CS469/569 - Linux and Unix Administration and Networking

Spring 2022

Mid-term Review

Commands

Command	What it does
passwd	Change your or someone elses password
chfn	Change your finger (identity) information
man	The online manual
ls	List files in a directory
cd	To change your current working directory (.)
cp	Copy files or directories (-r)
mv	Rename or move files / directories
rm	Delete files and / or directories (-r)
mkdir	Make a new directory
rmdir	Remove an empty directory
ssh	Login to a remote system
scp	Copy files to/from a remote system
rsync	Network copy, copying only changed information
sftp	Secure shell like remote copy command
jove/kate	The best Unix editors
vi	The default unix that editor that you have to use sometimes, and then never again
cat	Concatenates files to the output
more/less	Paginates output
grep	Print lines in a file/input that contain a regular expression
mount	Map a device/file-system onto the file-system tree
umount	Unmap a file-system from the file-system tree
df	Display the amount of used space/inodes on a file-system
chmod	Change the permissions on a file
chown	Change the owner of a file
chgrp	Change the group owner of a file
umask	Set/show the default file creation mask
getfacl	Get/show the ACL on a file or files
setfacl	Set/clear the ACL on a file or files
ln	Create a symbolic (-s) or hard link to another file/directory (symbolic only)
find	Searches for files given certain criteria
locate	Searches for files given a location database
ps	Reports a snapshot of current processes
pstree	Tree view of the processes
top / htop	View processes continuously
kill	Send a signal to a process
killall / pkill	Send a signal to a process by name or other information
limit / ulimit	Set/show process resource limits
nice / renice	Set the scheduling priority of a process
ionice	Set the I/O scheduling priority

Scripting commands

Command	What it does
head	Output just the beginning of a file / input
tail	Output just the end of a file / input, follow (-f) a file
touch	Updates the timestamps on files / creates new empty files
readlink	Gets the link destination for a symbolic link
realpath	Gets the real path for a given path (resolves all symbolic links)
sed	Stream editor
true	Always successful
false	Always fails
sort	sort lines in a file or input
uniq	truncates repeated lines
basename	Gives the filename component of a path
dirname	Gives the directory component of a path
tr	Translates or deletes characters
cut	Removes/extracts columns of a file
env	Run a program in a modified environment
date	Print or set date and time

Wildcards

Wildcards are NOT regular expressions

Wild-card	What it represents
~	Your home directory
~username	usernames home directory
`*`	Matches zero or more characters
`?`	Matches a single character
`[...]`	Matches any single character enclosed in the brackets

Regular Expressions

Regular expressions are not wildcards

File-system


/	The root of the file-system directory (not the root account)
.	The current directory
..	The parent directory
.*	A "hidden" file (not normally listed unless -a option used.)

Location	What goes there
`/bin`	Programs that required at boot time
`/boot`	Location of kernel and boot loader files
`/dev`	Location of the devices files
`/etc`	System wide configuration files
`/lib*`	Program libraries needed by programs at boot time
`/proc`	The process and system configuration pseudo-file-system
`/run`	The runtime files for system services
`/sbin`	Programs (usually used by the administrator) needed at boot time
`/sys`	System information pseudo-filesystem
`/tmp`	Temporary files not guaranteed to be preserved across reboots
`/usr`	Read only data not needed at boot time
`/usr/bin`	Most user programs
`/usr/lib*`	Most of the program libraries
`/usr/local`	Tertiary hierarchy for local system additions
`/usr/sbin`	non-essential system programs (daemons)
`/usr/share`	Shared data
`/var`	Variable data that changes during run-time
`/var/log`	Log files (syslogd and klogd)
`/var/mail`	User mailboxes
`/var/tmp`	Temporary files that are guaranteed to be preserved across reboots

Bash

Invocation: #!/bin/bash
chmod a+x ... to make executable
Pipelines: a | b, a |& b, ! a
Command lists: a ; b, a & b, a && b, a || b
if a; then echo "a was successful"; fi
if ! a; then echo "a failed"; fi
Compound commands:
- ( commands ) (subshell)
- { commands } (local shell)
- (( C-expression )) (as a command, evaluates to true/false)
- [ test-expression ]
- [[ test-expression ]]
I/O redirections:
- < file - reads file as input
- > file - outputs to file, overwriting it
- >> file - Appends to file
- &>, &>> - Outputs/appends both stdout/stderr to file
- Heredocs: <<, <<< string, < <( command )
Quoting:
- single quotes - no expansions
- double quotes - variable expansions
Bash data types: strings, integers, arrays and associative arrays
Variable expansions:
- Basic expansion: $foo, ${foo}
- Array size: ${#foo[*]}
- Default: ${foo:-default}
- $(( expression ))
Command expansion:
- $( expression )
- back-ticks
Functions - how to define them, how to make variables local
Loops:
- while program; do statements; done
- for variable; do statements; done - iterates over the positional parameters
- for variable in words; do statements; done - iterates over words
- for (( init ; test ; inc )) { statements } - C style loop
Conditional:
- if command ; then statements ; fi
- if command ; then statements ; else statements ; fi
- if command ; then statements ; elif statements ; else statements ; fi

Bash command	What it does
`let`	Define a integer variable/C like expression
`local`	Makes a variable local to a function
`export`	Make a variable part of the environment
`declare`	To declare variables of different types
`alias`	Make an command alias
`jobs`	List stopped / running jobs
`fg`	Bring a stopped or background job to the foreground
`bg`	Put a stopped job into the background
`echo` / `printf`	Print things to the output
`read`	read data into variables
`source`	read a file into a script

Bash special var	What it does
`$?`	Status of the last command run
`$#`	Number of positional parameters
`$1`, $2...	The positional parameters (the command line arguments )
`$*`, `$@`	Expands to the positional parameters ($1 and up )
IFS	The internal field separator
RANDOM	Random number from 0 to 32767
PATH	The colon delimited string of file-system paths to search for programs in

for var in $*; do .. done - Iterates over the positional parameters from $1 up.

Processes and threads

State	What it represents
R	Running: the process is executing in normal user-space.
S	Interruptible Sleep: The process has either given up it's time slice or been pre-empted by the kernel into giving up it's time.
D	Uninterruptible Sleep: The process is waiting on I/O to complete, usually means it is waiting for a kernel system call to return.
T	Stopped: The process execution has been temporarily suspended by a job control signal (SIGSTOP/SIGTSTP) until it is continued (via SIGCONT)
X/Z	Dead or Zombie: The process has completed and has either been reaped (X - should never actually be seen) or is waiting to be reaped (Z).

Attributes
Memory	How much memory the process is using (VSZ - Virtual Size, RSS - Resident Set Size - non-swapped heap+stack, Size - Heap+Stack
FS info	root FS, cwd, umask, etc
I/O Descriptors	Files, IPC, network objects
Namespace	Hierarchy of mounted file-systems
Signal Handlers	+ Signal dispositions
Credentials	Identifying info about process and parents
Scheduling Priority	Niceness: -19 (highest) - 20 (lowest)
Resource Limits
Processor affinity	How closely tied to specific CPUs/Memory
Additional meta info	Start / CPU time / Current state / CPU running on

Misc

Path resolution
Inode information about files (Owner uid/gid, mode (permissions), file-type, size, times (atime, ctime, mtime))
What each part of 'ls -l' output means
- file types: d, -, l
ACLs
Regular expressions
Signals SIGSTOP, SIGKILL cannot be caught blocked or ignored.
Process resource limits