Essential Commands (25%)

Log into graphical and text mode consoles

ssh user@host

Search for files

  • Main commands: find

Find files with depth 3 and size above 2 mb:

find . -maxdepth 3 -type f -size +2M

Find files with permission 777 and remove them:

find /home/user -perm 777 -exec rm '{}' +

Find files based on how many times they have been accessed (atime) or modified (mtime):

find /etc -iname "*.conf" -mtime -180 –print

List a directory with permissions:

ls –la /path/to/files

Find files in directory larger than 100 mb:

find /dir/ -type f –size +100m –depth

Find 100 larges files in a dir:

find . –type f | grep * | ls –li | sort –k1 –r | head –n 101 > biggestfiles.txt

Find files that match ‘rwx’:

find . – type f | ls –l | grep ‘rwx’ > files_that_match.txt

Evaluate and compare the basic file system features and options

  • Main commands: df

Printout disk free space in a human readable format:

df -h

See which file system type each partition is:

df -T

See more details with file command about individual devices:

file -sL /dev/sda1 (or other device)

File system fatures:

  • Ext: "Extended Filesystem". Old, deprecated.
  • Ext2: no journaling, max file size 2TB. Lower writes to disk = good for USB sticks etc.
  • Ext3: journaling (journal, ordered, writeback), max file size 2TB
    • Journaling: file changes and metadata are written to a journal before being committed. If a system crashes during an operation, the journal can be used to bring back the system files quicker with lower likeliness of corrupted files.
  • Ext4: From 2008. Supports up to 16TB file size. Can turn off journaling optionally.
  • Fat: From Microsoft. No journaling. Max file size 4 GB.

See more in Storage Management chapter (insert link)

Compare, create and edit text files

  • Main commands: diff

Compare two files:

diff -u <file1> <file2>

Compare two dirs:

diff -ur <dir1> <dir2>

Create a file:

touch <file>

Edit files:

Editors: vim, nano

In-line editing: sed

Compare binary files

Hex to string/bin conversion: 

xxd <infile> <outfile>

Dump binary file in hex/octal:

od <file> # octal
od -x <file> # hex

Compare files byte by byte:

cmp <file1> <file2>

Use input-output redirection

Output from command to file:

Command > <file>

Append output to file:

Command >> <file>

Output error from command to file:

Command 2> <error-file>

Pipe output from command as input to command:

Command1 | Command 2

Analyze text using basic regular expressions

  • Main commands: grep

Examples:

  • Quantifiers: * 0 or more, + 1 or more, ? zero or 1., single character (.)
  • [a-z]: match any lowercase character
  • \s whitespace or \S not a whitespace
  • \d digit or \D not a digit
  • \w word or \W not a word
  • \^word: search for word at start of line
  • \word$: search for word at end of line
  • Note: remember that special characters needs to be escaped (\)

Search regex with pattern:

grep <pattern> <file>

Find the word text in file.txt:

cat file.txt | grep “text”

Search case-insensitive:

cat file.txt | grep –i “text”

Search two different words with grep:

egrep –w ‘passw0rd’|‘otherpassword’ /etc/passwd

Display only the words containing search term

cat file.txt | grep –w ‘Test’

Archive, backup, compress, unpack, and uncompress files

  • Main commands: tar

To extract an uncompressed archive:

tar -xvf /path/to/foo.tar

To create an uncompressed archive:

tar -cvf /path/to/foo.tar /path/to/foo/

To extract a .gz archive:

tar -xzvf /path/to/foo.tgz

To create a .gz archive:

tar -czvf /path/to/foo.tgz /path/to/foo/

To list the content of an .gz archive:

tar -ztvf /path/to/foo.tgz

To extract a .bz2 archive:

tar -xjvf /path/to/foo.tgz

To create a .bz2 archive:

tar -cjvf /path/to/foo.tgz /path/to/foo/

To extract a .tar in specified Directory:

tar -xvf /path/to/foo.tar -C /path/to/destination/

Create, delete, copy, and move files and directories

  • Main commands: touch, mv, rm, cp
  • Main comands: ln

Hard links points to the contents of the file, while soft links point to the name/location of the file.

Hard link: ln <file> <hard-link>

Soft link: ln -s <file> <soft-link>

List, set, and change standard file permissions

  • Main commands: chmod, chown

Read, and use system documentation

  • Main commands: man, mandb, info, apropos, whatis

See /usr/share/doc

Manage access to the root account

Access root: su

Log on as another user: su <user>

Edit sudoers file: visudo or vim /etc/sudoers

File content structure:

Defaults   secure_path="/usr/sbin:/usr/bin:/sbin" # directories for sudo
root       ALL=(ALL) ALL # ALL hosts, user (root) can run as ALL users with ALL
commands
tecmint    ALL=/bin/yum update # user (tecmint) can run yum as root
gacanepa   ALL=NOPASSWD:/bin/updatedb # user (gacanepa) can run updatedb without
password
%admin     ALL=(ALL) ALL # same as first line, with group (admin)

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