Bash Index : S - The 'S' Bash commands : description, flags and examples

● smbd.service - Samba SMB Daemon
	Loaded: loaded (/lib/systemd/system/smbd.service; enabled; vendor preset: enabled)
	Active: active (running) since Thu 2018-11-29 08:36:29 CET; 1 weeks 0 days ago
	

dev-mapper-hostname\x2d\x2dvg\x2dswap_1.swap: Unit is bound to inactive unit dev-mapper-hostname\x2d\x2dvg\x2dswap_1.device. Stopping, too.
var.mount: Unit is bound to inactive unit dev-mapper-hostname\x2d\x2dvg\x2dvar.device. Stopping, too.
tmp.mount: Unit is bound to inactive unit dev-mapper-hostname\x2d\x2dvg\x2dtmp.device. Stopping, too.

halt

display a stop screen like System halted. At that step, it is safe to press the physical Power off button.

power-off

send an ACPI command to signal the PSU to disconnect main power.

• L : the corresponding stream will be line buffered. This option is invalid with standard input.
• 0 : the corresponding stream will be unbuffered

tail -f access.log | stdbuf -oL cut -d ' ' -f1 | uniq

journalctl -u ssh | cut -d ' ' -f6- | uniq
journalctl -u ssh | stdbuf -oL cut -d ' ' -f6- | uniq

	==> both _seem_ to behave the same. Maybe "journalctl" is not a good candidate to experiment on this ;-)

1. script /path/to/typescript/file
2. the commands you type and their output is recorded into /path/to/typescript/file but not recorded in real time, unless -f is used
3. stop recording :
   • exit
   • CTRL-d
4. You can now view the typescript :
   • cat /path/to/typescript/file
   • less -R /path/to/typescript/file

While debugging, it may be useful to prefix systemctl invocation with date :

systemctl status mysql

	● mysql.service - MySQL Community Server
	   Loaded: loaded (/lib/systemd/system/mysql.service; enabled; vendor preset: enabled)
	   Active: active (running) since Fri 2019-11-29 15:10:38 UTC; 2 days ago
	  Process: 1111 ExecStartPost=/usr/share/mysql/mysql-systemd-start post (code=exited, status=0/SUCCESS)
	  Process: 1050 ExecStartPre=/usr/share/mysql/mysql-systemd-start pre (code=exited, status=0/SUCCESS)
	 Main PID: 1110 (mysqld)

ps -o etime= 1110
	 2-17:50:46

==========================================
systemctl show mysql --property=MainPID | cut -d '=' -f 2
ps -o etime= $(systemctl show mysql --property=MainPID | cut -d '=' -f 2)
	 2-18:44:12

==========================================
systemctl show mysql --property=ActiveEnterTimestamp
	ActiveEnterTimestamp=Fri 2019-11-29 15:10:38 UTC

==========================================
ps -o lstart= $(systemctl show mysql --property=MainPID | cut -d '=' -f 2)
	Fri Nov 29 15:10:36 2019

echo $(($(date -d"now" +%s) - $(date -d"Fri Nov 29 15:10:36 2019" +%s)))

echo $(($(date -d"now" +%s) - $(date -d"$(ps -o lstart= $(systemctl show mysql --property=MainPID | cut -d '=' -f 2))" +%s)))
	==> uptime in seconds

• Read more about this
• Workaround :
  sudo su -s /bin/bash

as a member of the root group + su + right password

logged as root, as expected

as a member of the root group + su + wrong password

su: Authentication failure

not as a member of the root group + su + any password (right or wrong makes no difference)

su: Permission denied

This behavior is caused by this uncommented line of /etc/pam.d/su which forces users to be a member of group root before they can use su :

auth	required	pam_wheel.so	group=root

http://ss64.com/bash/su.html
	-, -l, --login
	Make the shell a login shell. This means the following:
		unset all environment variables except `TERM', `HOME', and `SHELL' (which are set as described above),
		and `USER' and `LOGNAME' (which are set, even for the super-user, as described above)
		set `PATH' to a compiled-in default value.
		Change to USER's home directory.
		Prepend `-' to the shell's name, intended to make it read its login startup file(s).


some tests as root :

su bob -c "whoami"
	bob

su bob -c "whoami; echo $HOME"
	bob	==> Expected, as we didn't change the environment.
	/root

su - bob -c "whoami; echo $HOME"
	bob
	/root ==> I expected /home/bob here. What happened ?

