Quote:
Originally Posted by i92guboj View Post

Code:
# Function to add a path
# Usage: add_to_path <path>
add_path () {
  if [ "$PATH" == "${PATH/$1/}" ]
  then
    PATH="$1:$PATH"
    export PATH
  fi
}
I also use a lot the AND and OR operators for many purposes, including when I don't want to bother an if...else...fi block for something simple.

Code:
$ [ -h /bin/sh ] && echo "/bin/sh is a symlink" || echo "/bin/sh is not a symlink"
/bin/sh is a symlink
Nice and neat. It saves some typing as well.

Another small trick I use quite a lot is "exec bash", when I want to reload .bashrc or something like that. It will close the actual shell and spawn a new one. That way you don't need to close and login again, or close urxvt and launch it again.
I like the add_to_path function. It's so simple, I had a "why didn't I think of that? moment.

I do use && and || fairly extensively.

When you use "exec bash", does it leave your history intact?

I was looking through the bash man pages today to see if there were any other nice little tidbits in there. I did run across this, which I will use:

Code:
  mkdir  -p {a,b,c}/asdf
This will create three directory trees:

a/asdf

b/asdf

c/asdf

Where I work, we have a production environment 'PROD' and a pre-production environment 'PREP'. I will often want to diff the same file between prep and prod, which have the same directory structure (both mounted at the root directory), using the same feature, I will be able to do this:

diff /{prep,prod}/path/to/some/files
 
Old 10-26-2008, 11:24 PM   #5
i92guboj
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Quote:
Originally Posted by bartonski View Post
I like the add_to_path function. It's so simple, I had a "why didn't I think of that? moment.

I do use && and || fairly extensively.

When you use "exec bash", does it leave your history intact?
It depends on what do you mean. It reloads bash, it's, to all effects, like if you open a new terminal with a new bash session. That means that the history will be reloaded, with all the latest additions.

When you use many shells, what you do on the shell on, let's say, termA, doesn't magically appear in the history for termB, and so on. But if you open a new bash session, then the commands that you ran on termA and termB are available in the history of this new session.

Of course, if you use the exec bash trick, then "exec bash" itself will be added to the history as well


I was looking through the bash man pages today to see if there were any other nice little tidbits in there. I did run across this, which I will use:

Quote:
Where I work, we have a production environment 'PROD' and a pre-production environment 'PREP'. I will often want to diff the same file between prep and prod, which have the same directory structure (both mounted at the root directory), using the same feature, I will be able to do this:

diff /{prep,prod}/path/to/some/files
Yep. Bash is really powerful.

Another thing you might want to look at is into bash string mangling capabilities if you haven't already. That will save you quite a few sed's and stuff in your scripts.

I use an example in the add_to_path function.

http://www.linuxjournal.com/article/8919
 
Old 10-27-2008, 08:02 AM   #6
bartonski
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I haven't played with string mangling yet... I know that it's there, and it's the next thing that I'm going to dig into in the man pages.
 
Old 10-28-2008, 08:32 AM   #7
bartonski
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I started playing with string mangling last night (known as 'Parameter Substitution' in the man pages). Parameter substitution acts as follows: given a variable, let's say '$foo' the parameter substitution will act on or modify the variable.

For instance ${foo:start:len} acts as a sub string operator:

Code:
foo='123456789012345678901234567890'
echo ${foo:15:10}
yields

Code:
6789012345
from the man pages:

Code:
       ${parameter:-word}
              Use  Default  Values.  If parameter is unset or null, the expan-
              sion of word is substituted.  Otherwise, the value of  parameter
              is substituted.
       ${parameter:=word}
              Assign  Default  Values.   If  parameter  is  unset or null, the
              expansion of word is assigned to parameter.  The value of param-
              eter  is  then  substituted.   Positional parameters and special
              parameters may not be assigned to in this way.
       ${parameter:?word}
              Display Error if Null or Unset.  If parameter is null or  unset,
              the  expansion  of  word (or a message to that effect if word is
              not present) is written to the standard error and the shell,  if
              it is not interactive, exits.  Otherwise, the value of parameter
              is substituted.
       ${parameter:+word}
              Use Alternate Value.  If parameter is null or unset, nothing  is
              substituted, otherwise the expansion of word is substituted.
'parameter' is the variable that you're testing, 'word' is subject to tilde expansion, command substitution, and arithmetic expansion.

so:

let's say that $foo is unset.

Code:
$ echo ${foo:-`echo "Hello World"`}
Hello World
now, let's set $foo, and then run the same substitution:

Code:
$ foo="asdf"
$ echo ${foo:-`echo "Hello World"`}
asdf
 
Old 10-28-2008, 05:40 PM   #8
bartonski
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Here's another one that I thought was kind of nice:

I have to wait on people to upload files to me via FTP {ftps, scp, http...} from time to time. Let's say that I know that the file will be named 'Population.txt' or 'population.txt'

Code:
while [ true ]; do [ -f [Pp]opulation.txt ] && echo -e '\0007' ; sleep 2; done
until the file arrives, it just sits there. Minimize the terminal, and it's out of site, out of mind. Then, when the file arrives, it rings the terminal bell every 2 seconds.
 
Old 11-25-2009, 05:19 PM   #9
bartonski
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Quote:
Originally Posted by bartonski View Post
Here's another one that I thought was kind of nice:

I have to wait on people to upload files to me via FTP {ftps, scp, http...} from time to time. Let's say that I know that the file will be named 'Population.txt' or 'population.txt'

Code:
while [ true ]; do [ -f [Pp]opulation.txt ] && echo -e '\0007' ; sleep 2; done
until the file arrives, it just sits there. Minimize the terminal, and it's out of site, out of mind. Then, when the file arrives, it rings the terminal bell every 2 seconds.
I don't use this as much as I did six months ago. If' I'm waiting for a particular condition, I'm more likely to use something like this:

Code:
while ! condition
do
    sleep 2
done; Echo "Blah Blah Blah just happened" | mail bchittenden@foo.com -s "Blah Blah Blah"
This allows me to minimize a terminal window, or even run the whole process in the background, then get an email when the condition is true (note that 'condition' above would be a statement in valid shell syntax which can evaluate to true or false).

If I actually want to see what's going on (for instance watch the size of a file grow), I'll use the 'watch' command, which executes a given command every n seconds (default is 2). It refreshes the screen at every new execution, and generally looks slick.
 
Old 11-25-2009, 05:24 PM   #10
i92guboj
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To monitor files you should look into inotifywatch and inotifywait, part of inotify-tools. Linux-only stuff though. That eliminates the need to do periodical checks.

You could monitor a file or a whole dir for modification and send a mail each time a given file is modified, or moved, or whatever else.
 
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