Bash: Let me bash that for you, Part 2

Is your buffalo wings so spicy ?

Lets juzz it up a bit more with bash !

other parts of «let me bash that for you» Part 0 and Part I

• var=
  creates a variable and assigns some value (e.g result command execution) to it for its future usages.
  Note: use a dollar sign ($) to reference a variable's value
  

  $ foo="FOO VARIABLE" ; echo "$foo $foo"
  FOO VARIABLE FOO VARIABLE

• export variable=value
  export the variable value for the future usage
  

  $ export A=5
  $ echo $A
  5

• set
  prints out all known (assigned) variables (embedded functions)
  
  Note: The list of the most commonly used and significant variables
  

  HOME home directory of the user, usually /home/username
  PATH paths to look for executable commands
  LD_LIBRARY_PATH - paths to look for dynamic shared libraries an application has been linked against
  PWD (actual) working directory
  OLDPWD old working directory
  JAVA_HOME usually sets as java (jdk) directory
  
  USER current user login name
  UID current user's user identification number, as recorded in /etc/passwd
  HOSTNAME the name of the current host
  
  
  TMOUT if set to a value greater than zero, bash terminates (log out) after that number of seconds if input does not arrive
  
  
  $? exit value of last executed command.
  $! pid of last background command.
  
  
  $0, $1, $2, etc - positional parameters, passed from command line to script, passed to a function
  $*, $@ all parameters passed from command line to script ($1 $2 etc)
  $# number of command-line arguments or positional parameters
  
  
  $$ pid of the current instance of shell
  PPID pid of parent process ( of the current instance of shell )
  

• unset variable
  discards the variable
  
• `command` and $(command)
variable assignment mechanism though a result of command execution

$ NOW=`date +"%m-%d-%Y"`; echo $NOW
03-16-2013
$ NOW2=$(date +"%m-%d-%Y"); echo $NOW2
03-16-2013

• df
  reports disk space capacity/usage
  

  $ df
  Filesystem     1K-blocks     Used Available Use% Mounted on
  rootfs          24776880 19823040   3715004  85% /
  /dev/root       24776880 19823040   3715004  85% /
  devtmpfs         1551036        0   1551036   0% /dev
  tmpfs            1551228     2720   1548508   1% /run
  cgroup_root        10240        0     10240   0% /sys/fs/cgroup
  tmpfs             540672     5156    535516   1% /dev/shm
  /dev/sda2       31814768 29868180    355852  99% /home
  

  Hint: specifying path get space usage of corresponding file system
  

  $ df ~/workspace/
  Filesystem     1K-blocks     Used Available Use% Mounted on
  /dev/sda2       31814768 29868180    355852  99% /home
  

  Hint: option -h prints in human readable format
  

  $ df -h ~/workspace/
  Filesystem      Size  Used Avail Use% Mounted on
  /dev/sda2        31G   29G  348M  99% /home
  

  Hint: use . (dot) to reffer actual file system
  

  $ df -h .
  Filesystem      Size  Used Avail Use% Mounted on
  /dev/sda2        31G   29G  348M  99% /home
  

  
  
• du
  estimates file space usage

  $ du myfile
  4 myfile
  $ du -h ~/workspace/docs/cs/java/langspec-3.0.pdf 
  7.8M /home/user/workspace/docs/cs/java/langspec-3.0.pdf
  

  Hint: option -s displays summarized info
  

  $ du -hs ~/workspace/docs/cs/java/
  25M /home/user/workspace/docs/cs/java/
  

• file determines a file type by its content
  

  $ file ~/workspace/docs/cs/java/langspec-3.0.pdf
  /home/bob/workspace/docs/cs/java/langspec-3.0.pdf: PDF document, version 1.3
  $ cp ~/workspace/docs/cs/java/langspec-3.0.pdf /tmp/xxx
  $ file /tmp/xxx
  /tmp/xxx: PDF document, version 1.3
  $ bzip2 /tmp/xxx
  $ file /tmp/xxx.bz2 
  /tmp/xxx.bz2: bzip2 compressed data, block size = 900k
  

• which
shows the full path of command, it looks for it a command within PATH environment variable:

$ which bash
/bin/bash
$ which java
/opt/sun-jdk-1.6.0.43/bin/java
$ which ls  
/bin/ls

• ldd - print shared library dependencies
  

  $ ldd /opt/sun-jdk-1.6.0.43/bin/java
   linux-gate.so.1 (0xffffe000)
   libpthread.so.0 => /lib/libpthread.so.0 (0xb76d4000)
   libjli.so => /opt/sun-jdk-1.6.0.43/bin/../jre/lib/i386/jli/libjli.so (0xb76cb000)
   libdl.so.2 => /lib/libdl.so.2 (0xb76c7000)
   libc.so.6 => /lib/libc.so.6 (0xb754a000)
   /lib/ld-linux.so.2 (0xb7713000)
  $ ldd `which sh`
   linux-gate.so.1 (0xffffe000)
   libreadline.so.6 => /lib/libreadline.so.6 (0xb7730000)
   libncurses.so.5 => /lib/libncurses.so.5 (0xb76e5000)
   libdl.so.2 => /lib/libdl.so.2 (0xb76e1000)
   libc.so.6 => /lib/libc.so.6 (0xb7564000)
   /lib/ld-linux.so.2 (0xb7794000)
  

That's fork bomb, dudes:

:(){ :|:& };: