fg

Overview

The fg command brings background or suspended jobs to the foreground. It’s essential for job control, allowing you to interact with processes that are running in the background or have been suspended.

Syntax

fg [job_spec]

Job Specification

Format Description
%n Job number n
%string Job whose command begins with string
%?string Job whose command contains string
%% or %+ Current job (default)
%- Previous job

Key Use Cases

  1. Bring background jobs to foreground
  2. Resume suspended processes
  3. Interactive job control
  4. Process management
  5. Terminal multitasking

Examples with Explanations

Example 1: Bring Current Job to Foreground

fg

Brings the most recent job to foreground

Example 2: Bring Specific Job

fg %1

Brings job number 1 to foreground

Example 3: Bring Job by Command Name

fg %vim

Brings the vim job to foreground

Common Workflow

  1. Start background job:

    long_command &
[1] 12345

  2. Bring to foreground:

    fg %1
long_command

  3. Or resume suspended job:

    # Job was suspended with Ctrl+Z
fg %1

Job Control Cycle

State Command Next State
Running (fg) Ctrl+Z Suspended
Suspended fg Running (fg)
Suspended bg Running (bg)
Running (bg) fg Running (fg)

Common Usage Patterns

  1. Quick foreground switch:

    # List jobs
jobs
# Bring job to foreground
fg %2

  2. Toggle between jobs:

    fg %1  # Switch to job 1
^Z     # Suspend
fg %2  # Switch to job 2

  3. Resume by partial command:

    fg %?backup  # Resume job containing "backup"

Advanced Usage

  1. Bring job and check status:

    jobs -l  # List with PIDs
fg %1    # Bring to foreground

  2. Conditional foreground:

    if jobs | grep -q "editor"; then
    fg %editor
fi

Performance Analysis

  • Instant operation
  • No resource overhead
  • Shell built-in command
  • Efficient process control
  • Real-time job switching

Best Practices

  1. Check job status before using fg
  2. Use specific job identifiers
  3. Understand job states
  4. Monitor job completion
  5. Handle job control signals properly

Error Handling

  1. Job not found:

    fg %99  # Error if job doesn't exist

  2. No current job:

    fg  # Error if no jobs available

  3. Job already in foreground:

    fg %1  # No effect if already foreground

Interactive Examples

  1. Editor workflow:

    vim file.txt    # Start editor
^Z              # Suspend (Ctrl+Z)
ls -la          # Do other work
fg              # Resume editor

  2. Compilation workflow:

    make &          # Start build in background
vim source.c    # Edit while building
^Z              # Suspend editor
fg %make        # Check build progress

Scripting Applications

  1. Job management function:

    resume_job() {
    local job_pattern="$1"
    if jobs | grep -q "$job_pattern"; then
        fg %"$job_pattern"
    else
        echo "Job not found: $job_pattern"
    fi
}

  2. Interactive job selector:

    select_job() {
    echo "Available jobs:"
    jobs
    read -p "Which job to foreground? " job_num
    fg %"$job_num"
}

Integration Examples

  1. With job monitoring:

    # Monitor and manage jobs
while true; do
    jobs
    read -p "Foreground job (or 'q' to quit): " choice
    case $choice in
        q) break ;;
        *) fg %"$choice" ;;
    esac
done

  2. Development environment:

    # Start development tools
code . &           # Editor in background
npm run dev &      # Dev server in background

# Work with tools interactively
fg %code          # Switch to editor
^Z                # Suspend
fg %npm           # Check dev server

Signal Handling

When bringing job to foreground: - Job receives SIGCONT if suspended - Terminal control is transferred - Job can receive keyboard signals - Ctrl+C sends SIGINT to foreground job - Ctrl+Z sends SIGTSTP to suspend job

Shell Compatibility

Shell Support Features
Bash Full Complete job control
Zsh Enhanced Advanced job management
Fish Modern User-friendly syntax
Dash Basic Limited job control

Troubleshooting

  1. Job control disabled:

    set +m  # Disable job control
set -m  # Enable job control

  2. No controlling terminal

  3. Job has exited

  4. Permission issues

  5. Shell doesn’t support job control

Security Considerations

  1. Verify job ownership
  2. Check process legitimacy
  3. Monitor resource usage
  4. Validate job commands
  5. Control process privileges

Alternative Methods

  1. Direct process control:

    kill -CONT $PID  # Resume process

  2. Screen/tmux sessions:

    screen -r session_name
tmux attach -t session_name

  3. Process substitution:

    command < <(background_process)

Real-world Scenarios

  1. System administration:

    # Start system monitor
htop &
# Do other work
ps aux | grep problem_process
# Return to monitor
fg %htop

  2. Development debugging:

    # Start debugger
gdb program &
# Edit source
vim source.c
^Z
# Return to debugger
fg %gdb

Job State Transitions

[Start] → Running(fg) → [Ctrl+Z] → Suspended
   ↓           ↑                        ↓
   &          fg                       bg
   ↓           ↑                        ↓
Running(bg) ←─────────────────── Running(bg)

Monitoring and Control

  1. Job status checking:

    jobs -l | grep Stopped
fg %1  # Resume stopped job

  2. Resource monitoring:

    # Before bringing to foreground
ps -p $(jobs -p %1) -o pid,pcpu,pmem,cmd
fg %1