bg

Overview

The bg command resumes suspended jobs in the background. It’s part of shell job control, allowing you to continue stopped processes without bringing them to the foreground.

Syntax

bg [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. Resume suspended processes
  2. Multitasking in terminal
  3. Job control management
  4. Process workflow optimization
  5. Shell session efficiency

Examples with Explanations

Example 1: Resume Current Job

bg

Resumes the most recently suspended job in background

Example 2: Resume Specific Job

bg %1

Resumes job number 1 in background

Example 3: Resume Multiple Jobs

bg %1 %2 %3

Resumes multiple jobs in background

Common Workflow

  1. Start a command:

    long_running_command

  2. Suspend it (Ctrl+Z):

    ^Z
[1]+  Stopped    long_running_command

  3. Resume in background:

    bg %1
[1]+ long_running_command &

Job Control Sequence

Action Command Result
Start job command Runs in foreground
Suspend Ctrl+Z Job stopped
Background bg Job runs in background
Foreground fg Job returns to foreground
List jobs jobs Show all jobs

Common Usage Patterns

  1. Quick background resume:

    # Suspend current job
^Z
# Resume in background
bg

  2. Manage multiple jobs:

    jobs  # List jobs
bg %2  # Resume job 2

  3. Resume by command name:

    bg %vim  # Resume vim job

Advanced Usage

  1. Resume all stopped jobs:

    for job in $(jobs -s | awk '{print $1}' | tr -d '[]+-'); do
    bg %$job
done

  2. Conditional resume:

    if jobs -s | grep -q "backup"; then
    bg %backup
fi

Performance Analysis

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

Best Practices

  1. Check job status before using bg
  2. Use specific job numbers for clarity
  3. Monitor background jobs regularly
  4. Combine with job listing commands
  5. Understand job control implications

Error Handling

  1. Job not found:

    bg %99  # Error if job doesn't exist

  2. Job already running:

    bg %1  # No effect if already running

  3. No current job:

    bg  # Error if no current job

Scripting Applications

  1. Automated job management:

    #!/bin/bash
# Start job
long_process &
JOB_PID=$!

# Later, if needed to suspend and resume
kill -STOP $JOB_PID
kill -CONT $JOB_PID

  2. Interactive job control:

    manage_jobs() {
    echo "Stopped jobs:"
    jobs -s
    read -p "Resume which job? " job_num
    bg %$job_num
}

Integration Examples

  1. With job monitoring:

    # Check and resume stopped jobs
if jobs -s | grep -q .; then
    echo "Resuming stopped jobs..."
    jobs -s | while read line; do
        job_num=$(echo "$line" | awk '{print $1}' | tr -d '[]+-')
        bg %$job_num
    done
fi

  2. Workflow automation:

    # Start multiple tasks
task1 &
task2 &
task3 &

# If any get suspended, resume them
for job in $(jobs -s | awk '{print $1}' | tr -d '[]+-'); do
    bg %$job
done

Shell Compatibility

Different shells support bg: - Bash: Full support - Zsh: Enhanced features - Fish: Modern syntax - Dash: Basic support - Tcsh: C-shell style

Troubleshooting

  1. Job control not enabled
  2. No jobs to resume
  3. Job already running
  4. Shell doesn’t support job control
  5. Process has exited

Security Considerations

  1. Monitor background processes
  2. Check process ownership
  3. Verify job legitimacy
  4. Resource usage monitoring
  5. Process privilege levels

Alternative Methods

  1. Start directly in background:

    command &

  2. Use nohup for persistence:

    nohup command &

  3. Use screen/tmux for session management:

    screen -S session_name
command
# Ctrl+A, D to detach

Real-world Examples

  1. Development workflow:

    # Start editor
vim file.txt
# Suspend to test
^Z
# Resume editor in background
bg
# Run tests in foreground
make test

  2. System administration:

    # Start backup
backup_script
# Suspend if needed
^Z
# Resume in background
bg
# Continue other tasks

Monitoring Background Jobs

  1. Regular status check:

    watch -n 5 jobs

  2. Job completion notification:

    (sleep 100; echo "Job completed") &

  3. Resource monitoring:

    jobs -l | while read job; do
    pid=$(echo "$job" | awk '{print $2}')
    ps -p $pid -o pid,pcpu,pmem,cmd
done