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File Management

Everything is file in linux.

  • ordinary file - binary/text file
  • directory file - contains filename and there inode number

  • device file

  • pwd - print working directory

  • cd - change directory

  • cd ~ - change to the home directory

  • cd - - cahnge to previous directory(used for toggeling)

  • mkdir - make directory

  • rmdir - remove directory(empty directory)

  • ls - list files

    • Options and flags::
    • -x multicolumnar output
    • -F mark executables with *, directories with /, symbolic links with @
    • -a show all files
    • -R recursive list
    • -r sort in reverse
    • -l long listing
    • -t sort by last modification time
    • -u sort by last access time
    • -i display inode number
    • ls -l - time of last file modification
    • ls -lu - time of last access
    • ls -lc - time of last inode modification

  • mv - moving files

  • rsync - sync files
    • rsync -z|--compress -v|--verbose /foo /bar Copy directory, overwrites destination
    • rsync --ignore-existing -a|--archive-a|--archive -z|--compress -v|--verbose /foo /bar Copy directory, without overwriting destination
    • rsync -avz /foo username@hostname:/bar Copy local directory to remote directory
    • rsync -avz username@hostname:/foo /bar Copy remote directory to local directory
  • rm - remove files
  • inode number - the unique number given to a file
  • ls -i - show inode number
  • Making hard links
    • ln filename new-filename - make hardlink
  • Making soft links (symbolic links)
    • ln -s file new-filename - make softlinks

umask

umask is the number that is subtracted from the default file and directory permissions, to set permission for newly created directory or file. The default permission for directory is 777 and for file is 666. This cannot be changed.

  • umask - show default umask
  • umask no - set umask as no

touch

  • touch filename - creates a new file
  • touch [MMDDhhmm] file - modifies the last time of modification

handling text files

  • cat [filename] - displays the contents of the file
  • cat -v [filename] - displays the non-graphical characters also
  • cat -n [filename] - displays the line number also
  • cp [file] [destination] - copy file to the destination
  • cp -i - interactive copy(provides warning while overwriring)
  • cp -R - recursive copy
  • mv - renames a files
  • rm - remove a file

archiving and compressing

gzip to compress tar to archive and zip to archive and compress

gzip and gnuzip (.gz)

  • gzip -d [filename] - decompress the file
  • gzip -r [filename] - recursive compression
  • gzip -l [filename] - list the compression ratio

tar - archival program (.tar file)

  • -c create an archive
  • -x extract the archive
  • -t view the archive
  • -x extract the files form archive
  • -t displays files in archive
  • -f arch - specifies the archive arch
  • -v verbose mode

tar -cvf [archive name] [files to add...]

now to compress the file use gzip

gzip [archive name] - creates a tar-gzipped file

  • tar -xvf [archive name] - extracts the archive
  • tar -tvf [archive name] - view the archive

Doing archiving and compression together (-z)

tar -cvzf [compressed archive] - create a compressed archive

zip AND unzip - compressing and archiving together (.zip)

  • zip [archive name] [input files] - creates an archive
  • zip -r [archive name] - recursive compressing
  • unzip [archive name] - unzips an archive
  • unzip -v [archive name] - viewing detaill about the compressed file

Basic File Attributes

ls -l - long listing files The columns are as folows::

  • File Type and Permissions
  • Links - the number of links associated with that particular file
  • Ownership - owner of the file
  • Group Ownership
  • File Size in bytes
  • Last Modification Date
  • Filename

File Ownership

  • The user-id (UID)
  • The group-id (GID)

id - view UID and GID associated with particular user

File Permissions

|0|1|2|3|4|5|6|7|8|9|
|d|r|w|x|r|w|x|r|w|x|
  • r - read permission
  • w - write permission

  • x - execute permission

  • 0 - defines the type of file d for directory, - for normal file

  • 1,2,3 - permission granted to owner of file
  • 4,5,6 - permission granted to group owner of file
  • 7,8,9 - permission granted to other users

chmod - used to change the permissions of the file

chmod [category][operation][permission] [filename...]

  • category - u-user, g-group, o-other, a-all(ugo)
  • operation - + assign permission, - remove permission, = absolute permission
  • permission - r, w, x

chmod [ocatal-code] [filename...] - used to assign absolute permission

        a        b        c
|0|  |4 2 1|  |4 2 1|  |4 2 1|
|d|  |r|w|x|  |r|w|x|  |r|w|x|

octal-code - a three digit code for permission of the file

How directory permission affect the file permissions:

  • if the directory has write permission for all than they can delete the files in the directory

Changing file ownership

  • chown [options] [owner] [files...] - change owner of file to owner
  • chgrp [group] [files...] - change group of file to group

file permissions

  • chmod 777 file this octal notation don't show all the flags

  • chmod 0777 file this is the actual octal notation

  • there are three flags more

  • 1 - setuid - s
  • 2 - setgid - s
  • 4 - stickybit - t

setuid

  • setuid
    • is a permission bit flag
    • when setuid flag is set, it allows user to run an executable with the permission of the file owner
  • Do ls /bin/passwd, you will see the permission as .rwsr-xr-x
    • that means the setuid bit is set
    • and other users can also run this file with the permission of the root user

Demonstration

  • shell script can't use the setuid, if you use #!/bin/bash at start of script, bash throws the root power if the effective uid don't match the uid.
  • here is a c program
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>

int main(){
    setuid(0); //sets the uid to that of the root user
    system("ls -lh /srv/setuid/private")
    return 0;
}

how to setuid

  • chmod u+s file_name - +s for the setuid bit
  • chmod 4755 filename - 4 for the setuid bit

setgid

  • enabling setgid for a directory sets the group of all new files created in directory to be the group of the directory

  • chmod g+s filename - set the setgid flag

  • chmod g-s filename - set the setgid flag
  • chmod 2770 filename - set the setgid flag, 2 indicates setgid

stickybit

  • when this bit is set for a directory only the owner of files can remove the files in that directory
  • when set for a directory, the file will be not remove from swap for fast startup times (now not used due to fast secondary storage).
  • chmod 4770 filename - set the sticky bit 4 indicates the sticky bit
  • chmod +t filename - set the sticky bit

ACL - access control list

  • should be turned on to be used, some don't even support it
  • files with ACL permission turned one show a + in the ls -lh
  • getfacl - get file ACL
  • setfacl - set file ACL
    • -m indicates modify
    • setfacl -m g:group_name:permissions files
      • sets the group permissions for the files
    • setfacl -Rm g:group_name:permissions directory
      • sets the permissions recursively to all files inside a directory
    • setfacl -dm g:group_name:permissions files
      • sets the default permissions for the directory
      • it will apply to the newly created files too
    • -k remove default ACL permissions
    • -x remove ACL permissions
    • -Rx remove ACL permissions recursively
    • -Rb remove all ACL permissions recursively