Week 4 #

Shell Scripting #

Running Scripts #

$ sh myscript

Alternatively, in your myscript.sh file, do the following

#! /bin/sh
chmod u+x myscript # sets 'executable' flag on the file

and run it with…

$ ./myscript

Positional Parameters #

$ ./myscript foo bar baz # come after filename, spaced

• $#: Number of arguments (3)
• $n: Parameter name (n is the number)
  • eg. $1 = "foo"
• $*: One string with all parameter names
  • "foo bar baz"
• $@: Comma seperated strings for each parameter name
  • "foo", "bar", "baz"
• shift: Shifts Positional Parameters by 1

  $# = 2
  $1 = "bar"
  $2 = "baz"
  $* = "bar baz"
  # good for looping over parameters
  

Example Script #

#! /bin/sh
chmod u+x pydelete # sets exec flag on pydelete.sh

dryrun=
verbose=
while [ $# -gt 0 ]; do # while the number of params > 0
  case "$1" in # takes the first parameter/argument
    -n) # if the argument is -n
      dryrun=y # sets the program as a dryrun "doesnt actually delete files"
      ;;
    -v)
      verbose=y # sets the program as verbose "deletes verbosely"
      ;;
    *) # as soon as there is another different parameter, the while loop breaks
      break
  esac # end of case
  shift # basically $# -= 1
done # end of while

for f in "$@" ; do # for each parameter (after remove the arguments)
  case "$f" in # begins case
    *.py) # if the file is .py extension
      [ -n "$verbose" ] && echo "deleting $f" # if user used `-v`
      [ -z "$dryrun" ] && rm "$f" # if user did not use `-n`, then it will delete
      ;;
    *) # any other file extension
      [ -n "$verbose" ] && echo "not deleting $f" # if the user set verbose
  esac # end of case
done # end of for

exit #

• Terminates script and/or shell process
• Useful for:
  • Early exit
  • Non-zero exit codes: exit 1

Functions #

• Example definition

  myfunc() {
    echo "Hot diggity dog"
  }
  

• Example function call

  myfunc foo bar baz # positional parameters are now function arguments
  

• Returning

  return    # early return
  return 1  # with exit code
  

• Variable Scopes
  • Local variables

    myfunc() {
      local x y z
      x=69
      y=351
      z=$(expr $x + $y)
      echo $z
    }
    

  • Dynamic Scoping

    $ cat myscript
    x=0
    printnum() {
      local x
      echo $x
    }
    func1() {
      local x
      x=1
      printnum
    }
    func2() {
      local x
      x=2
      printnum
    }
    func1 ; func2
    $ sh myscript
    1 # from func1
    2 # from func2
    

Feeding multi-line text into stdin #

$ cat << EOF # could be any string
﹥Hi I'm Navinn
﹥Variables also work \$x=$x
﹥EOF # tells shell its the end of file
Hi I'm Navinn
Variables also work $x=42069

• if end-marker is declared in quotes, $ is no longer special.

Command Substitution #

• Run a command and take its stdout in-place

  for i in $(cat myfile) ; do 
  # if wrapped in double-quotes, `cat myfile` is just one string instead of multiple
    echo "$i"
  done
  

Environment Variables #

• Every process has a collection of environment variables
  • eg. PATH, CLASSPATH, USER, PWD, PS1 etc.
  • Convention is all caps
  • Same syntax: eg. $PATH, PATH=...
• printenv: prints current env. vars.

  $ printenv
  HOME=/Users/home
  PWD=/Users/home/desktop
  PATH=/usr/local/cms/jdk1.8.0_31/bin:/usr/bin:/bin # colon-seperated list of directories
  ANDROID_HOME=/Users/home/Library/Android/sdk
  SHELL=/bin/bash
  ...
  

• Creating a new env. var. is different

  LOGNAME=navn          # same init
  export LOGNAME        # different
  export LOGNAME=navinn # alt
  

File Attributes #

General Attributes #

• Size
• Type
• Last modified time
• Last access time
• Last change time
• Owning user
• Owning group …

Unix Account Organization #

Two main accounts: user, groups

• user: the user (home, navinn etc.)
• groups: A user can be in multiple groups
  • groups command displays which groups the current user is in

  $ groups
  staff everyone localaccounts _appserverusr admin # ...
  

Permission Flags #

• Each file is assigned one owning user and one owning group. Default is the user who created it and their default group

  • Permissions: take the form rwxrwxrwx
    • r: read
    • w: write
    • x: execute
  • File permission notation

    $ ls -l
    rwxr-x--x 1 home  staff  73966  1 Jun 23:07  myscript.sh
    # perms.    user  group  bytes  date_created name
    # rwxr-x--x: user gets `rwx`, users in group get `x`, other users get `x` 
    

• For directories:

  • r: list files in the dir ls
  • w: add, delete, modify files in the dir
  • x: cd into dir., use paths mentioning dir

• To Check Perms on a file/dir:

  [ -r path ] # exists and readable by current user
  [ -w path ] # exists and writable
  [ -x path ] # exists and executable
  

• Changing Ownership/Permission

chown user path1 path2        # changes the owning user for both paths
chown user:group path1 path2  # changes owning user and owning group
chown :group path1 path2      # changes group
chmod u=rw,g=r,o= path1 path2 # changes specific perms (u=user, g=group, o=other)

find #

Literally finds a path and operates on selected files with automatic recursion

$ find path ...expression

For each given path, recurse down and pick out a file based on the expression

eg. Find python files, print their path, then delete

$ find . -name ’*.py’ -exec rm ’{}’ ’;’ -print