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sept_13.md

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Sept 13 - CS246

Using output of program as arg to another program

  • Eg. echo "Today is $(date) and I am $(whoami)
    • date and whoami are both commands
  • Using double quotes for arguments
    1. echo Today is $(date) and I am $(whoami)
    • This is multiple arguments but they still all get printed
    1. echo "Today is $(date) and I am $(whoami)"
    • Single arg
    1. echo 'Today is $(date)'
    • Output: Today is $(date)
    • Single quotes will suppress embedded commands
    1. echo date
    • It will print the date. date is treated as a command and is evaluated
    • Older syntax, won't be used much

Searching within text files

man grep

  • tool: egrep (grep -E)
  • print every line in files provided as args that contain the regex pattern

Regex additional examples

  • Match zero or one of the precending identifier
    1. To match cs246 and cs 246
    • cs ?246
    1. To match cs246 and 246
    • (cs)?246
  1. Match zero or more of the preceding identifier
  • *
  • Not the same as globbing wildcard
  • (cs)*246
    • Match cs246 cscs246 246
  1. Match any character (only a single char)
  • .
  • To match any number of anything
    • .*
  1. To search for special identifiers you can escape with \
  • \(\) will match ()
  1. Match one or more occurences
  • +
  • sc+ will match sc sccc sccccc but not s
  1. Match the start of the line
  • ^
  • ^tobe will match tobesssss but not sssstobe
  1. Match end of the line
  • $
  • tobe$ will match ddddddtobe$ but not dddtobeddd
  • ^tobe$ will match tobe but not dddtobeddd

Examples

  1. grep searchtext myfile.txt
  • Search for searchtext in myfile.txt
  1. grep "searchtext|Searchtext" myfile.txt
  • Or grep "(s|S)earchtext" myfile.txt
  • Search for searchtext or Searchtext in myfile.txt
  1. grep "[sS]earchtext" mufile.txt
  • Same as above, [] mean to match any char between them
  • [^sS] means to match not s and not S
  1. grep -i "searchtext" myfile.txt
  • Same as above, but -i forces case-insensitivity
  1. Print all words from /usr/share/dict/words that start with an e and consist of 5 characters
  • egrep "^e.{4}$" /usr/share/dict/words
  • egrep "^e....$" /usr/share/dict/words
  1. Lines of even length
  • egrep "^(..)*$" /usr/share/dict/words
  1. Filenames from cur directory whose name contains exactly one a
  • ls | egrep "^[^a]*a[^a]*$"

File Permissions

  • ls -l - long listing
    • Eg.
    -rw-r--r--  1 rethy  staff  2433 Sep 13 15:29 sept_13.md
-rw-r--r--  1 rethy  staff  1351 Sep  9 15:37 sept_6.md
    • see below - shortcut - owner - group - size - date last modified - filename
    • -rw-r--r--
      • first bit - type of file - ordinary or directory (- for ordinary file, d for directory)
      • Nine following bits of three bits each (owner bits - single owner, group bits - all users that belong to the same group to which the file belongs, other bits - all other users not in group)
      • r (read) w (write) x (execute)
    • Ordinary files
      • r read the file, cat the file
      • w modify the contents
      • x can execute the program
    • Directories
      • r ls, tab completion
      • w add/remove files
      • x can search the directory, enter it, navigate into it

Changing permissions

  • chmod {mode} {file}+
    • Add or remove mode to file
    • u (owner) g (group) o (other) a (all three)
    • chmod a+x {file}+ - add executable permission for everyone
    • chmod u-rw {file}+ - remove read/write permission from owner
    • chmod u= {file}+ - remove all permissions from owner (still has permission to re-add these permissions)