/!\ this _could_ be caused by the fact that logging as root was ALREADY made via "su" (?)

su bob -c 'whoami; echo $HOME'		==> changed to single quotes
	bob
	/home/bob

1. set -- whatever
   This construct actually turns whatever into positional parameters. whatever can be :
   • a list of files : file1 file2 file3
   • a shell expansion corresponding to a list of files : /path/to/*jpg
   • the result of a command : $(ls -1)
2. You can then "read" these as if they were previously sent as parameters :

   set -- $(ls -1); echo $1 $2 $3

   file1 file2 file3		3 files of the current directory

   set -- $(ls -1); echo $@

   ... ... ... ...		all of them

State filters :

Address filters :

strerror
%s: I/O error
error setting limit (%s)
Syntax error: %s

• someFile is executed even though its execution bit is not set
• someFile is executed within the current shell context, which allows :
  • loading variables into the current interactive shell session :
    1. source myConfigFile
    2. other commands using variables
  • using someFile as a configuration file for a script, without leaving variables in the shell environment once the script is over :
    sourcedFile=$(mktemp sourcedFile.XXXXXXXX); echo 'value=42' > "$sourcedFile"; chmod -x "$sourcedFile"; scriptFile=$(mktemp scriptFile.XXXXXXXX); echo "source $sourcedFile; echo 'sourced'; echo \"during the script : \\\$value='\$value'\"" > "$scriptFile"; chmod +x "$scriptFile"; ls -l "$sourcedFile" "$scriptFile"; cat "$sourcedFile" "$scriptFile"; ./"$scriptFile"; echo "after the script : \$value='$value'"; rm "$sourcedFile" "$scriptFile"

    value=42	cat "$sourcedFile"
    source sourcedFile.akr6paIl; echo 'sourced'; echo "during the script : \$value='$value'"	cat "$scriptFile"
    sourced		started running the script
    during the script : $value='42'	the variable exists in the script context
    after the script : $value=''	no value anymore

    Since a dedicated subshell (having its own context) is spawned when executing a script, if you want variables to survive for future scripts or commands after the script ends, you'll have to :

    • export variables within the script (details)
    • or source the script
• The return status is the exit status of the last command executed from someFile, or zero if no commands are executed.

Trying to source a file having DOS line endings led the shell to complain for syntax errors on EVERY line. Consider converting line endings into DOS format.

configFile='./script.conf'; scriptFile='./script.sh'; echo -e '#!/usr/bin/env bash\nvar1=value1\nexport var2=value2' > "$configFile"; echo -e '#!/usr/bin/env bash\necho "\tvar1 = $var1"\necho "\tvar2 = $var2"' > "$scriptFile"; echo -e "\n'source'd config file :"; cat "$configFile"; source "$configFile"; echo -e "\nCommand line (current shell context) :\n\tvar1 = $var1\n\tvar2 = $var2\n\nScript (subshell context) :"; bash "./$scriptFile"; rm "$configFile" "$scriptFile"

'source'd config file :
#!/usr/bin/env bash
var1=value1			not exported
export var2=value2

Command line (current shell context) :
	var1 = value1		exists 
	var2 = value2

Script (subshell context) :
	var1 =			unset 
	var2 = value2

• script.conf :

  variable='value'

• script.sh :

  #!/usr/bin/env bash
  
  source script.conf
  
  ... do something with "$variable" ...

Then no shebang is required in the configuration file script.conf. It may also be a safe practice to remove its execution permissions.

grep ' 500 ' 2012-07-03-apache-access.log | cut -d ' ' -f 11 | sort | uniq -c | sort -nr | less

All the magic is in the uniq -c prior to sorting.

1. the kernel supports them : zgrep POSIX_ACL /boot/config* (used to be /proc/config.gz), which may output :

   CONFIG_EXT2_FS_POSIX_ACL=y
   CONFIG_EXT3_FS_POSIX_ACL=y
   CONFIG_EXT4_FS_POSIX_ACL=y
   CONFIG_REISERFS_FS_POSIX_ACL=y
   CONFIG_JFS_POSIX_ACL=y
   CONFIG_XFS_POSIX_ACL=y
   CONFIG_BTRFS_FS_POSIX_ACL=y
   CONFIG_FS_POSIX_ACL=y
   CONFIG_TMPFS_POSIX_ACL=y
   CONFIG_JFFS2_FS_POSIX_ACL=y
   CONFIG_9P_FS_POSIX_ACL=y

2. the filesystem we're about to play with is mounted with the acl flag (which enables support of the ACL. Just in case : noacl does the opposite).
   Have a look at /etc/fstab. (details : mount man page)

• prefix :
  • u: to change user rights
  • g: to change group rights
  • o: to change other rights. No need to specify userOrGroup
  • d:u: to declare default user rights
  • d:g: to declare default group rights
  • d:o: to declare default other rights
• rights : specified with r, w, x or - (=not granted), exactly like in chmod.

• [ + ] : service is running
• [ - ] : service is stopped
• [ ? ] : unknown / doesn't reply to the "service status" command