The super tip for the fg command: when Ctrl+Z saves your terminal session

$ ./gradlew build
> Building...

@startuml
skinparam backgroundColor #FEFEFE

actor Développeur as dev
participant "Terminal" as term
participant "Gradle\n( process )" as gradle

dev -> term : ./gradlew build
term -> gradle : Lance le build
gradle -> term : Occupe le terminal\n(pas de prompt)
dev -> term : veut taper une commande
term --> dev : ❌ Terminal bloqué

note right of dev
  Le développeur est
  paralysé pendant
  toute la durée du build
end note
@enduml

@startuml
skinparam backgroundColor #FEFEFE
autonumber

actor Développeur as dev
participant "Terminal" as term
participant "Gradle" as gradle
database "State\nprocess" as state

dev -> term : ./gradlew build
term -> gradle : Lance le build
gradle -> state : En cours (FG)
dev -> term : Ctrl+Z
term -> state : Suspendu (job [1])
term --> dev : Prompt libre $

dev -> term : git status
term --> dev : working tree clean

dev -> term : fg
term -> state : Reprend (FG)
gradle --> dev : Build réussi ✓

note right of state
  Le processus Gradle
  n'a jamais été tué
  Il reprend exactement
  où il s'était arrêté
end note
@enduml

# Dans vim, en mode commande :
:shell
$ cat /path/to/other/file.txt
$ exit
# Retour dans vim

# Dans vim, n'importe quel moment :
Ctrl+Z
[1]+  Stoppé    vim README.md

$ cat /path/to/other/file.txt
# ... lire le fichier ...

$ fg
vim README.md
# Retour exact là où on en était, curseur en place

@startuml
skinparam backgroundColor #FEFEFE
autonumber

actor Développeur as dev
participant "vim" as vim
participant "Shell" as shell

dev -> vim : édition en cours
dev -> vim : Ctrl+Z
vim -> shell : Suspendu (job [1])
dev -> shell : cat autre_fichier.txt
shell --> dev : contenu du fichier
dev -> shell : fg
shell -> vim : Reprend (job [1])
vim --> dev : Curseur exactement\nà la même position

note right of vim
  vim restaure
  intégralement l'état :
  position du curseur,
  buffer, undo stack
end note
@enduml

@startuml
skinparam backgroundColor #FEFEFE
skinparam defaultFontName sans-serif
skinparam componentStyle rectangle

actor Développeur as dev
participant "opencode\n(LLM agent)" as agent
participant "Shell" as shell
participant "git" as git

dev -> agent : Session active
agent -> agent : Tour de parole\ndu LLM en cours

note over agent
  Ctrl+Z arrive
  en plein milieu
  d'une analyse de code
end note

dev -> agent : Ctrl+Z
agent -> shell : Suspendu (job [1])
dev -> git : git diff
git --> dev : changements visibles
dev -> shell : fg
shell -> agent : Reprend (job [1])
agent --> dev : Reprend exactement\nl'analyse en cours

note right of agent
  Aucune perte de contexte
  Le LLM reprend sa réponse
  exactement où il en était
end note
@enduml

$ opencode
# ... session en cours, LLM analyse le code ...
^Z
[1]+  Stoppé    opencode

$ git diff HEAD~1
# vérifier les changements récents

$ fg
opencode
# Le LLM reprend là où il s'était arrêté

$ vim config.yml
^Z
[1]+  Stoppé    vim config.yml

$ ./gradlew test
^Z
[2]+  Stoppé    ./gradlew test

$ htop
^Z
[3]+  Stoppé    htop

$ jobs
[1]   Stoppé    vim config.yml
[2]-  Stoppé    ./gradlew test
[3]+  Stoppé    htop

$ fg %2
./gradlew test
# Le build reprend en avant-plan

$ bg %3
[3] htop &
# htop reprend en arrière-plan

@startuml
skinparam backgroundColor #FEFEFE

rectangle "Suspended jobs" as jobs #LightYellow {
  card "[1] vim config.yml" as j1
  card "[2] ./gradlew test" as j2
  card "[3] htop" as j3
}

rectangle "Actions" as actions #LightGreen {
  card "fg %2\n→ foreground" as a1
  card "bg %3\n→ background" as a2
  card "fg %1\n→ foreground" as a3
}

jobs --> actions : fg/bg + numéro de job
actions --> j2 : Reprend
actions --> j3 : En arrière-plan

note bottom of jobs
  Chaque job conserve
  son état exact
  entre les suspend/resume
end note
@enduml

