Bash scripting course notes post

Shell/bash course

https://urfist.unistra.fr/uploads/media/command_memento_fr.pdf

BSD: berkely univsersity. public/research
system V: private, constructors: DEC, SUN
POSIX: standard on all system

Change shell

usermod -s /bin/bash formation
chsh

[root@localhost ~]# useradd ben
[root@localhost ~]# passwd ben
Changement de mot de passe pour l'utilisateur ben.
Nouveau mot de passe : 
MOT DE PASSE INCORRECT : Le mot de passe comporte moins de 8 caractères
Retapez le nouveau mot de passe : 
passwd : mise à jour réussie de tous les jetons d'authentification.
[root@localhost ~]# usermod -s /bin/ksh ben
[root@localhost ~]# tail -4 /etc/passwd
tcpdump:x:72:72::/:/sbin/nologin
formation:x:1000:1000:Formation:/home/formation:/bin/bash
vboxadd:x:988:1::/var/run/vboxadd:/bin/false
ben:x:1001:1001::/home/ben:/bin/ksh

Switch user: su

Make use of the - to get user env [root@localhost ~]# su - ben Dernière connexion : lundi 7 octobre 2019 à 09:46:27 CEST sur pts/0

Text shell

ctrl + alt + F1 => graphical shell mode
ctrl + alt + F[2-6] => text shell mode

File type

- file
d directory
l link
c char I/O hardware
b block I/O hardware
p pipe : inter process (local)
s socket : inter process (network)

tty

terminal teletype

[formation@localhost ~]$ tty
/dev/pts/0
[formation@localhost ~]$ echo "hello"
hello
[formation@localhost ~]$ echo "hello" > /dev/pts/1

Env var

available in:

new window
new shell
new script

env: list env var set: env + pre param shell

File command

[formation@localhost ~]$ file /bin/ls
/bin/ls: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.32, BuildID[sha1]=c5ad78cfc1de12b9bb6829207cececb990b3e987, stripped
[formation@localhost ~]$ file foo.pdf 
foo.pdf: ASCII text
[formation@localhost ~]$ mv foo.pdf foo.md
[formation@localhost ~]$ file foo.md 
foo.md: ASCII text

special bits

setuid

when present on an executable file, the programm is executed with the identity of the file owner

[formation@localhost ~]$ which passwd 
/usr/bin/passwd
[formation@localhost ~]$ ls -l /usr/bin/passwd 
-rwsr-xr-x. 1 root root 27832 10 juin   2014 /usr/bin/passwd

chmod 4555 prog.sh 
[formation@localhost ~]$ ls -l prog.sh 
-r-sr-xr-x. 1 formation formation 0  7 oct.  10:56 prog.sh

setgid

idem for group

sticky bit: t

only owner can destroy folder (add resistance to destruction).

ls -ld /tmp/
drwxrwxrwt. 15 root root 4096  7 oct.  10:59 /tmp/

Groups

create first group then create user, and assign group

groupadd friends
groupadd painters

useradd foo -g firends -G painters

$PATH

contains paths of executables path

PATH=$PATH:/new/location/exec

find command

find /dev - type l -name "*or*" -exec file {} \;

locate command

locate file2Search updatedb

Command IO

stdin: <1
stdout: >1
stderr: >2

2>&1: recirect stderr to stdout 1>&2: redirect stdout to stderr

ls -l file 1> out
ls -l file doesNotExist 2> error 1>out
ls -l file &> out

tree

tree of files

pstree

tree of ps

ps clax
ps -ef

bash debug mode

bash -vx script.sh

command return code

$? == 0 ok $? != 0 ko

logical operators

AND &&

if left part is OK

ls && echo "ls done"

OR ||

if left part fails

ls -l inexistant || echo "inexistant file does not exist"

Together

concatenate if exists,else copy

ls -l mypasswd && cat /etc/passwd >> mypasswd || cp /etc/passwd mypasswd



cut -d":" -f1,3 /etc/passwd | tr ':' ' ' > users-id

Terminal movement utilities

ctrl+a ctrl+e alt+d

Vim

filetype indent plugin on gg=G => indent all file

source VS ./

source script: execute in current shell ./script: execute in a child shell

nohut

run a script whithout deconnexion nohup ./script.sh &

exec command

exec date print date and kill exit current process

signals

ctrl + c:
ctrl + z:
ctrl + d: When killing a parent process with child process, children are adopted by systemd PID #1. example eyes & ctrl+d ps aux eyes has PPID=1

jobs command

foreground + ctrl-c

[formation@localhost ~]$ xeyes &
[1] 19078
[formation@localhost ~]$ jobs
[1]+  En cours d'exécution   xeyes &
[formation@localhost ~]$ fg %1
xeyes
^C
[formation@localhost ~]$

kill it

[formation@localhost ~]$ xeyes &
[1] 19207
[formation@localhost ~]$ kill %1
[1]+  Complété              xeyes

trap

[root@localhost ~]# trap -l
 1) SIGHUP   2) SIGINT   3) SIGQUIT  4) SIGILL   5) SIGTRAP
 6) SIGABRT  7) SIGBUS   8) SIGFPE   9) SIGKILL 10) SIGUSR1
11) SIGSEGV 12) SIGUSR2 13) SIGPIPE 14) SIGALRM 15) SIGTERM
16) SIGSTKFLT   17) SIGCHLD 18) SIGCONT 19) SIGSTOP 20) SIGTSTP
21) SIGTTIN 22) SIGTTOU 23) SIGURG  24) SIGXCPU 25) SIGXFSZ
26) SIGVTALRM   27) SIGPROF 28) SIGWINCH    29) SIGIO   30) SIGPWR
31) SIGSYS  34) SIGRTMIN    35) SIGRTMIN+1  36) SIGRTMIN+2  37) SIGRTMIN+3
38) SIGRTMIN+4  39) SIGRTMIN+5  40) SIGRTMIN+6  41) SIGRTMIN+7  42) SIGRTMIN+8
43) SIGRTMIN+9  44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12 47) SIGRTMIN+13
48) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14 51) SIGRTMAX-13 52) SIGRTMAX-12
53) SIGRTMAX-11 54) SIGRTMAX-10 55) SIGRTMAX-9  56) SIGRTMAX-8  57) SIGRTMAX-7
58) SIGRTMAX-6  59) SIGRTMAX-5  60) SIGRTMAX-4  61) SIGRTMAX-3  62) SIGRTMAX-2
63) SIGRTMAX-1  64) SIGRTMAX

Loop

sequence command

seq 5
1
2
3
4
5

for i in $(seq 5); do echo $i; done

Return code

$? value ok: 0

shift: process & remove first value of array parameter shift.sh

#!/bin/bash
while test $# -ne 0
do
    echo "current paramater: $1 - nb remaning paramter "$#
    shift 1
done

bash shift.sh aa bb cc

current paramater: aa - nb remaning paramter 3
current paramater: bb - nb remaning paramter 2
current paramater: cc - nb remaning paramter 1

arithmetic expression : (( expr ))

echo $(( 2+2+6+6 ))
16

((i=i+1))

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