@startuml
skinparam backgroundColor #FEFEFE
skinparam defaultFontName sans-serif

rectangle "**Ctrl+Z**\nSIGTSTP" as ctrlz #LightYellow {
  card "Suspend the process\n(it is still in memory)\nCan be used with fg/bg" as c1
}

rectangle "**Ctrl+C**\nSIGINT" as ctrlc #LightCoral {
  card "Kill the process\n(no return possible)\nLoss of all state" as c2
}

rectangle "**& **\n(background)" as ampersand #LightGreen {
  card "Launch directly\nin the background\nTerminal free immediately" as c3
}

rectangle "**fg**\n(foreground)" as fg_ #SkyBlue {
  card "Bring a suspended job\nto the foreground\nTerminal busy" as c4
}

rectangle "**bg**\n(background)" as bg_ #Lavender {
  card "Resumes a job\nsuspended in the background\nFree terminal" as c5
}

ctrlz -right-> fg_ : fg pour reprendre
ctrlz -right-> bg_ : bg pour continuer\nen arrière-plan
ctrlc -right-> c2 : Pas de retour\nprocessus tué

note bottom of ctrlz
  Le processus est **figé**
  mais **pas tué**
end note

note bottom of ampersand
  Alternative : lancer
  ./gradlew build &
  directement en arrière-plan
end note
@enduml

@startuml
skinparam backgroundColor #FEFEFE
skinparam defaultFontName sans-serif

title Complete lifecycle of a shell job
state "Non-existent" as none
state "Foreground\n(occupies the terminal)" as foreground #LightCoral
state "Suspended\n(Ctrl+Z / SIGTSTP)" as suspended #LightYellow
state "Background\n(bg or &)" as background #LightGreen
state "Done\n(exit code 0)" as done #LightGray

[*] --> none
none --> foreground : tape une commande
foreground --> suspended : Ctrl+Z
foreground --> done : fin normale

suspended --> foreground : fg
suspended --> background : bg
suspended --> done : kill %n

background --> foreground : fg
background --> done : fin normale\n(en arrière-plan)
background --> suspended : Ctrl+Z\n(si fg puis Ctrl+Z)

done --> [*]

note right of suspended
  L'état suspendu est
  le pivot de tout le
  système de jobs
end note
@enduml

@startuml
skinparam backgroundColor #FEFEFE
skinparam defaultFontName sans-serif
skinparam ArrowColor #444444

title "fg — The tip that changes everything"

rectangle "**Problem**" as problem #LightCoral {
  card "Terminal blocked\nby a process" as p1
  card "Opening a new\nterminal = heavyweight solution" as p2
}

rectangle "**Solution**" as solution #LightGreen {
  card "Ctrl+Z → suspend" as s1
  card "fg → resumes" as s2
  card "bg → in the background" as s3
  card "jobs → see suspended" as s4
}

rectangle "**Advantages**" as advantages #SkyBlue {
  card "No loss of context" as a1
  card "No restart" as a2
  card "A single terminal is enough" as a3
}

problem -right-> solution : "The native solution\nUnix for 40 years"
solution -right-> advantages : "Result"

note bottom of advantages
  fg n'est pas un tips obscur
  C'est un mécanisme fondamental
  du shell Unix
end note
@enduml

@startuml
skinparam backgroundColor #FEFEFE
skinparam defaultFontName sans-serif

rectangle "Why is fg\nunderestimated?" as why #LightCoral {
  card "IDE with multi-tab terminal" as w1
  card "Graphical tools\nthat replace the CLI" as w2
  card "tmux/screen\nsolve the problem\ndifferently" as w3
}

rectangle "Why is fg\nsuperior?" as superior #LightGreen {
  card "Universal: bash,\nzsh, fish, sh..." as s1
  card "No dependencies:\nno installation" as s2
  card "Two keystrokes:\nCtrl+Z then fg" as s3
  card "Preserved context:\nzero state loss" as s4
}

why -right-> superior : "Simplicity\nalways wins"
@enduml

@startuml
skinparam backgroundColor #FEFEFE
skinparam defaultFontName sans-serif

participant "User" as user
participant "Shell" as shell
participant "Process" as proc

user -> shell : Ctrl+Z
shell -> proc : SIGTSTP (signal 18)
note right of proc : Processus figé\nen mémoire

user -> shell : fg
shell -> proc : SIGCONT (signal 19)
note right of proc : Processus reprend\nen avant-plan

user -> shell : bg
shell -> proc : SIGCONT (signal 19)
note right of proc : Processus reprend\nen arrière-plan

user -> shell : Ctrl+C
shell -> proc : SIGINT (signal 2)
note right of proc : Processus terminé\nétat perdu
@enduml